Pressure is defined as the amount of force applied over a unit area. Usually involving liquids and gases, pressure is a critical component of a diverse array of applications,
both those that rely on accurate pressure control as well as those that derive other values (such as depth/level or flow) based upon pressure. Pressure sensors, transmitters, and
transducers are the class of instruments which convert applied pressure into a measurable electrical signal which can be used for display or to trigger a control function.
Transmitters vs. transducers vs. sensors
Although many users—and even some manufacturers— use the terms sensors, transmitters and transducers interchangeably, technically there are differences between them.
Pressure transmitters are current output devices that, generally, have two or three wires that are used to both power the device as well as transmit the output
signal. They are often scaled from 4-20 mA although other ranges are available. Transmitters are ideal for longs runs as wiring is simpler and cheaper and there is no significant
degradation of the output signal with distance.
Pressure transducers are voltage output devices that, generally, have three or four wires to power the device as well as transmit the output signal. Transducers are
better for short runs as electrical resistance can cause errors on longer runs. They are also sensitive to electromagnetic interference.
Pressure sensor is a catch-all term that includes both pressure transmitters as well as pressure transducers.
More about pressure…
As pressure is defined as force over a unit area, there are a number of ways to articulate pressure readings depending upon the unit of force and the unit of area. Most commonly,
we will see PSI ( pounds per square inch) or bar. Other units of measure include kg/cm2, inH2O, mmHg, Pa, and many others.
There are also different types of pressure to consider. The type of pressure refers to the zero reference point of a measurement. For example:
Gauge pressure: The sensor is referenced against atmospheric pressure so it does not include the effects of that pressure. It is equal to absolute pressure minus
ambient air pressure. Sealed gauge sensors may use a fixed pressure different than ambient atmospheric temperature.
Absolute pressure: The sensor is referenced against a perfect vacuum so it, therefore, includes the effects of atmospheric pressure. It is equal to gauge pressure
plus atmospheric pressure.
Differential pressure: Similar to gauge pressure although the reference point is another pressure point rather than ambient pressure. The sensor measures the
difference between two pressures, such as each side of a filter to measure pressure drop.
Pressure Sensing Technology
Though there are many sensing technologies available, most pressure sensors, transmitters, and transducers use force collector type sensors. These electronic sensors employ a force
collector such as a diaphragm or piston to measure the strain caused by force applied over an area. Simply put, pressure applies force to a diaphragm or piston which causes the piston
or diaphragm to move in relation to the amount of pressure. Sensors detect that movement (the strain) and convert to a unit of pressure.
Among force collector sensors, there are two types that we see most often:
Piezoresistive sensors are based upon the piezoresistive effect which describes changes in the electrical resistivity of a semiconductor or metal —commonly silicon,
polysilicon thin film, bonded metal foil, thick film, or sputtered thin film—when mechanical strain (pressure) is applied. Increasing pressure results in changes in the resistivity
of the strain gauges which is detected and converted into an electrical signal proportional to pressure. Generally, the strain gauges are connected to form a Wheatstone bridge circuit
to maximize the output of the sensor and to reduce sensitivity to errors.
Capacitive sensors generally feature two closely spaced, electrically-isolated metallic surfaces one of which acts as a diaphragm by slightly flexing under applied
pressure. The flexing alters the gap between the plates creating, in effect, a variable capacitor. The resulting changes in capacitance can be measured and converted into an electrical
signal proportional to pressure.
Things to consider when purchasing a pressure sensor:
Pressure sensors, transmitters, and transducers can differ dramatically in design, performance, and cost. Some of the factors that may influence your decision on which sensor to
purchase may include:
- What type of output is required?
- What accuracy is required?
- What is the type and range of the pressure?
- What units of measurement are preferred?
- Which process connection is required?
- Are there any issues with material compatibility or chemical resistance?
- What is the temperature range? Is compensation needed?
- Will a local or remote display be used? If remote, what is the distance between the display and the sensor?
- What burst pressure is required?
- Is electrical interference a concern?
- Are any agency approvals needed?
- Is a submersible unit needed or not?
- Does the unit need to be bidirectional?
If you have any questions regarding pressure sensors, transmitters, or transducers please don't hesitate to speak with one of our engineers by e-mailing us at sales@instrumart.com
or calling 1-800-884-4967.
