You may not surprise, in the process of measurement and control, people have been thinking and talking about pressure transmitter, pressure transducer and pressure sensor many times, all these three devices can touch, sense the environment we lived, then transfer kind of electrical output signal, it is series activities from physical deformation to electrical signal sensing.
In case of that, sometime some customers or even some junior engineer make no difference among these three device, they always take the three as the same instrument, and some time you may clear know you need pressure sensor only, what on earth is the difference? How to make correct decision to choose the right detector items without cost much?
To comprehending the difference in what these 3 terms mean is necessary to make sure the measurable device picked is right for the end application; particularly when it comes to cost, power excitation, vulnerability to noise, and also constraints around electrical wiring and installment.
In this article, I will certainly disclose the truth of how to conveniently determine which among 3 is best for you and what is the key features to make them differ with each other, all you find here is the industry rarely know, let’s begin.
If it is just one rule, you can say that the properties of the output tell which kind the sensor is, to distinguish among the various types, it can be useful to think about
Pressure sensor is the most preliminary but critical sensing unit which followed by the features including the millivolt output (always less than 100mv when 5v or 1.5mA excitation) , relative small size, a little higher accuracy compare to it after assembled as pressure transmitter of transducer. The working principles of pressure sensor can be categorized silicon piezo-resistive, thin-film sputtered, ceramic piezo-resistive (thick-film), metal capacitive and others, click here to find more details about different pressure sensing technology.
Pressure transduceras kind of pressure detector has been given a voltage output, which might have a size of a couple of millivolts or numerous volts (always 5v, 0.5-4.5v or 100mv). With signal amplified pcb inside the transducer body, voltage output pressure transducer always work out 0.5-2.5v, 4.5v, 5v or even 10v output, on the contrary, millivolt output pressure transducer with no such amplification process.
Pressure transmitter, on the other hand, takes 10-30vdc as power excitation, and have a present output, normally developed for connecting to the standard 4-20mA present loop, it’ been widely utilized in industrial sensing and control.
- The preliminary sensing unit
- 0-100 millivolt output
- Signal without amplified
- Small and compact size
- Millivolt or voltage output
- Longer connection distance
- Lower power consumption compares
- Moderate susceptible to noise
- Has more linear ability than traducer
- The longest connection distances
- Strong immunity to noise
- Higher power consumption
Millivolt-output pressure transducer
As remains in the name, these transducers output in millivolts (mV). The output signal is proportional to the power excitation, as an example, if it is 5Vdc power excitation, the output can be 0-50mV with a 10mv/v output signal.
The traditional foil-type strain-gauge sensing units can create an output of concerning 2-3mV/ V, whereas today’s MEMS sensors can give about 20mV/V with excellent linearity. Any variant in pressure is determined by measuring small changes in this voltage, which is a result of tiny modifications in resistance (about 0.1%) in the strain gauges themselves.
The layout listed below programs a half-bridge strain-gauge type pressure sensor , showing the excitation voltage and also output voltage. A bigger excitation voltage, say 10V as opposed to 3V, creates a bigger output voltage.
Although the output signal of mv type pressure transducer hasn’t been amplified, it also featured a lot benefits such as competitive cost, small but compact size, on the other hand, the mv type pressure transducer provides engineers more adaptability to design the most suitable interface pcb and make it the most suitable performance in measuring process.
However, there are also some disadvantages to think about for mv pressure transducer.
What is the drawback of millivolt-output pressure transducers
- Need regulated excitation: Since the major output is straight proportional to the excitation, the excitation voltage have to normally be produced using a regulated power supply.
- Low noisy sensitivity: Due to the unamplified output signal, the capability of signal transfer is poor which easy affected by noisy, so millivolt-output pressure transducer is not typically suitable for use in electrically noisy environments.
- Short distance connection: As well as due to the fact that the output voltage is attenuated by the resistance in attaching wires, these wires have to be kept short, suggesting that the sensing unit must be close to the surveillance instrumentation. Regarding 3 to 6 meters is normally the maximum practicable distance.
If the link range is short, and noise is not a trouble, a millivolt-output pressure transducer or even pressure sensor can be easy to design-in, please keep it in mind, millivolt-output sensor or transducer needs a regulated power supply to avoid variations in the excitation which may impact the output in response.
You can also click here to check our previous content : Why you need Millivolt Output Pressure Transducer
Voltage-output pressure transducer
A voltage-output transducer contains extra signal boosting to boost the output voltage of the bridge to a larger amount such as 5V or 10V.
Having a larger output, voltage-output transducers are less susceptible to noise, allowing for usage in harsher electric atmospheres. Longer attaching wires can be made use of, enabling the sensor to be additionally from the panel.
Supply voltages are typically from 8-28VDC. This allows the use of a lower-cost unregulated power supply, other than where the output is 0.5-4.5 V, which calls for a 5VDC controlled supply. Reduced current usage implies voltage output pressure transducer can be powered by battery.
A while back, no matter the traditional voltage-output pressure transducer or current-output pressure transmitter, they both no “live zero”, that means zero pressure, zero output.
However the risk with these is that the system can’t identify the difference between zero pressure or a malfunction, both case will provide zero output.
