When selecting a pressure sensor, what factors should I consider?
There are three primary considerations when selecting a pressure sensor; pressure values the system will have, temperature of the process material and the compatibility of the sensor with the process fluid.
System pressure values – the normal working pressure should be below the maximum range of the sensor. The overload pressure, which is the pressure at which the measurement cell is damaged, and the bursting pressure or the pressure at which the measurement cell is destroyed should be high enough to provide an adequate margin of safety in any event of overpressure.
Temperature – the normal temperature of the process fluid should fall in range of the compensated temperature range (TEMPCO) for transmitters and in the temperature drift range for switches. The minimum and maximum process fluid temperatures should always stay with in the stated limits.
Process fluid compatibility – since many types of fluids can be corrosive, it is very important to identify and refer to compatibility charts to assure that the process fluid will not harm the sensor, and that the sensor material will not have any affect on the process fluid.
What technical specification features are most important when selecting a pressure sensor?
There are many types of pressure sensors in the market today with a varied list of technical specifications. Some of the more important specs a user should examine would be repeatability and accuracy. Also important are burst pressure, overload pressure, life cycles, material compatibility and ease of use.
Repeatability, or the precision by which repeat measurements of the same pressure sample give the same output value of a sensor, may be one of the most important specifications that a user may want to review. The better the repeatability a sensor has, the more consistently a process will be monitored.
Accuracy, or the closeness of an output of set value to the actual value, is typically considered to be the most important specification. Accuracy of a sensor ensures confidence that the process will be controlled appropriately, and is a good indication of the quality and precision of the sensor and its internal components. A good sensor is one that can utilize its accuracy with a repeatable output value.
What is the difference between Gauge pressure (psig), Absolute pressure (psia) and Differential pressure (psid)?
The pressure of a system is defined as the force exerted by the system on a unit area of its boundaries such as pounds per square inch (psi).
Gauge pressure (psig) is the comparison of system pressure to atmospheric pressure. This is the most common type of pressure that is measured. A sensor measuring gauge pressure would read zero if the measuring cell (element) is exposed to the atmosphere (room pressure) regardless of elevation or other barometric influences.
Absolute pressure (psia) is the comparison of system pressure to an absolute vacuum. As an example, a sensor measuring absolute pressure would read approximately 14.696 psi if the measuring cell (element) is exposed to the atmosphere (room pressure) at sea level.
Differential pressure (psid) is the comparison of one system pressure to another, so the sensor would have two measurement cells that could be connected to two different pressure systems. This is most common in filter type monitoring systems.
Should I select a pressure switch or pressure transmitter for my pressure measurement?
When deciding on whether to use a pressure transmitter or a pressure switch for your system, the main consideration should be the way the output will be processed. If you have a system that just needs to know that a setpoint has been reached, a switch would be well suited for this type of application. If your system is going to monitor the pressure over the complete range of the system pressure and be tied to a monitoring system such as a PLC or other type of controller or recorder, a pressure transmitter is required.
What are the benefits of using an electronic style pressure switch compared to a mechanical style design?
There are many differences in construction between an electronic pressure switch versus a mechanical pressure switch.
A typical mechanical pressure switch consists of either bellows, diaphragms, pistons and springs or some combination of these elements. Since these elements will wear, the actual life span is a typical 1 million switching cycles. A mechanical switch theoretically has a large working range, but each application requires the user to balance switch life against setpoint accuracy. If the monitored system pressure is close to the maximum range of the pressure switch, the user will gain accuracy, but the life expectancy of the switch shortens. If the system pressure is on the low side of the range of the switch, then the accuracy is poor, but the user could expect higher life cycles from the switch. Therefore, the mechanical switch really needs to be operated in a small range in the middle to gain good accuracy and maximum life expectancy.
An electronic pressure switch basically has no moving parts to wear out or break; it utilizes strain gauge technology that measures the strain induced on the sensing element due to changing pressure. This strain is proportional to the applied pressure. Since there are no true moving parts to wear or break, the life cycle of an electronic pressure switch can be 50 million or more switching cycles. Also, the accuracy and repeatability stay consistent and
accurate through the entire allowable pressure range.
What is the difference between a pressure transmitter and pressure transducer?
A pressure transducer is basically a device that converts an applied pressure into an electrical signal. This is typically a millivolt output signal which will vary as the pressure varies. A typical pressure transducer output signal is generated by the primary sensing element such as thick film, ceramic, thin film or semiconductor strain gauge. The transducer also typically contains a circuit to trim the zero and span offsets and compensate for errors over the operating temperature range.
A pressure transmitter is basically a pressure transducer with electronics added to transmit an amplified output signal. This enables signal transmission over large distances and reduces electromagnetic interference. Typically, a pressure transmitter will supply a 4–20 mA signal. Since a pressure transducer that has an amplified voltage output, such as 0-5 VDC or 0-10 VDC, can transmit over large distances, they can be classified as pressure transmitters as well.
View products from AutomationDirect that relate to this article: ProSense Pressure Sensors
By Lenny Filipkowski
AutomationDirect
Product Manager, Industrial Components
Originally Published: June 1, 2008