A pressure transducer
, often called a pressure transmitter
, is a transducer that converts pressure into an analog electrical signal. Although there are various types of pressure transducers, one of the most common is the strain-gage base transducer.
The conversion of pressure into an electrical signal is achieved by the physical deformation of strain gages which are bonded into the diaphragm of the pressure transducer and wired into a wheatstone bridge configuration. Pressure applied to the pressure transducer produces a deflection of the diaphragm which introduces strain to the gages. The strain will produce an electrical resistance change proportional to the pressure.
The Electrical Output of Pressure Transducers
Pressure transducers are generally available with three types of electrical output; millivolt, amplified voltage and 4-20mA. Below is a summary of the outputs and when they are best used.
Millivolt Output Pressure Transducers
Transducers with millivolt output are normally the most economical pressure transducers. The output of the millivolt transducer is nominally around 30mV. The actual output is directly proportional to the pressure transducer input power or excitation. If the excitation fluctuates, the output will change also. Because of this dependence on the excitation level, regulated power supplies are suggested for use with millivolt transducers. Because the output signal is so low, the transducer should not be located in an electrically noisy environment. The distances between the transducer and the readout instrument should also be kept relatively short.
Voltage Output Pressure Transducers
Voltage output transducers include integral signal conditioning which provide a much higher output than a millivolt transducer. The output is normally 0-5Vdc or 0-10Vdc. Although model specific, the output of the transducer is not normally a direct function of excitation. This means unregulated power supplies are often sufficient as long as they fall within a specified power range. Because they have a higher level output these transducers are not as susceptible to electrical noise as millivolt transducers and can therefore be used in much more industrial environments.
4-20 mA Output Pressure Transducers
These types of transducers are also known as pressure transmitters. Since a 4-20mA signal is least affected by electrical noise and resistance in the signal wires, these transducers are best used when the signal must be transmitted long distances. It is not uncommon to use these transducers in applications where the lead wire must be 1000 feet or more.
What is the most common indicator that a transducer has been overpressured?
The most common indcation that a transducer has been overpressured is a shift in the zero reading usually in an increasing direction. It may read 5-6 ma or even higher. It can even saturate at a maximum value which is typically around 24 ma.
Choosing Pressure Transducer
Still wondering how to decide what type of pressure transducer or pressure transmitter you need? To learn more about the criteria you should think about when making your decision, this article, Practical Considerations when Choosing a Pressure Transducer
will help you pick out the right device for your application.