Pressure Transmitter

Pressure Transmitter

A Pressure Transmitter is an instrument connected to a Pressure Transducer. The output of a Pressure Transmitter is an analog electrical voltage or a current signal representing 0 to 100% of the pressure range sensed by the transducer.

Pressure transmitter
Pressure transmitter

Inductance Type PT

Resistance Type PT

Parts of Pressure Transmitter

Measuring Diaphragm

The actual sensor part of the transducer which makes contact with the pressure being sensed is based on several technologies and materials such as Strain Gauge, Capacitance, and Potentiometric. The actual process is separated from the sensing material by a measuring diaphragm. The type of sensor chosen is decided by the application and environment it is being used in.

Output Of transmitter

                                                                                                                          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. 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 

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 milli volt 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.

 

Selection of pressure Transmitter

1.   Application and measurement type

Common type of pressure measurement include: Absolute, gauge, differential, vacuum. The application will determine the most appropriate measurement type.

2.   Pressure range

Probably the single most important decision in selecting a pressure transducer is the range. One must keep in mind two conflicting considerations:

1. The instrument’s accuracy and its protection from overpressure. From an accuracy point of view, the range of a transmitter should be low (normal operating pressure at around the middle of the range), so that error, usually a percentage of full scale, is minimized.

2. On the other hand, one must always consider the consequences of overpressure damage due to operating errors, faulty design (water hammer), or failure to isolate the instrument during pressure-testing and start-up. Therefore, it is important to specify not only the required range, but also the amount of overpressure protection needed.

 

3.   Process media

The process fluid that is going to be measured

4.   Temperature range and installation environment

Extremes in temperature or vibration will limit what transmitters will function properly. For temperature extremes, thin-film technology is superior. The extreme temperatures also create errors in the output of the transducer. The error is often expressed in percent full scale over 1°C (%FS/°C).

5.   Accuracy

Pressure gauges come in many different accuracies. Accuracy of common pressure transducers could range from 0.5% to 0.05 % of the full-scale output. Higher accuracies are desired when it is necessary to read very low pressures for critically demanding applications.

6.   Output

There are several types of outputs for pressure transducers. These include: mV output, amplified voltage output, mA output.