A pressure transmitter is a device used to measure the pressure in a liquid or gas. The working principle of the Resistive pressure transmitter is simple, it consists of the diaphragm which is the core element used to measure pressure. The diaphragm is connected to a strain gauge. The pressure created by the liquid or gas makes the diaphragm to deflect which in turn produces strain in the gauge and there will be a change in resistance and is converted into equivalent Electrical signal ( 4 to 20 ma). The change in resistance is directly proportional to the change in pressure.
Some of the types of pressure transmitters are,
- Gauge pressure transmitter.
- Absolute pressure transmitter
- differential pressure transmitter.
- Multivariable pressure transmitter.
One can easily understand the gauge and absolute pressure transmitter by comparison. The working principle is pretty much the same for both the transmitters.
The Absolute pressure transmitter shows pressure reading with pressure in the atmosphere. i.e. in an ideal state, an absolute pressure transmitter shows 1 atm. When pressure acts on the diaphragm it increases from 1 atm.
Standard atmospheric pressure = 1atm
Absolute pressure = Atmospheric pressure + Gauge pressure.
Gauge pressure is nothing but the pressure produced by the fluids.
So basically a Gauge pressure transmitter displays a 0 as in the ideal state. And increases when a pressure acts on the diaphragm. For a better understanding, the Gauge pressure assumes that the atmospheric pressure is Zero.
The Gauge pressure is denoted by g at the end of the pressure unit. ( eg: psi(g) )
The Absolute pressure is denoted by A at the end of the pressure unit ( eg: psi(A) )
The Vacuum pressure can be measured by both absolute and Gauge pressure transmitters. But mostly Absolute transmitters are used to measure vacuum and depending on range specification, Gauge transmitters can be used.
As explained already in an ideal state the absolute pressure transmitter shows 1 atm. So when a vacuum is detected on a pipeline to which the absolute pressure transmitter is connected then the pressure displayed will be less than 1 atm and reaches zero when the vacuum pressure is maximum. So zero is the perfect Vacuum or the maximum vacuum pressure. Thus there is no negative range in Absolute pressure transmitter.
Depending on the range specification Gauge pressure transmitter is used. Here the negative range is also possible and the negative range indicates the vacuum pressure. When there is a vacuum pressure the reading will range from 0 to -760mm/hg. -760mm/hg being the maximum vacuum pressure.