Pressure
Pressure is a measure of the force exerted on a surface per unit area. It is expressed in units such as psi (pounds per square inch), kPa (kilo Pascal), millibar and many others.
Pressure can be exerted by all states of matter. The atmosphere exerts a pressure of 101 kPa on everything at the surface of the earth. Pumping blood exerts a pressure of 120 mmHg against the walls of your arteries.
The pressure exerted by solids is mainly of interest to mechanical or structural engineers and has to be calculated. You cannot simply “sample a bit” of a building to understand the pressure it is exerting on it’s foundation. I’ll be focusing on fluid pressure in this article since that’s what I understand and deal with in my profession.
Why measure pressure?
Pressure is hugely important when dealing with gases. A gas is compressible — it’s volume is a function of temperature and pressure. As pressure increases the volume of a gas is reduced. To put it another way, the relationship between volume and mass is not a constant, i.e. density is a function of pressure. What this means is that “how much” of a gas is contained in a given container (volume) is dependent on temperature and pressure! So if you are buying or selling a gas, it is critical to know the temperature and pressure of the gas.
A standard bedroom (120 sq.ft with 8ft. ceiling) has a volume of 960 ft³ or 27 m³. This room will contain 32.4kg of air (1.2 kg/m³ x 27 m³)¹ at 21C and 101 kPa, i.e. a typical bedroom at a location close to sea level.
The same room in Tibet (elevation 14,370 ft.) at the same temperature will contain only 20.7kg² of air (0.77 kg/m³ x 27 m³) a reduction of 37%. Why is this? This is because the air pressure at 14,370 ft is 58 kPa, 42% less than at sea level!
[1] The quantity 1.2 kg/m³ is density of air. Density is simply the ratio of mass-to-volume of a substance.
[2] The density of air in Tibet was estimated using a model. This is why the reduction in density/mass as a % does not exactly match the reduction in pressure.
Now is a good time to mention absolute and relative units. Pressure in absolute units is a value relative to vacuum, which logically has zero pressure. However pressure readings from most instruments (such as a tire pressure gauge) are relative to local ambient conditions. A tire pressure reading of 30 psi means 30 psi OVER the ambient air pressure at the location of measurement. So a car tire filled to 30psi at the top of a mountain, will not read 30psi at the bottom, with the same gauge!
Unlike gases liquids are incompressible, i.e. it’s density/volume does not change with applied pressure. A glass of water at sea level, is still the same glass of water in Tibet, it’s density and material properties does not change. However pressure of a liquid is of interest because it affects flow rates. Pressure is like Voltage, and it drives flow in a pipe. Pressure is also monitored for safety reasons — an unexpected drop in pressure could indicate a leak in the system.
Types of pressure sensors
The ‘type’ of a pressure sensor can refer to either the type of measurement (absolute, gauge or differential pressure) or the measurement technology (piezoelectric, capacitive, strain gauge).
Types of measurement
Absolute: This is the actual pressure at a given point, measured relative to a perfect vacuum. An absolute PT (pressure transducer) actually contains a sealed vacuum chamber which is used as the reference point for readings produced by the transducer.
Differential: This is the difference in pressure between two points in a system.
A differential PT has two inlets, and the reading reported by the transducer is the pressure difference between the HI and LO ports of the transducer. Differential transducers are extremely sensitive devices and have limitations on the maximum pressure differential that can exist across the two inlets.
For example: A differential PT can be used to measure a differential pressure of 2psi with inlet pressures of 100 and 98 psi, but could be damaged by an unbalanced low pressure input of just 6 psi on the HI side and zero pressure on the LO side.
Gauge: A gauge PT is constructed similar to an absolute PT but instead of a sealed vacuum chamber there is a sealed ambient pressure chamber that has a constant positive pressure reference equivalent to pressure at sea level. In this case the gauge pressure reading would be referenced to sea-level conditions.
Alternately the transducer could actually have a small filtered vent which exposes the sensing element to the actual local ambient conditions and all pressure readings would be referenced to the local ambient conditions.
