What are absolute pressure sensors?
We have so many kinds of pressure sensors available, it is not easy for most people to tell what are the differences and which one will be the best for their application, let alone the absolute pressure sensors.
Everyone knows that the weight of atmosphere has its own pressure, called ambient pressure, such pressure amount goes up and down when altitude vary in response, the higher location altitude, the less pressure amount it will be, except altitude, the ambient pressure also affected by whether conditions such as temperature, the hot areas always produce less pressure amount then cold areas.
Having said that, if you want to measure air pressure, for example, to measure the weather barometric, which kind of pressure sensors do you really need? Yes, the absolute pressure sensors will do the job very well.
What is the difference between gauge pressure and absolute pressure?
For most pressure measurement, such as car tire pressure that need gauge pressure detector to measure pressure using ambient air as reference point, in other cases, the measurement always be processed by other reference point like vacuum instead of ambient air.
More time than not, we use pressure sensors to measure air or gas, however that not always the case, in this regard, we make below distinction to explain the difference between gauge and absolute pressure sensors.
- Gauge pressure sensors use air ambient pressure as its zero reference to measure tire or blood pressure, it equal to absolute pressure minus atmosphere pressure (14.7psi).
- Absolute pressure sensors use vacuum as its zero reference to measure altimeter pressure or vacuum pressure, it equal to gauge pressure plus atmosphere pressure(14.7psi).
What affect the accurate readings of absolute pressure sensors?
Differ with gauge pressure which measure pressure in the open atmosphere, the absolute pressure should be measured in a relative vacuum, such process is kind of similar with measure temperature with kelvin at 0°K.
Before measurement, if we call the vacuum as reference point, so all absolute pressure we measured will be bigger than the reference point.
On the other hand, we need to find way to guarantee the measurement accuracy, so all factors that can affect the precision need to be taken into consideration.
We all know Law of Boyle, it described the absolute pressure exerted by a given mass of an ideal gas inversely proportional to the volume it occupies if the temperature and amount of gas remain unchanged with a closed system. —Wikipedia.
That means when at constant temperature, the gas pressure is proportional to its volume, hence all absolute pressure measurement will vary with temperature no matter how perfect the vacuum is.
Can we get perfect vacuum when measure?
Vacuum can be a vessel or space that exhausted by air artificial method such as air pump, in our real world, the perfect 100% vacuum is hardly to achieve, we can only produce, at the most possible degree, human-made vacuum that approximately close to the perfect value.
Above figure shows the relationship between gauge and absolute pressure, 0psia on behalf the highest possible vacuum (0.0005psi around), in theory, vacuum can be any pressure range from 0pisa to 14.7psia, if we make further definition, absolute pressure sensors typically take range of 0.003kpa-0.005kpa as reference points.
How do absolute pressure sensors work?
In order to get the highest possible vacuum as reference point, we need to get a sealed space at first, then many sensing technology can be used to measure absolute pressure, take piezo-resistive as example, it uses four resistors in Wheatstone bridge, one of the resistors has been embedded into the material like silicone or metal thin film.
On top of piezo-resistive, capacitive sensing can also do the job by measure the variation when deformation, for most of the sensing technology, it is common to include temperature compensation pcb on single substrate, such compensation circuitry can definitely assure the accurate measurement at relative large temperature range.
For more sensing technology compassion, please click below links to find details:
- 5 Pressure Sensor Working Principles You Need to Know
- Pressure Sensor Technology Comparison
- The Growth of MEMS Technology and the Benefits to our Life
How do I integrate an absolute pressure sensors into my circuit?
As we are talking about above, so many sensing technologies we are adopting to measure absolute pressure at different circumstances, so there is no single way to make absolute pressure sensors connected with pcb, anyway in most cases, the microcontroller will be indispensable.
For analog signal output
You may need many circuitries and make them work together to condition the output signal, the absolute pressure sensors offer the access to Wheatstone bridge pcb may need to be amplified to produce a larger signal (0-5v or 4-20mA).
When consider the tolerances of different material, you may also need your pcb be calibrated well individually, like the figures shows below, A quad op-amp perform the function of amplification.
You need to design such pcb carefully, sometime other necessary process like low-noise and screening will be require for stable signal output.
For digital signal output
Digital output protocol can integrate the circuitries together including conditioning, compensation, amplification, so digital output probably be some extent easier when connect to microcontroller and store the data into register.
In the modern industry line, many digital communication protocols are available, they differ with each from speed, noise immunity, to distance and connectivity, below figure illustrates the differences.
In case of transmission speed is critical, but noisy not so sensitive, I2C interface will be the ideal choice for digital output, if the absolute pressure sensors have been designed with I2C output, they can read data and control power on and off to reduce power consumption through microprocessor operation.
ESS319-I2C pressure sensors is designed to fulfill the integrated mission of digital compensation, pressure sensor offset, sensitivity, temperature drift compensation and nonlinearity.
SPI protocol can do the similar job with a little shorter distance and slower speed when compare to I2C. However, both I2C and SPI protocols only support certain amount of data to transfer per second, the more devices hanging on the bus, the more drop of bandwidth will happen, so they are not suitable to measure pressure with very quick variation.
You can refer to kind of tailor-made analog to digital controller with reasonable conversion requirement.
Can I use an absolute pressure sensors in my design?
For some light consumer products, like sport watch or altimeter, you need absolute pressure sensors that can be fixed with printed circuit board (pcb) through holes.
In other industry case, high temperature, chemical medium, dust and moisture is normal, the industry level absolute pressure sensors, which designed with robust and anti-corrosive material like SS316, connected with different kind of threads (Click to find difference among BSP, NPT, M and other types of thread), packed robustly, will be good at that environment.
As industry engineers, they know clearly which kind of performance will be ideal for their application, so in large extent they hope to find the correct absolute pressure sensors with the right data they need, right process thread connect with pipe line, right cost within their budget, right time before the manufacturer delivery the absolute pressure sensors at place, that will be great.
If there were other things need to be done, or even taken into consideration like circuitry customization, temperature drift compensation, response time test, maybe they are not so interested in building their own sensors any more.
What applications are absolute pressure sensors used in?
In consumer Electronics, absolute pressure sensors can help to measure elevation above sea level, like altimeter, sport watch.
For weather forecast, absolute pressure sensors can be used for barometric pressure measurement.
For auto vehicle industry, the vehicle ECU need accurate information to diagnose the optimal time for ignition and optimal proportion between air and fuel, absolute pressure sensors are absolutely necessary.
In food packing process, the remained pressure within the food bag can determine the food expiration date, to keep the equal vacuum pressure in each bag is so critical, then absolute pressure sensors have been used to measure if the vacuum in each food package is the same.