Pressure Measurement (Part-1)

What is Pressure

Pressure is defined as force per unit area that a fluid exerts on its surroundings. 

P = F/A

Pressure, P, is a function of force, F, and area, A.

The SI unit for pressure is the pascal (N/m2), but other common units of pressure include pounds per square inch (psi), atmospheres (atm), bars, inches of mercury (in. Hg), millimetres of mercury (mm Hg), and torr.

A pressure measurement can be described as either static or dynamic. The pressure in cases with no motion is static pressure. Examples of static pressure include the pressure of the air inside a balloon or water inside a basin. Often, the motion of a fluid changes the force applied to its surroundings. Many factors including flow, compressibility of the fluid, and external forces can affect pressure.

 The three methods for measuring pressure are

1) Absolute

2) Gauge and 

3) Differential

Absolute Pressure

The absolute measurement method is relative to 0 Pa, the static pressure in a vacuum. The pressure being measured is acted upon by atmospheric pressure in addition to the pressure of interest. Therefore, absolute pressure measurement includes the effects of atmospheric pressure. This type of measurement is well-suited for atmospheric pressures such as those used in altimeters or vacuum pressures.

Gauge Pressure

Gauge pressure is measured relative to ambient atmospheric pressure. This means that both the reference and the pressure of interest are acted upon by atmospheric pressures. Therefore, gauge pressure measurement excludes the effects of atmospheric pressure. These types of measurements include tire pressure and blood pressure measurements. Similar to absolute pressure, the abbreviations Pag (Pascal’s gauge) or psig (pounds per square inch gauge) are used to describe gauge pressure.

Differential Pressure

Differential pressure is similar to gauge pressure; however, the reference is another pressure point in the system rather than the ambient atmospheric pressure. You can use this method to maintain relative pressure between two vessels such as a compressor tank and an associated feed line. Also, the abbreviations Pad (Pascal’s differential) or PSID (pounds per square inch differential) are used to describe differential pressure.

How do pressure sensors work?

Different measurement conditions, ranges, and materials used in the construction of a sensor lead to a variety of pressure sensor designs. Often you can convert pressure to some intermediate form, such as displacement, by detecting the amount of deflection on a diaphragm positioned in line with the fluid. The sensor then converts this displacement into an electrical output such as voltage or current. Given the known area of the diaphragm, you can then calculate pressure. Pressure sensors are packaged with a scale that provides a method to convert to engineering units.

The three most universal types of pressure transducers are the bridge (strain gage based), variable capacitance, and piezoelectric.

Bridge-Based

Of all the pressure sensors, Wheatstone bridge (strain based) sensors are the most common because they offer solutions that meet varying accuracy, size, ruggedness, and cost constraints. Bridge-based sensors can measure absolute, gauge, or differential pressure in both high- and low-pressure applications. They use a strain gage to detect the deformity of a diaphragm subjected to the applied pressure.

 

When a change in pressure causes the diaphragm to deflect, a corresponding change in resistance is induced on the strain gage, which you can measure with a conditioned DAQ system. You can bond foil strain gages directly to a diaphragm or to an element that is connected mechanically to the diaphragm. Silicon strain gages are sometimes used as well. For this method, you etch resistors on a silicon-based substrate and use transmission fluid to transmit the pressure from the diaphragm to the substrate.

Capacitive Pressure Sensors

A variable capacitance pressure transducer measures the change in capacitance between a metal diaphragm and a fixed metal plate. The capacitance between two metal plates changes if the distance between these two plates changes due to applied pressure.

Piezoelectric Pressure Sensors

Piezoelectric sensors rely on the electrical properties of quartz crystals rather than a resistive bridge transducer. These crystals generate an electrical charge when they are strained. Electrodes transfer the charge from the crystals to an amplifier built into the sensor. These sensors do not require an external excitation source, but they are susceptible to shock and vibration.

Conditioned Pessure Sensors

Sensors that include integrated circuitry, such as amplifiers, are referred to as amplified sensors. These types of sensors may be constructed using bridge-based, capacitive, or piezoelectric transducers. In the case of a bridge-based amplified sensor, the unit itself provides completion resistors and the amplification necessary to measure the pressure directly with a DAQ device. Though excitation must still be provided, the accuracy of the excitation is less important.

 Optical Pressure Sensors

Pressure measurement using optical sensing has many benefits including noise immunity and isolation.