What is more, the voltage-output pressure transducer enhances the bridge signal, making it a great selection where longer cable television lengths are needed. Reduced noise vulnerability, and a lower-cost uncontrolled power supply are added advantages.
Still have question about the output for pressure transducer, you can refer to our previous article about: What do you use for pressure transducer output?
Current -output pressure transmitter
In contrast to a voltage-output pressure transducer, the pressure transmitter has a low-impedance current output, it usually has been designed to transfer analogue 4-20mA signals. The output might be developed for usage with either a 2-wire or 4-wire present loophole, as both types are popularly adopted throughout our daily life.
4-20mA pressure transmitter supply good electrical noise resistance (EMI/RFI), making them the best choice when the signal must be transfer in very long distance. The current-output pressure transmitter can be powered by an unregulated excitation, so accordingly the current-output type pressure transmitter is generally inappropriate for battery powered devices when running at complete pressure.
A pressure transmitter converts the voltage output to an existing signal, commonly 4-20mA. Noise sensitivity is exceptionally low as well as wire sizes can be a number of hundred meters (1000m-5000m) if no more consideration or power consumption.
Case Study: 4-20mA output pressure transmitter
Like the diagram show above, when the fluid passing through the duct produce 10bar pressure, a 4-20mA current output pressure transmitter has been calibrated to measure which process, the output signal can be sent from pressure transmitter that is proportional to the pressure exerted by the fluid.
| ||mA||Percentage |
If no fluid passing through, there will be no pressure at all, the pressure transmitter then generate 4mA current output signal, on the other hand, if the full fluid through the duct, it may occur the maximal 10bar pressure which sensed by the pressure transmitter through diaphragm deformation extent, a 20mA current output signal will be sent out to the terminal, in case of that, it is easily get the conclusion that the input car of controller may be broken if no current produced.
At the same time, if the input card of controller receives any amount of current output from 4 to 20mA, a 250 Ohm resistor can make current signal converted to certain voltage proportionally. As usual, the ADC can process the voltage from range 1-5V, so here we can easily understand 4-20mA is reasonable to use without any doubt.
We can then make the conclusion that, 4-20mA currently type can help us
- Easily detect open circuit problem, if 0mA, it will difficulty to know whether wire problem or no pressure exerted at all.
- Easily convert current signal to 1-5Vdc.
Click to find out more details of how 4-20mA pressure transmitter works
Why need 4mA instead of 3mA or 1mA?
The pressure transmitter was working on basis of pneumatic measurement when it is firstly introduced to the industry, later on, when electronics come up to manufacture, it require at very least of 3mA to active the function, on the safe side, 4mA with little margin has been proved workable.
We use 4mA as live zero, if it too small mA like 1mA, the output current will be too undetectable or even no current at all, then more wires (3 or 4 wires) instead of 2 will requested for filed instrument which will rise the cost at some extent.
Wy need 20 mA instead of 21mA or 22mA?
Taken 20mA as maximal level because on hand hand, it is energy efficiency, on the other hand, 20mA is most safety range for human hart to withstand (30mA will be kind of dangerous)
When to use current signal output instead of voltage
- 4-20mA current output signals often has more linear ability than the voltage output signal.
- 4-20mA current output signals has low impedance and be better noise immunity.
- 4-20mA current output signals pressure transmitter can transfer the signal to long distance usually more than 1km.
- 4-20mA current output signals is safe for human to withstand even the excitation is 24v or 30vdc
- 4-20mA current output signals is intrinsically safe for hazardous location.
You can also click here to check our previous content :
What is The Difference between Pressure Transducer and Pressure Transmitter?
How to choose the right one
You may now understand that both pressure transducer and pressure transmitter are derive from pressure sensor, they all can be produced out based on the material of silicon or ceramic, designed abide by the principle of piezo-resistive or capacitive, used to measure pressure or level, sent signal either voltage or current.
For better understanding, we wrap up the main features of each in below chart.
|Millivolt pressure transducer||Voltage pressure transducer||Current pressure transmitter |
|Signal without amplified||Moderate susceptible to noise||4-20mA is used in everywhere |
|The lowest cost||Longer connection distance than millivolt pressure transducer||Pressure transmitter has more linear ability than pressure traducer |
|For short distance connection (always 3-6 meters)||Shorter connection distances than pressure transmitter||The longest connection distance (always 1-5km) |
|Sensitive to noisy||Lower power consumption compares to pressure transmitter||Strong immunity to noise |
|Needs regulated voltage (stable bridge-excitation)||Needs unregulated bridge-excitation voltage||Higher power consumption than pressure transducer |
Except the output types for pressure measurement, you can also find more details by clicking here to find out what else need to keep in mind when choose pressure transmitter.
We can help you in Eastsensor
In Eastsensor, our experienced technical expert will be on your assistance anytime, and to help you get out of the question and problem with all they have.
ESS3 series pressure sensor always produces 50 to 100 mv as output signal (before amplified), like ESS319 for gauge pressure, ESS320 for differential pressure, ESS319T for pressure and temperature together, they all with mv output less than 100 usually.
EST3 series pressure transducers always adopt either 10-30v unregulated voltage or 5v regulated voltage as excitation, produce output signal of 4-20mA and 0.5-4.5v in response.
