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Pressure Transmitter Vent fittings

Pressure Transmitter Vent fittings

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.

DP Transmitters Applications

DP Transmitters Applications

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:

DP Transmitters 01-Eastsensor

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

DP Transmitters 03-Eastsensor

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:

DP Transmitters 04-Eastsensor

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:

DP Transmitters 05-Eastsensor

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.

Measuring vacuum

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:

DP Transmitters 06-Eastsensor

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

Capacitance Differential Pressure Transmitter Working Principle

Capacitance Differential Pressure Transmitter Working Principle

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℃

Capacitance Differential Pressure Transmitter 01 - Eastsensor

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

Basics of DP Transmitter

Basics of DP Transmitter

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):

DP Transmitter 01 – Eastsensor

Two models of electronic differential pressure transmitter appear in the following photographs, the Rosemount model 1151 (left) and model 3051 (right):

DP Transmitter 02 – Eastsensor

Two more models of electronic differential pressure transmitter are shown in the next photograph, the Yokogawa EJA110 (left) and the Foxboro IDP10 (right):

DP Transmitter 03 – Eastsensor

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.

DP Transmitter 04 – Eastsensor

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.

DP Transmitter 05 – Eastsensor

The concept of differential pressure instrument port labeling is very similar to the “inverting” and “noninverting” labels applied to operational amplifier input terminals:

DP Transmitter 06 – Eastsensor

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):

DP Transmitter 07 – Eastsensor

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:

DP Transmitter 08 – Eastsensor

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 :

DP Transmitter 09 – Eastsensor

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:

DP Transmitter 10 – Eastsensor

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

How 4-20mA Transmitter Works

How 4-20mA Transmitter Works

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.

4-20mA Transmitter 001 Eastsensor

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.

4-20mA Transmitter 002 Eastsensor

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.

4-20mA Transmitter 003 Eastsensor

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

What do you use for pressure transducer output?

What do you use for pressure transducer output?

Transducer Input / Output

An electrical input, called the excitation voltage, is required to operate a pressure transducer.  This DC excitation can vary from 5-10VDC for unamplified transducers with millivolt output, to 8-36VDC for amplified transducers producing voltage or current output.

Eastsensor offer three electrical output options:

  1. Millivolt (mV)
  2. Volt (VDC)
  3. Current (4-20mA)

Below is a summary of these outputs with their pros and cons.

Millivolt (mV) Output

Pressure transducers with millivolt output are generally the most economical pressure transducers. They are often called “low-level” transducers because they are unamplified and only contain passive electronics necessary to develop and thermally compensate the low electrical output of the Wheatstone bridge.  This also means they tend to be smaller and lighter than voltage or current output transducers.

EST330V is one of Eastsensor Millivolt output product, for details please refer to the article of Why you need Millivolt Output Pressure Transducers? 

The actual output of these instruments is directly proportional, or ratiometric, to the excitation voltage. This means that if the excitation fluctuates, the output will change proportionally. As a result of this dependence on a steady excitation voltage, regulated power supplies are highly recommended.

Because the output signal is so low, a millivolt output transducer tends to be more affected by EMI and should not be located in an electrically noisy environment (hand radios, switch gear, electric motors, etc.). The distances between the transducer and the readout instrument should also be kept relatively short.

Transducers with a mV output signal typically have a better Response Time than most high-level output transducers because there is less electronic circuitry and no isolation of the excitation voltage from the output signal.

The basic passive electronics in these low-level transducers can withstand higher and lower temperatures than the active amplifying circuits used in high-level transducers. As a result, millivolt output pressure transducers are popular for use in high heat (+400°F) and cryogenic (-450°F) applications.

Voltage (VDC) Output Pressure Transducer 

Voltage output pressure transducers are amplified and add higher level electronics to the low level passive circuit discussed above. This permits the integration of noise filtering, voltage regulation, excitation-to-output isolation, and advanced signal conditioning circuitry.

The additional components of high level output pressure transducers mean they are typically longer and heavier than low level transducers of the same pressure range.

A voltage output transducer provides a much higher output than a millivolt transducer (normally 0-5VDC) and its output can be isolated or non-isolated from the excitation voltage.  Because the output of this transducer is not a direct function of excitation an unregulated power supply is sufficient, provided that it falls within a specified power range.

In addition, the higher level output of this type of electronic circuit is not as susceptible to electrical noise as millivolt transducers and can be used in many more industrial and aerospace environments where greater levels of EMI are found.

The compensated temperature ranges of these transducers generally extend from a low of -65°F [-54°C], to a high of +250°F [+121°C]. This is the normal maximum operational temperature range for active electronic circuits.

For most of Eastsensor product, they can be made of both voltage and 4-20mA output products, you can find model of  EST330F, EST383, EST370, EST3110

Current (4 – 20 mA) Output Pressure Transducer

This type of high-level pressure transducer is also known as a pressure transmitter. Since a 4-20mA current signal is least affected by electrical noise and resistance in the signal wires, these transducers are best used when the signal must be transmitted long distances. It is not uncommon to use these transducers in applications where the lead wire might be 1000 feet [305 meters] or more.

The excitation range of a Taber Industries 4-20mA unit is wider (8-36VDC) than that of transducers with voltage output, and elaborate EMI protection electronics are not necessary due to the nature of the current loop signal arrangement.  Because of this, there is less electronic circuitry and the Response Time is on a par with 0-5VDC non-isolated units.

For most of Eastsensor product, they can also be made of 4-20mA output products.

Wrapping Up 

Voltage output transducers include integral signal conditioning which provide a much higher output than a millivolt transducer. The output is normally 0-5Vdc or 0-10Vdc. Although model specific, the output of the transducer is not normally a direct function of excitation. This means unregulated power supplies are often sufficient as long as they fall within a specified power range. Because they have a higher level output these transducers are not as susceptible to electrical noise as millivolt transducers and can therefore be used in much more industrial environments.

These types of transducers are also known as pressure transmitters. Since a 4-20mA signal is least affected by electrical noise and resistance in the signal wires, these transducers are best used when the signal must be transmitted long distances. It is not uncommon to use these transducers in applications where the lead wire must be 500 metres or more.

So, in short, you must look at your application to find out which output is the best to choose. For higher temperature applications the mV output should be the first consideration. For industrial environments with potentially unregulated power supplies, the voltage outputs will suffice, and for applications where your signal needs to transmit over long distances then a 4-20mA transmitter is perfect.

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