Terminologies and Technologies
Over the years, George has done articles examining numerous types of sensors that measure various physical aspects of our world. But one measurement type he’s not yet discussed in the past is pressure. Here, George looks at pressure sensors in the context of using them in an electronic monitoring or control system. He looks at the math, physics and technology associated with pressure sensors.
Great efforts are expended by numerous R&D laboratories on development of new sensors capable of detection of just about any physical aspect of our world. After all, sensors are what give systems their intelligence. An important physical quantity whose measurement we have not yet discussed in the past is pressure. In this article I’ll look at sensors capable of providing electrical output signal so that it can become a part of an electronic monitoring or control system.
By definition a pressure sensor is a transducer whose purpose is to measure pressure of gases or liquids. A gas or a liquid pressure is equal to the force required to stop that gas or fluid from expanding. It is expressed as a force per unit area.
PHYSICS AND UNITS
Let’s start with the fundamental physics. The SI (metric) system designates pressure as a derived unit called Pascal (Pa), named after the French mathematician and physicist Blaise Pascal (1623–1662). The pressure of 1 Pa represents the force of one Newton (N) exerted per one square meter (m2) area. In the metric system, Pa, as the measure of gas or liquid pressure, is frequently substituted by units called atmosphere (atm) or a Torr—where 1 atm = 101,325 Pa = 760 Torr. In the industry 1 atm is often considered to be a reference pressure.
A bar is a metric unit of pressure equaling to exactly 100,000 Pa. That said, bar hasn’t been approved by the International System of Units for use as a bona fide metric unit. A bar is slightly less than the average atmospheric pressure on Earth at sea level. A common unit of pressure used in North America is PSI, which stands for “pounds per square inch” and is equivalent to 6.894 × 103 Pa in SI units. In North America you always can encounter a unit referred to as PSIA, which represents the absolute pressure in pounds per square inch relative to vacuum—as opposed to the atmospheric pressure at sea level, which is 14.7 PSIA = 1 bar. Similarly, the PSIG (PSI gauge) designation indicates that the atmospheric pressure is included in the measurement.
Torr is a unit of pressure named in honor of the 17th century Italian mathematician and physicist Evangelista Torricelli (1608 –1647). Torricelli is the inventor of the Mercury (Hydrargyrum – Hg) barometer. The principle of the Torricelli barometer is shown in Figure 1. Atmospheric pressure acting on a pool of Mercury in a vessel causes the Mercury to rise inside an evacuated tube to a height corresponding to the atmospheric pressure. This is typically 760 mm at the sea level but it also depends on the temperature and altitude. In fact, barometric pressure has been commonly used to establish altitude. Typically, an altitude of an object is:
This measurement is quite accurate up to about 11,000 m (36,090’). Altitude measurement even with an inexpensive barometric sensor  can achieve resolution of about 0.3 m (approximately 1’)—better than most GPS systems. In general, however, just remember that “normal” barometric pressure is around 760 mm Hg, that is 760 Torr or 1 atm.
I have fond memories of my high school days when building the Torricelli barometer was the first experiment we conducted in the physics class. Those were the “good old days” when the teacher didn’t mind us splashing our bare hands in Mercury in an open vat. Fortunately, those days are over—but the barometer worked and I never forgot how and why.
A millimeter of mercury is also used as a manometric unit of pressure. Most of us are familiar with blood pressure monitors, although most modern instruments use electronic transducers rather than a column of Mercury. The unit was originally defined as the required pressure to raise a column of Mercury by 1 mm, but the definition has been changed to exactly 133.322387415 Pa. It is denoted by the symbol “mmHg.” As one might expect, the inch-of-Mercury pressure is also used and can still be found in aviation and some industries in North America.
Confused? Even though we’ve been talking the same physical quantity, numerous units and methods of measurement have been used over the time based on history, convenience or application. And there are more—we just don’t have the space to discuss them all here. If you are interested, do your research. There are many articles on the Internet explaining different pressure gauges, units, their conversion and practical use.