EST4 series smart pressure transmitter is based on metal capacitance sensing, the output can be 4-20mA with HART.
|Span and Measuring Range || || || ||
|Span ||Pressure Range||Level Range||GP||AP||DP||HP
Before removing a pressure transmitter from live service, the technician must “bleed” or “vent” accumulated fluid pressure to atmosphere in order to achieve a zero energy state prior to disconnecting the transmitter from the impulse lines. Some valve manifolds provide a bleed valve for doing just this, but many do not. An inexpensive and common accessory for pressure-sensing instruments (especially transmitters) is the bleed valve fitting or vent valve fitting, installed on the instrument as a discrete device.
The most common bleed fitting is equipped with 1/4 inch male NPT pipe threads, for installation into one of the 1/4 inch female NPT pipe ports typically provided on pressure transmitter flanges. The bleed fitting is operated with a small wrench, loosening a ball-tipped plug off its seat to allow process fluid to escape through a small vent hole in the side of the fitting.
The following photographs show close-up views of a bleed fitting both assembled (left) and with the plug fully extracted from the fitting (right). The bleed hole may be clearly seen in both photographs:
When installed directly on the flanges of a pressure instrument, these bleed valves may be used to bleed unwanted fluids from the pressure chambers, for example bleeding air bubbles from an instrument intended to sense water pressure, or bleeding condensed water out of an instrument intended to sense compressed air pressure.
The following photographs show bleed fittings installed two different ways on the side of a pressure transmitter flange, one way to bleed gas out of a liquid process (located on top) and the other way to bleed liquid out of a gas process (located on bottom):
NOTE : The standard 3-valve manifold, for instance, does not provide a bleed valve – only block and equalizing valves.
With the bleed plug completely removed, the open bleed fitting provides a port through which one may apply air pressure for testing the response of the pressure transmitter. A special test fitting called a bleed port adapter or DP transmitter calibration fitting – colloquially known as a stinger – threads into the opened bleed fitting.
A photograph of a bleed port adapter is shown here:
This special fitting allows a compression-style tube to be temporarily connected to the opened bleed port, which then allows the connection of an air pump and test pressure gauge to the transmitter. Thus, the bleed port adapter enables a technician to conveniently apply test pressures to the DP transmitter without having to loosen any of the instrument manifold bolts, tapered thread pipe connections, or impulse tube compression fittings.
When performing field checks of pressure transmitters, bleed port adapters substantially reduce the amount of time necessary to field-test pressure instruments. The following sequence of illustrations show how a bleed port adapter may be used in conjunction with a three-valve instrument manifold to isolate a DP transmitter from a process and then subject it to test pressures from a hand pump:
Note how both bleed vents must be opened, and the equalizing valve shut, in order to apply a test pressure to the DP transmitter.
Although it is possible to safely bleed pressure from both sides of a DP instrument through just one bleed fitting (through the open equalizing valve), both bleeds must be open in order to perform a pressure test. If the “L” side bleed fitting is left in the shut position, some pressure may be trapped there as pressure is applied to the “H” side by the hand pump. If the equalizing valve is left open, no difference of pressure will be allowed to form across the DP instrument.
The combination of two differential pressure ports makes the DP transmitters very versatile as a pressure-measuring device. This one instrument may be used to measure pressure differences, positive (gauge) pressures, negative (vacuum) pressures, and even absolute pressures, just by connecting the “high” and “low” sensing ports differently.
In every DP transmitters application, there must be some means of connecting the transmitter’s pressure-sensing ports to the points in a process. Metal or plastic tubes (or pipes) work well for this purpose, and are commonly called impulse lines, or gauge lines, or sensing lines. This is equivalent to the test wires used to connect a voltmeter to points in a circuit for measuring voltage. Typically, these tubes are connected to the transmitter and to the process by means of compression fittings which allow for relatively easy disconnection and reconnection of tubes.
Measuring Process Vessel Clogging
We may use the DP transmitters to measure an actual difference of pressure across a process vessel such as a filter, a heat exchanger, or a chemical reactor. The following illustration shows how DP transmitters may be used to measure clogging of a water filter:
Note how the high side of the DP transmitters connects to the upstream side of the filter, and the low side of the transmitter to the downstream side of the filter. This way, increased filter clogging will result in an increased transmitter output. Since the transmitter’s internal pressure sensing diaphragm only responds to differences in pressure between the “high” and “low” ports, the pressure in the filter and pipe relative to the atmosphere is completely irrelevant to the transmitter’s output signal.
The filter could be operating at a line pressure of 10 PSI or 10000 PSI – the only variable the DP transmitters measures is the pressure drop across the filter. If the upstream side is at 10 PSI and the downstream side is at 9 PSI, the differential pressure will be 1 PSI (sometimes labeled as PSID, “D” for differential). If the upstream pressure is 10000 PSI and the downstream pressure is 9999 PSI, the DP transmitters will still see a differential pressure of just 1 PSID.
Likewise, the technician calibrating the DP transmitters on the workbench could use a precise air pressure of just 1 PSI (applied to the “high” port, with the “low” port vented to atmosphere) to simulate either of these real-world conditions. The DP transmitters imply cannot tell the difference between these three scenarios, nor should it be able to tell the difference if its purpose is to exclusively measure differential pressure.
