A physical process value, such as a temperature, pressure, liquid level, or other physical quantity, is transferred via 4-20 mA signals. The corresponding physical process value is represented by the mA current in the loop.
Example:
A 200 degree temperature is measured via a 4-20 mA temperature transmitter with a 0 to 1000 degree Celsius temperature range. The signal loop will then generate a current of 7.2 mA.
The 4-20mA scaling calculator below converts the current to the measured process value:
The following calculations provide the basis for the 4-20mA to PV scaling calculator:
Formula for converting a current (I) value to a physical process value (PV):
Pv=[(Pv high-Pv low)/(I high-I low)]*(I-I low)+Pv low
In this article, scaling is utilised to establish the connection between a field value (in this case, mA) and a process value (e.g. Temperature, Pressure, Level, flow and etc.).
A linear characteristic is used to convert the electrical mA current loop signal to a physical value. When describing the relationship between the present mA signal and physical process value, this feature is used.
The conversion from a physical value with a starting value other than zero requires extra care. A pressure signal, for instance, ranging from -1 to 1 bar. The current is 12 mA at 0 bar of process pressure. It is suggested that you utilise a scaling calculator in this case because the characteristic between the physical process value and current is irrational due to the 4 mA starting current.
Other typical examples of measuring transmitters with this issue include pH sensors and temperature signals ranging from -80 to 20 degrees Celsius.
When dealing with actual-life environments, it could be required to test instruments using a 4-20 mA simulator. Scaling can be verified and checked quite easily because the simulator permits manual control of the current in the loop.
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