Choosing the Right Pressure Sensor
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 adiaphragm 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 Sensors
Bridge-based sensors operate by correlating a physical phenomena, like pressure, to a change in resistance in one or more legs of a Wheatstone bridge. They are the most common type of sensor 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.
Because of the simple construction and durability these sensors are lower cost and makes them ideal for higher channel systems. In general, foil strain gages are used in high-pressure (up to 700M Pa) applications. They also have a higher operating temperature than silicon strain gages (200 °C versus 100 °C, respectively), but silicon strain gages offer the benefit of larger overload capability. Because they are more sensitive, silicon strain gages are also often preferred in low-pressure applications (~2k Pa).
Capacitive Pressure and Piezoelectric 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 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.
Capacitive and piezoelectric pressure transducers are generally stable and linear, but they are sensitive to high temperatures and are more complicated to set up than most pressure sensors. Piezoelectric sensors respond quickly to pressure changes. For this reason, they are used to make rapid pressure measurements from events such as explosions. Because of their superior dynamic performance, piezoelectric sensors are the least cost-effective, and you must be careful to protect their sensitive crystal core.
Conditioned Pressure 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.
Conditioned sensors are typically more expensive because they contain components for filtering and signal amplification, excitation leads, and the regular circuitry for measurement. This is helpful for lower channel systems that do not warrant a dedicated signal conditioning system. Because the conditioning is built in, you can connect the sensor directly to a DAQ device as long as you provide power to the sensor in some way. If you are working with nonconditioned pressure bridge-based sensors, your hardware needs signal conditioning. Check the sensor’s documentation so that you know whether you need additional components for amplification or filtering.