Measuring Positive Gauge Pressure
DP transmitters may also serve as simple gauge pressure instruments if needed, responding to pressures in excess of atmosphere. If we simply connect the “high” side of DP transmitters to a process vessel using an impulse tube, while leaving the “low” side vented to atmosphere, the instrument will interpret any positive pressure in the vessel as a positive difference between the vessel and atmosphere:
Although this may seem like a waste of the transmitter’s abilities (why not just use a simpler gauge pressure transmitter with just one port?), it is actually a very common application for DP transmitters. This usage of a differential device may not actually be a “waste” if true-differential applications exist at the same facility for that pressure transmitter, which means only one spare transmitter need be stocked in the facility’s warehouse instead of two spare transmitters (one of each type).
Most DP transmitters manufacturers offer “gauge pressure” versions of their differential instruments, with the “high” side port open for connection to an impulse line and the “low” side of the sensing element capped off with a special vented flange, effectively performing the same function we see in the above example at a slightly lesser cost. A close-up photograph of a Rosemount model 1151GP gauge pressure transmitter shows the port-less flange on the “low” side of the pressure sensing module. Only the “high” side of the sensor has a place for an impulse line to connect:
So in this regards, the DP Transmitters can serve both differential pressure process and gauge process. This is also the main reason why people can’t tell the obvious difference between DP transmitters and Gauge transmitter if only distinguishes them from appearance.
Also read: Basics of DP Transmitter & Capacitance Differential Pressure Transmitter Working Principle
A closer look at this flange reveals a vent near the bottom, ensuring the “low” side of the pressure-sensing capsule always senses ambient (atmospheric) pressure:
Measuring Absolute Pressure
Absolute pressure is defined as the difference between a given fluid pressure and a perfect vacuum, as opposed to gauge pressure which is the difference between a fluid’s pressure and the atmospheric air pressure. We may build an absolute pressure sensing instrument by taking a DP instrument and sealing the “low” side of its pressure-sensing element in connection to a vacuum chamber. This way, any pressure greater than a perfect vacuum will register as a positive difference:
Most absolute pressure transmitters resemble “gauge pressure” adaptations of DP transmitters, with only one port available to connect an impulse line. Unlike gauge pressure transmitters, though, absolute pressure transmitters do not have vent holes on their “low” sides. The “low” side of an absolute pressure transmitter must be a sealed vacuum in order to accurately measure the “high” side fluid pressure in absolute terms.
Absolute pressure measurement is important for a variety of process applications, including boiling-point control and mass flow measurement of gases. The boiling temperature of any liquid is a function of the absolute pressure it experiences, and in applications where boiling temperature must be precisely controlled in order to achieve a certain outcome (e.g. vacuum distillation of crude oil, for example) the best type of pressure measurement to use absolute. When computing the mass flow rate of gases in a pipe, the relationship between volume and molecular count is a function of both temperature and pressure (both absolute), and so absolute pressure measurement is indispensable here as well.
The same principle of connecting one port of DP transmitters to a process and venting the other works well as a means of measuring vacuum (pressures below that of atmosphere). All we need to do is connect the “low” side to the vacuum process and vent the “high” side to atmosphere:
Any pressure in the process vessel less than atmospheric will register to the DP transmitters as a positive difference (with Phigh greater than Plow). Thus, the stronger the vacuum in the process vessel, the greater the signal output by the transmitter.
This last statement deserves some qualification. It used to be, the way analog pneumatic and electronic transmitters were designed many years ago, that the only way to obtain an increasing signal from a DP transmitters was to ensure the “high” port pressure rose in relation to the “low” port pressure (or conversely stated, to ensure the “low” port pressure dropped in relation to the “high” side pressure). However, with the advent of digital electronic technology, it became rather easy to program DP transmitters with a negative range, for example 0 to −10 PSI. This way, a decreasing pressure as interpreted by the transmitter would yield an increasing output signal.
It is rare to find a pressure transmitter calibrated in such a way, but bear in mind that it is possible. This opens the possibility of using a regular “gauge” pressure transmitter (where the “high” port connects to the process vessel and the “low” port is always vented to atmosphere by virtue of a special flange on the instrument) as a vacuum instrument. If a gauge pressure transmitter is given a negative calibration span, any decreasing pressure seen at the “high” port will yield an increasing output signal.
Article reposted with permission from Instrumentation Tools
Another common electrical pressure sensor design works on the principle of differential capacitance, most of capacitance differential pressure transmitter use it. In this design, the sensing element is a taut metal diaphragm located equidistant between two stationary metal surfaces, comprising three plates for a complementary pair of capacitors. An electrically insulating fill fluid (usually a liquid silicone compound) transfers motion from the isolating diaphragms to the sensing diaphragm, and also doubles as an effective dielectric for the two capacitors:
Eastsensor use three types of fluid compound to fill according to the requirement of measurement process, each has different temperature performance.
- Silicone Oil 200 Temp. Range -40～149℃
- Modified silicone oil Temp. Range 15～315℃
- Fluorocarbon oil Temp. Range -45～205℃
Any difference of pressure across the cell causes the diaphragm to flex in the direction of least pressure. The sensing diaphragm is a precision-manufactured spring element, meaning that its displacement is a predictable function of applied force. The applied force in this case can only be a function of differential pressure acting against the surface area of the diaphragm in accordance with the standard force-pressure-area equation F = PA.
