The need to measure pressure accurately is a requirement in many industries, such as plastic injection molding, automotive, aviation, oil and gas, and HVAC. Pressure measurement is used not only to monitor equipment and processes but can also control and maintain pressure requirements in certain applications.
Depending on the application required for the pressure measurement, there several different types of equipment that can be used. These include pressure gauges and pressure transducers.
The terms pressure gauge and pressure transducer are often used interchangeably, however, pressure gauges and pressure transducers are used for different applications and the proper instrument needs to be chosen based on the equipment being monitored and the process requirements.
A pressure gauge is a pressure indicating device designed to measure equipment or system pressures. Pressure gauges are reliable and relatively inexpensive compared to other pressure measurement devices. Pressure gauges are the simplest method to measure pressure. however, they only provide a measurement at the location where they are installed, so someone physically needs to look at the gauge to obtain the pressure reading.
There are both mechanical and digital types of pressure gauges. Both types can be purchased in a wide variety of pressure ranges and accuracies. Pressure gauges have accuracies of as low as 0.25% of the range of the gauge. Most mechanical pressure gages only provide pressure readings in one pressure unit, such as PSI, but the digital gauges can convert between multiple units. Mechanical pressure gauges are sensitive to vibration and condensation, so they should not be used in applications where those elements are present.
A pressure transducer is an instrument that converts pressure into an electronic output signal. Although there are various types of pressure transducers, one of the most common types is based on the strain-gage principle. The strain-gage transducer contains a type of resistor network that is arranged in the form of a Wheatstone Bridge circuit. The pressure is converted into an electrical signal due to the change in resistance as the pressure fluctuates on the resistor network. The resistance variations change the flow of current passing through the network which is measured on an indicator.
Pressure transducers typically provide higher accuracies than do pressure gages and can measure better than 0.1 % of the transducer reading.
The output signal is typically either a 4–20 mA or 1 to 5-volt output signal and is proportional to the indicated pressure. For example, a 1000-psi pressure transducer with a 0 to 1-volt output will indicate 0.5 volts @ 500 psi and 1 volt at 1000 psi.
This proportional signal can be used in various automated data acquisition or computer control systems to continuously monitor and manage the output parameters of the system. These output signals also provide flexibility to use scaling factors to format the output signal to read any type of pressure units.
Pressure transducers require an external ‘Excitation voltage’ to provide the proper source voltage for the transducer to operate. The source voltage is typically 10 VDC but can range anywhere from 8-28VDC depending on the transducer. Most transducer indicators have the excitation voltage built into the unit, so you do not need a separate power supply for the excitation voltage.
The electrical output signals of most pressure transducers are relatively weak and should be used within 10 to 20 feet of the indicator to maintain adequate signal strength. If the signals are required to be transmitted over longer distances, a pressure transmitter is required that contains a signal conditioner that amplifies the output signal. That signal amplification allows the transducer to be used in digital data acquisition systems or controllers that are located at further distances from the transducer. These longer signal paths must also be protected against both electromagnetic and radio frequency interference.