What’s the difference between all the available pressure sensors and how to choose one? This is what you should consider.
There might be times, as in the COVID-19 crisis, where measuring pressures becomes a question of life and death. What is it that one must keep in mind when selecting a pressure sensor then?
In the Open Source Ventilator – Open Lung initiative we’ve faced this issue, and along the way we’ve learned many important things, let’s give a look at some of them.
Pressure Sensors: what’s the difference?
There’s a wide variety of pressure sensors in the market, and there’s a reason for it: each is optimized for a specific purpose, these are the most common ones:
Note: to ease the explaination and drawings I’m going to show sensors with barbed tubing connections, they are clearly not the only ones available in the market, more later in the article.
- Absolute Pressure Sensors
Absolute pressure sensors are chosen each time comparison between vacuum and the system is needed. They are obtained by proceeding with the assembly of the package substrate in a high vacuum area, effectively sealing the package with a vacuum empty area inside. On the vented side, this small vacuum tank will be exposed by means of a very thin silicon diaphragm which will be slightly bent depending on the measured pressure. This effect is then measured with various techniques, one of which is by applying a strain gauge, read, decoded, and then sent to the external world.
A usage example would be when measuring atmospheric pressure to compute dew points and forecast the weather. The output of this sensor is then similar to the equation: Pout(t)=Pin(t)-Pvacuum.
- Gauge Pressure Sensors
Gauge pressure sensors are chosen each time comparison between system pressure and the local environment air pressure is needed. This is achieved by taking the absolute pressure sensor and… drilling a hole in the vacuum area (poetic license abused). This way, by leaving vented to the environment what was before the vacuum area, we have: Pout(t)=Pin(t)-Penvironment(t).
Note that this time the difference between the two pressures has both elements dependent on time. This means that as the environment pressure varies if the system pressure is constant, the output will change. Gauge sensors are used every time a system working is affected by the surrounding environment and not by its absolute pressure, therefore, gauge sensors are the ones used to measure pressure in patient airways. Gauge sensors are also used in all those applications where there’s a structural limit to the maximum pressure a system can withstand, which may be the case for combustion engines and almost everything that goes under water.
- Differential Pressure Sensors
Differential pressure sensors are used every time a differential reading is needed, which means every time two pressures must be compared. The differential sensor is therefore a sibling of the gauge sensor, for the only difference lies in its vented connection being redirected to another port. To detail this affirmation a bit more, there is indeed another difference. Since gauge sensors are usually installed in systems where high pressures are applied, the range and maximum pressure a gauge sensor can be exposed to are quite broad and high. This is usually not the case for differential sensors where tiny differences are the ones to look for, but many exceptions apply.
Differential sensors are used in a broad list of applications, including measurement of airflow by exploiting the venturi effect or, for the ones capable of withstanding high pressures, all the applications a gauge sensor could handle, especially if in the final application both differential and gauge sensors are required. This has mainly to do with economies of scale. If you can buy 1 part 1000 times instead of 2 parts 500 times each, the latter will be more expensive. Another effect not to underestimate is noise and disturbances. By having 2 ports instead of one, differential sensors enable the system designer to route the environment reference port (when the differential sensor is used as a gauge one) where air is stable.
What would you think we’ve used for the OSV ventilator then? We went for differential sensors only: low pressures(lungs are not engines), lower noise (controlled reference), lower price (scale).
What package should I use?
As with many other components, pressure sensors come in a variety of packages. Each of them has its pros and cons. The choice is then, as always in engineering, dictated by your necessities.
Smaller sensors tend not to have any kind of port one can connect to. An example are the BOSCH BMP280 and BMP180. They are made to measure the absolute environmental pressure in smartphones and other tiny applications, if that’s what you need, then they are a very good choice.
If your system is more complicated than a smartphone or requires tubing connections, I would advise to think carefully before jumping on the BMPx80 train and thinking “we’ll build the tubing connections”. That’s just not so easy, especially if your application is concerned with high pressures, high sampling frequencies, or, worst of all, human lives. In this case, everything that affects the airway must be controlled and checked for outgassing, the release of particles and other substances. When taking into consideration these factors, the 20ish € mark for some sensors with integrated barbed port (as in this article’s cover) starts making sense.
Pressure sensor selection checklist
Generally speaking, the questions you should ask yourself when choosing a pressure sensor are:
- What kind of pressure reading do I need?
Absolute, Gauge, differential, others…
- What is the maximum and minimum pressure level I want to read?
PSI – mmH20 – mm of mercury
- What is the target pressure resolution?
PSI or mmH20 etc per LSB (least significant bit) or equivalent Signal/Noise ratio
- How many samples per second would I like to have? Does it make sense (Physics!)?
Does the airway even allow for such frequencies to be used? Whan phenomena do I want to monitor?
- What are my size constraints?
- How am I going to connect the pressure sensor to my system?
tubing, vented to enclosure air, vented outside, sealed
- What is the maximum budget I can allocate for this system part, including integration?
- How much time do I have to develop pressure sensor integration?
- Do I have specific contaminant constraints?
plastics, epoxy, silicon, etc
- What is going to happen to my monitored system if the sensor fails open?
There are no important or not important questions. Many systems will find some of these points critical and others just superficial, but there’s where you come into the picture. To select what to use for a system must always come after a clear assessment of what’s needed, and especially, of what we do not want to happen.