In this case, we have two forces caused by two fluid pressures working against each other, so our force-pressure-area equation may be rewritten to describe resultant force as a function of differential pressure (P1 − P2) and diaphragm area: F = (P1 − P2)A. Since diaphragm area is constant, and force is predictably related to diaphragm displacement, all we need now in order to infer differential pressure is to accurately measure displacement of the diaphragm.
The diaphragm’s secondary function as one plate of two capacitors provides a convenient method for measuring displacement. Since capacitance between conductors is inversely proportional to the distance separating them, capacitance on the low-pressure side will increase while capacitance on the high-pressure side will decrease:
A capacitance detector circuit connected to this cell uses a high-frequency AC excitation signal to measure the different in capacitance between the two halves, translating that into a DC signal which ultimately becomes the signal output by the instrument representing pressure.
These pressure sensors are highly accurate, stable, and rugged. An interesting feature of this design – using two isolating diaphragms to transfer process fluid pressure to a single sensing diaphragm through an internal “fill fluid” – is that the solid frame bounds the motion of the two isolating diaphragms such that neither one is able to force the sensing diaphragm past its elastic limit.
As the illustration shows, the higher-pressure isolating diaphragm gets pushed toward the metal frame, transferring its motion to the sensing diaphragm via the fill fluid. If too much pressure is applied to that side, the isolating diaphragm will merely “flatten” against the solid frame of the capsule and stop moving. This positively limits the isolating diaphragm’s motion so that it cannot possibly exert any more force on the sensing diaphragm, even if additional process fluid pressure is applied. This use of isolating diaphragms and fill fluid to transfer motion to the sensing diaphragm, employed in other styles of differential pressure sensor as well, gives modern differential pressure instruments excellent resistance to over-pressure damage.
It should be noted that the use of a liquid fill fluid is key to this overpressure-resistant design. In order for the sensing diaphragm to accurately translate applied pressure into a proportional capacitance, it must not contact the conductive metal frame surrounding it. In order for any diaphragm to be protected against overpressure, however, it must contact a solid backstop to limit further travel. Thus, the need for non-contact (capacitance) and for contact (overpressure protection) are mutually exclusive, making it nearly impossible to perform both functions with a single sensing diaphragm. Using fill fluid to transfer pressure from isolating diaphragms to the sensing diaphragm allows us to separate the function of capacitive measurement (sensing diaphragm) from the function of overpressure protection (isolation diaphragms) so that each diaphragm may be optimized for a separate purpose.
A classic example of a pressure instrument based on the differential capacitance sensor is the Rosemount model 1151 capacitance differential pressure transmitter, shown in assembled form in the following photograph:
By removing four bolts from the transmitter, we are able to remove two flanges from the pressure capsule, exposing the isolating diaphragms to plain view:
A close-up photograph shows the construction of one of the isolating diaphragms, which unlike the sensing diaphragm is designed to be very flexible. The concentric corrugations in the metal of the diaphragm allow it to easily flex with applied pressure, transmitting process fluid pressure through the silicone fill fluid to the taut sensing diaphragm inside the differential capacitance cell:
The interior of the same differential capacitance sensor (revealed by cutting a Rosemount model 1151 sensor in half with a chop saw) shows the isolating diaphragms, the sensing diaphragm, and the ports connecting them together:
Here, the left-side isolating diaphragm is clearer to see than the right-side isolating diaphragm. A feature clearly evident in this photograph is the small clearance between the left-side isolating diaphragm and the internal metal frame, versus the spacious chamber in which the sensing diaphragm resides.
Recall that these internal spaces are normally occupied by fill fluid, the purpose of which is to transfer pressure from the isolating diaphragms to the sensing diaphragm. As mentioned before, the solid metal frame limits the travel of each isolating diaphragm in such a way that the higher pressure isolating diaphragm “bottoms out” on the metal frame before the sensing diaphragm can be pushed past its elastic limit. In this way, the sensing diaphragm is protected against damage from overpressure because the isolating diaphragms are simply not allowed to move any farther.
The capacitance differential pressure transmitter inherently measures differences in pressure applied between its two sides. In keeping with this functionality, this pressure instrument has two threaded ports into which fluid pressure may be applied. A later section in this chapter will elaborate on the utility of capacitance differential pressure transmitter. All the electronic circuitry necessary for converting the sensor’s differential capacitance into an electronic signal representing pressure is housed in the blue-colored structure above the capsule and flanges. A more modern realization of the differential capacitance pressure-sensing principle is the Rosemount model 3051 capacitance differential pressure transmitter:
As is the case with most of capacitance differential pressure transmitter, this instrument has two ports through which fluid pressure may be applied to the sensor. The sensor, in turn, responds only to the difference in pressure between the ports.
The differential capacitance sensor construction is more complex in this particular pressure instrument, with the plane of the sensing diaphragm perpendicular to the plane of the two isolating diaphragms. This “coplanar” design is more compact than the older style of sensor, and more importantly it isolates the sensing diaphragm from flange bolt stress.