Gauge pressure can be converted to absolute pressure by adding the value of the reference pressure to the reading. Example: For a gauge pressure of 6 psig measured at sea level the absolute pressure is calculated as 6 + 14.7 = 20.7 psia. 14.7 psi is the pressure of the earth’s atmosphere at sea level relative to a vacuum.
Applications: Absolute transducers are useful in vacuum applications or applications where accuracy is important. The readings from a vented, gauge reference PT is a function of local ambient conditions. The readings will drift (in absolute terms) with changes in ambient atmospheric pressure.
Absolute and sealed gauge PTs might be useful to standardize production processes at factories that exist at different elevations. Differential PTs are useful in applications that require high sensitivity such as leak testing. Vented Gauge PTs are simply digital versions of the standard manual pressure gauge.
Types of Measurement Technology
Capacitive pressure transducer: A capacitive pressure transducer works by measuring the change in capacitance between two electrodes as a result of the application of pressure which deflects the electrodes and changes the distance between them. The deflection of the capacitor electrodes causes the capacitance to change since capacitance is inversely proportional to distance between the electrodes.
In the simplest case a capacitive pressure transducer consists of two metal electrodes that are separated by a thin dielectric material. One side of the capacitor is connected to the diaphragm sensing element that is deflected by the applied pressure. The other side is rigidly connected to the transducer housing and does not move.
When pressure is applied to the transducer, the distance between the electrodes decreases, which causes the capacitance between the electrodes to increase. The change in capacitance is measured by a capacitance detection circuit and converted to a DC current or voltage signal.
A differential transducer can be constructed by using two complementary capacitors as shown in the picture below. In this setup there are two complementary capacitors with a shared sensing diaphragm which also serves as a capacitor plate/electrode. The differential transducer has two inlets and the sensing diaphragm is subject to pressure from both sides.
If the pressures on the two sides is unbalanced the sensing diaphragm will be deflected towards the side with lower pressure. The difference in capacitance between the complementary capacitors is used to measure differential pressure.
Strain gauge pressure transducer: To understand the strain gauge pressure transducer we must first understand strain, and the principle behind the strain gauge. Engineering strain is defined as the ratio of the change in length of an object ΔL relative to its original length(L0) when subject to an applied stress (load).
ΔL / L0 = ε
ε is engineering strain, a dimensionless number (i.e. a ratio/percentage).
In simple terms, all materials ‘stretch’ when pulled, even materials like steel. This change in length due to applied force is the fundamental operating principle of the strain gauge.
Electrical resistance depends on multiple variables such as temperature, Length of conductor and cross sectional area. 💡
A strain gauge is made of extremely thin metal foil that is shaped into a coiled serpentine shape as shown below. This foil is glued to a plate/diaphragm that interacts with the object being measured. The resistance as measured across the wide strips at the bottom changes with applied load due to a change in length/shape of the strain gauge.
Note that a strain gauge is most sensitive when the applied load is parallel to the serpentine grid pattern, because this maximizes the deformation in the strain gauge.
The change in resistance due to the change in length is extremely small, but these small changes can be amplified and converted into a useful signal that can be used to measure weight or pressure.
Piezoresistive pressure transducer: Piezoresistive transducers operate on the same principle as the strain gauge PT where resistance of the sensing element is proportional to applied pressure. However unlike strain gauges which are made of metal alloys, the piezoresistive transducer uses a monocrystalline semiconductor material.
Piezoresistive transducers offer higher sensitivity, in a more compact package than traditional strain gauge transducers.
Industrial PTs are compatible with gases and liquids. The type of fluid is not a factor in the transducer’s accuracy or performance.
Suppliers
Some of the most prominent manufacturers of pressure transducers are listed below:
- https://www.bakerhughes.com/druck/industrial-pressure-sensors
- https://www.stssensors.com/products/gp-sts-pressure-2/
- https://www.emerson.com/en-ca/automation/measurement-instrumentation/pressure-measurement/pressure-transmitters-and-transducers
- https://www.omega.ca/en/pressure-measurement/c/pressure-transducers
- https://www.wika.ca/landingpage_pressure_sensor_en_ca.WIKA