All those details are secondary for the engineer faced with a task of designing a pressure-based electronic system. The sensor type, its specification and electrical interface should—and usually is—selected by the system designer and included in the system specification. The circuit designer just has to make sure the specification and especially the units of pressure are correct.
Pressure sensors, whether we call them transducers, gauges, indicators or something else have many uses in automatic control. Besides direct measurement and control of pressure their outputs can be used to determine altitude, tire pressure, liquid level, fluid or gas flow speed and many others.
Two subcategories I should mention at this time are similar to ones I mentioned with many other transducers. Many transducers can be divided between sensors and switches. Sensors provide continuous analog or digital signal in some defined way proportional to the magnitude of the measured quantity. Switches, on the other hand, generate a discrete on/off signal when a specific magnitude threshold has been reached. Since the switches are primarily just sensors equipped with some kind of a threshold detecting logic, we can concentrate on sensors only.
When selecting a pressure sensor, you need to consider a number of characteristics. Pressure range, operating temperature, the type of pressure, liquid or gas, size, cost, output signal and others are among the most obvious ones. Do we need an absolute or relative measurement? Absolute sensors provide pressure measurement with respect to perfect vacuum. Relative pressure gauges measure pressure with respect to the existing atmospheric pressure. The relative measurement can be both above or below the atmospheric pressure. In the latter case we usually call such transducers vacuum sensors. If you need to measure differential pressure—such as occurring across pumps, blowers, filters and so forth—differential pressure transducers will do the job.
Pressure transducers can be divided into two basic categories. Force collector and special. Force collector transducers use some mechanical arrangement to convert the pressure acting on a known area into a movement, displacement, strain or a distortion of a mechanical component which can be subsequently converted into an electrical signal. Typical examples would be diaphragms, pistons, bourdon tubes, bellows and others. Various technologies are used to convert the results of the mechanical movement or strain into electrical, usually analog signal, which can be and quite often is these days, digitized. Special type transducers are, as their name suggests, not very common. Some rely on resonant frequency changing with pressure, thermal conductivity, ionization stream and so forth.
Strain or displacement conversion methods are similar to the ones I described in my previous articles on transducers, such as “Accelerometers Revisited” (Circuit Cellar 334, May 2018) . Piezoresistive strain gauge is a popular technology, commonly employed for general purpose measurement. However, it is sensitive to temperature and, therefore requires appropriate compensation. Consequently, piezoresistive transducers such as NXP Semiconductors’ MPL115A2 must contain a temperature sensor as well.
Internally, the strain gauge is usually a part of a Wheatstone bridge (Figure 2). Its resistance increases with the increasing strain. Capacitive sensors rely on a diaphragm being a part of a variable capacitor. Here, the capacitance usually decreases with the rising pressure. Many other methods of displacement or distortion detection are used—some of which I described in my previous articles. Among these are LVDT (linear variable differential transformer), inductance change Hall Effect and others. Imagination has no limits. Piezoelectric effect—due to its very nature—makes piezoelectric sensors unsuitable for measurement of static forces and relegates those transducers to dynamic measurements.
In practical terms, it is easy to experiment with pressure measurement. Various pressure sensors on break-out boards can be purchased from vendors such as Adafruit , SparkFun  and others for literally just a few dollars. Most break-out boards, such as the MPL115A2 I²C Barometric Pressure/Temperature Sensor Board in Figure 3 are available with I2C interface and can be readily used with platforms such as Arduino. Order one of these break-out boards and have fun!
 Adafruit Industries Barometric Pressure/Temperature Sensor https://www.adafruit.com/?q=pressure
 Accelerometers Revisited George Novacek, Circuit Cellar # 334, May 2018
 SparkFun Pressure Sensor MS5637 https://www.sparkfun.com/products/14688
Pressure Sensors, Wikipedia https://en.wikipedia.org/w/index.php?title=Pressure_sensor&oldid=846577974
PUBLISHED IN CIRCUIT CELLAR MAGAZINE • MAY 2019 #346 – Get a PDF of the issue