Take particular note of how the sensor assembly is not embedded in the solid metal frame as was the case with the original Rosemount design. Instead, the sensor assembly is relatively isolated from the frame, connected only by two capillary tubes joining it to the isolating diaphragms. This way, stresses inside the metal frame imparted by flange bolts have virtually no effect on the sensor.
A cutaway model of a Rosemount model 3051S (“supermodule”) Capacitance Differential pressure transmitter shows how this all looks in real life:
Process fluid pressure applied to the isolating diaphragm(s) transfers to fill fluid inside the capillary tubes, conveying pressure to the taut diaphragm inside the differential capacitance sensor. Like the classic Rosemount model 1151 design, we see the fill fluid performing multiple functions:
- The fill fluid protects the delicate sensing diaphragm from contact with unclean or corrosive process fluids
- The fill fluid allows the isolating diaphragms to provide overpressure protection for the sensing diaphragm
- The fill fluid provides a medium of constant permittivity for the differential capacitance circuit to function
The “supermodule” series of Rosemount capacitance differential pressure transmitters shares the same coplanar design as the earlier 3051 models, but adds a new design feature: inclusion of the electronics within the stainless-steel module rather than the blue-painted upper housing. This feature allows the transmitter size to be significantly reduced if needed for applications with limited space.
EST4300 Capacitance Differential Pressure Transmitter has the same function which Rosemount 3051 has. Find out details of EST4300 series, please click below links.
Also read: Basics of DP Transmitter
Article reposted with permission from Instrumentation Tools
One of the most common, and most useful, pressure measuring instruments in industry is the differential pressure transmitter (DP Transmitter). This device senses the difference in pressure between two ports and outputs a signal representing that pressure in relation to a calibrated range. Differential pressure transmitters may be based on any of the previously discussed pressure-sensing technologies, so this section focuses on application rather than theory.
Please Click Here to find out more details about Pressure sensing technology, each pros and cons.
DP Transmitter construction and behavior
Differential pressure transmitter, also name DP Transmitter, constructed for industrial measurement applications typically consist of a strong (forged metal) body housing the sensing element(s), topped by a compartment housing the mechanical and/or electronic components necessary to translate the sensed pressure into a standard instrumentation signal (e.g. 3-15 PSI, 4-20 mA, digital fieldbus codes):
Two models of electronic differential pressure transmitter appear in the following photographs, the Rosemount model 1151 (left) and model 3051 (right):
Two more models of electronic differential pressure transmitter are shown in the next photograph, the Yokogawa EJA110 (left) and the Foxboro IDP10 (right):
In each of these DP transmitter examples, the pressure-sensing element is housed in the bottom half of the device (the forged-steel structure) while the electronics are housed in the top half (the colored, round, cast-aluminum structure).
Regardless of make or model, every differential pressure (“DP”, “d/p”, or ΔP) transmitter has two pressure ports to sense different process fluid pressures. These ports typically have 1/4 inch female NPT threads for convenient connection to the process.
Find out more Thread types for process connection please refer to below post
One of these ports is labeled “high” and the other is labeled “low”. This labeling does not necessarily mean that the “high” port must always be at a greater pressure than the “low” port. What these labels represent is the effect any increasing fluid pressure applied to that port will have on the direction of the output signal’s change.
The most common sensing element used by modern DP transmitter is the diaphragm. One side of this diaphragm receives process fluid pressure from the “high” port, while the other receives process fluid pressure from the “low” port. Any difference of pressure between the two ports causes the diaphragm to flex from its normal resting (center) position. This flexing is then translated into an output signal by any number of different technologies, depending on the manufacturer and model of the transmitter:
For EST4300 Smart DP transmitter, it adopts the most precise and stable Capacitance Sensing Technology by Eastsensor.
The concept of differential pressure instrument port labeling is very similar to the “inverting” and “noninverting” labels applied to operational amplifier input terminals:
The “+” and “−” symbols do not imply polarity of the input voltage(s); i.e. it is not as though the “+” input must be more positive than the “−” input. These symbols merely represent the different direction each input tends to drive the output signal. An increasing potential applied to the “+” input drives the opamp’s output positive, while an increasing potential applied to the “−” input drives the opamp’s output negative. Phrasing this in terms common to closed-loop control systems, we could say that the “+” input is direct-acting while the “−” input is reverse-acting.
Similarly, the “H” and “L” labels on a DP transmitter ports do not imply magnitude of input pressures; i.e. it is not as though the “H” port’s pressure must be greater than the “L” port’s pressure. These symbols merely represent the different effects on the output signal resulting from pressure applied to each port. An increasing pressure applied to the “high” port of a DP transmitter will drive the output signal to a greater level (up), while an increasing pressure applied to the “low” port of a DP transmitter will drive the output signal to a lesser level (down):
The ability to arbitrarily connect a DP transmitter to a process in such a way that it is either direct-acting or reverse-acting is a great advantage.
In the world of electronics, we refer to the ability of a differential voltage sensor (such as an operational amplifier) to sense small differences in voltage while ignoring large potentials measured with reference to ground by the phrase common-mode rejection. An ideal operational amplifier completely ignores the amount of voltage common to both input terminals, responding only to the difference in voltage between those terminals. This is precisely what a well-designed DP instrument does, except with fluid pressure instead of electrical voltage. A DP instrument ignores gauge pressure common to both ports, while responding only to differences in pressure between those two ports. Stated in other words, a differential pressure instrument (ideally) responds only to differential pressure while ignoring common-mode pressure.
To illustrate, we may connect the “high” and “low” ports of a DP Transmitter together using pipe or tube, then expose both ports simultaneously to a source of fluid pressure such as pressurized air from an air compressor. If the transmitter is in good working order, it should continue to register zero differential pressure even as we vary the amount of static pressure applied to both ports. So long as the applied pressures to each port are equal, the transmitter’s sensing diaphragm should experience zero net force pushing left or right. All force applied to the diaphragm from the “high” port’s fluid pressure should be precisely countered (canceled) by force applied to the diaphragm from the “low” port’s fluid pressure.
An electrical analogy to this would be connecting both red and block test leads of a voltmeter to a common point in an electrical circuit, then varying the amount of voltage between that point and earth ground. Since the voltmeter only registers differences of potential between its test leads, and those test leads are now electrically common to one another, the magnitude of common-mode voltage between that one point of the circuit and earth ground is irrelevant from the perspective of the voltmeter:
In each case the differential measurement device rejects the common-mode value, registering only the amount of difference (zero) between its sensing points.
The same common-mode rejection principle reveals itself in more complex fluid and electrical circuits. Consider the case of a DP Transmitter and a voltmeter, both used to measure differential quantities in a “divider” circuit :
In each case the differential measurement device responds only to the difference between the two measurement points, rejecting the common-mode value (97.5 PSI for the pressure transmitter, 97.5 volts for the voltmeter). Just to make things interesting in this example, the “high” side of each measuring instrument connects to the point of lesser value, such that the measured difference is a negative quantity. Like digital voltmeters, modern DP transmitter are equally capable of accurately measuring negative pressure differences as well as positive pressure differences.
A vivid contrast between differential pressure and common-mode pressure for a DP instrument is seen in the pressure ratings shown on the nameplate of a Foxboro model 13A differential pressure transmitter:
This nameplate tells us that the transmitter has a calibrated differential pressure range of 50” H2O (50 inches water column, which is only about 1.8 PSI). However, the nameplate also tells us that the transmitter has a maximum working pressure (MWP) of 1500 PSI. “Working pressure” refers to the amount of gauge pressure common to each port, not the differential pressure between ports.
Taking these figures at face value means this transmitter will register zero (no differential pressure) even if the gauge pressure applied equally to both ports is a full 1500 PSI! In other words, this differential pressure transmitter will reject up to 1500 PSI of common-mode gauge pressure, and respond only to small differences in pressure between the ports (1.8 PSI differential being enough to stimulate the transmitter to full scale output).
Article reposted with permission from Instrumentation Tools
4-20mA Transmitter Basics
A sensor is an input device that provides a usable output in response to the input measurand. A sensor is also commonly called a sensing element, primary sensor, or primary detector. The measurand is the physical parameter to be measured.
An input transducer produces an electrical output that is representative of the input measurand. Its output is conditioned and ready for use by the receiving electronics like PLC or DCS.
The receiving electronics can be an indicator, controller, computer, PLC, DCS etc. The term “transmitter,” as commonly used with industrial process control instrumentation, has a more narrow deﬁnition than those of a sensor or transducer:
A transmitter is a transducer that responds to a measured variable by means of a sensing element and converts it to a standardized transmission signal (like 4-20mA) that is a function only of the measured variable.
Transmitters can have any of several electrical connection schemes. The most common and easiest to use is the two-wire, loop-powered conﬁguration. This is generally the basic conﬁguration for industrial process control systems when digital communication is not required. As shown in Below Figure, only two wires are used to accommodate both power to the transmitter and output signal from the transmitter.
To facilitate a closed-loop control system, information from the process must be obtained before a controller can determine what action may be required by a control element. Some popular names for the sensing devices that provide the information are sensors, transducers, and/or transmitters.
The standard loop current is usually 4 to 20 mA. Important calibration parameters with a current loop are Zero, full scale, and span. With the 4- to 20mA range, the loop current is normally 4 mA when the measurand or Process Variable is at zero, and 20 mA when the measurand or Process Variable is at full scale. The difference between Zero and full scale, 16 mA, is called the span. Thus, the span corresponds to the indicated range of the measurand or Process Variable.
When considering a flow transmitter, for example, the range of the measurand or Process Variable is 0.0 to 100.0 m3/hr, corresponding to a 4- to 20-mA loop current (output span is then 16 mA); the output scaling factor is 0.16 mA/(m3/hr) (which is 100 m3/hr 16 mA).
4-20mA Transmitters Working Animation
Assumptions : Standard +24V DC with 20mA
In General PLC / DCS Analog Input card channel supplies more than 20mA current to power the loop.
Case 1 : Process Variable @ 0%
The PLC / DCS Analog Input card transmits a standard +24 V DC, 20 mA signal to Power the Transmitter.
A one pair cable is used to power the transmitter and the same cable is used to receive the data in the 4-20mA current range.
Transmitter receives +24V DC, more than 20mA signal in the loop. A minimum +5V DC, 20mA signal is required to proper functioning of transmitter. In Practical there will be a voltage drop in the loop.
Transmitter have an inbuilt voltage regulator function which is used to regulate the loop current. The Transmitter will be configured with LRV, URV and other details of Process variable. The loop current will be varied / changed by the transmitter as per the measured Process variable.
The 4-20mA current will be converted into standard 1 – 5 V DC using a precision 250 ohms resistor. The Analog to Digital converter will be used to convert the voltage into digital signal which is used to indicate the Process Variable value in the DCS / PLC HMI.
Example: A Flow Transmitter with a range of 0 to 100 m3/hr. Transmitter indicates 0 m3/hr as there is no flow in the line. DCS / PLC powers the transmitter with +24V DC, 20mA. As Process Variable is 0 m3/hr, the transmitter regulates the loop current to 4mA and its equivalent voltage is 1 V DC which is measured by A/D Converter which indicates 0% of Process Variable.
Note: The loop current will be same either at starting or end or any point in the loop. For easy understanding only, both 20mA & 4 mA signals are shown in the animation. In Practical, When we measure the current at any point in the loop, the transmitter output current will be found i.e. 4mA as per above figure, so just assume DCS/PLC system powers the loop with +24V DC, 20mA (generally systems supplies more than 20mA) while transmitter regulates the loop current within 4mA to 20mA as per its configuration & real time process variable value. If you want to measure the PLC/DCS loop power i.e. 20mA (as per above figure) signal then disconnect the transmitter from the loop & Connect the multimeter in series to measure the loop current. And the one pair cable is a twisted pair cable.
Case 2: Process Variable @ 50%
Same Principle applies. The Transmitter adjust the loop current as per the Process Variable.
The only difference is the transmitter sensing element changes its output as process variable varied from 0% to 50%. The transmitter regulates the loop current as per the sensing element.
Example : Process variable indicates 50 m3/hr. Transmitter regulates output to 12mA in the loop as per the configured range and measured process variable and its equivalent voltage is 3 V DC which is measured by A/D Converter which indicates 50% of Process Variable.
Case 3 : Process Variable @ 100%
Same Principle applies. The Transmitter adjust the loop current as per the Process Variable.
The only difference is the transmitter sensing element changes its output as process variable varied from 50% to 100%. The transmitter regulates the loop current as per the sensing element.
Example : Process variable indicates 100 m3/hr. Transmitter regulates outputs to 20mA in the loop as per the configured range and measured process variable and its equivalent voltage is 5 V DC which is measured by A/D Converter which indicates 100% of Process Variable.
Measured or Process Variables
Although almost any type of transducer can be conﬁgured as a transmitter, the most common types for industrial process control comprise measurands or Process Variables like temperature, pressure, flow, level etc. Transmitters for measuring other parameters will have the same possibilities for connection and communication methods, with the main differences being in the sensing element design of transmitter and electronic equipment will be same with slightly software modification.
Two Wire Loops
The main advantage of a two-wire loop is that it minimizes the number of wires needed to run both power and signal. The use of a current loop to send the signal also has the advantages of reduced sensitivity to electrical noise and to loading effects.
The electrical noise is reduced because the two wires are run as a twisted pair, ensuring that each of the two wires receives the same vector of energy from noise sources, such as electro- magnetic ﬁelds due to a changing current in a nearby conductor or electric motor. Since the receiving electronics connected to the transmitter is designed to ignore common-mode signals, the resulting common-mode electrical noise is ignored. The sensitivity to loading effects is reduced because the current in the twisted pair is not affected by the added resistance of long cable runs. A long cable or other series resistance will cause a greater voltage drop but does not affect the current level as long as enough voltage compliance is available in the circuit to supply the signal current.
The circuit compliance to handle a given voltage drop from additional loop devices depends on the transmitter output circuit and on the power supply voltage. The typical power supply for industrial transmitters is +24 VDC. If 6 volts, for example, are needed to power the transmitter and its output circuit, then 18 volts of compliance remain to allow for wire resistance, load resistance, voltage drops across intrinsic safety (IS) barriers and remote displays, etc.
Where the current loop signal is connected to the main receiving equipment (PLC/DCS) or data acquisition system, a precision load resistor of 250 ohms is normally connected. This converts the 4 to 20 mA current signal into a 1 to 5 volt signal, since it is standard practice to conﬁgure the analog-to-digital converter of the receiving equipment (PLC/DCS) as a voltage-sensing input.
Three and Four Wire Loops
In contrast to the two-wire current-loop conﬁguration, some current-loop devices require a three or four wire connections. These are not loop powered and therefore have a separate means for providing power by adding one or two more wires.
In a four-wire conﬁguration, the current-loop wires can be a twisted pair, and the power supply wires a separate twisted pair. This preserves the ability to reject electrical and magnetic common-mode interference. This is not so effective in a three-wire conﬁguration due to the common connection for the return current path. Typically, though, when an instrumentation engineer speciﬁes a current-loop transmitter for industrial process control, it is assumed that a two-wire, loop-powered 4 to 20 mA device is intended. Other data signals may also be impressed upon the same wire pair, or alternatively, various digital communication techniques can be used instead of a current loop, if required.
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Article reposted with permission from Instrumentation Tools