Pressure Transducer 4-20ma Output Calculator
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User Guide
This pressure transmitter 4 to 20 milliamp current output calculator provides a way to determine the output reading from the applied pressure and pressure transmitter range. A conversion scale graphic for each pressure and 4-20mA current output range combination entered is also displayed below the calculated values.
Formula
This pressure transmitter output calculator uses the following formula to calculate the output over a 4-20mA range:
Linear mA out = 4 + (16 x ((Rdg – Low Limit) / (High Limit – Low Limit)))
Unit
Enter the unit used for measuring pressure, e.g. bar, psi, kPa, inH2O etc…
ReadingThis is the applied pressure measured by the pressure transmitter that you wish to convert to a 4 to 20 milliamp current output reading.
Low LimitEnter the lowest pressure that the pressure transmitter will measure. For example zero would be a typical value for most pressure transmitters, but you can also add negative (-) or positive (+) values as well. Please enter the value in the same pressure units as specified for all other input values.
High LimitEnter the maximum pressure that the pressure transmitter will measure.
Output Reading (mA)This is the expected ideal pressure transmitter current output signal in milliamps for the applied pressure specified.
Why choose 4-20MA as the main signal output for the remote transmission of pressure transmitter?
The traditional output DC signals of pressure transmitters are 0-5V, 0-10V, 1-5V, 0-20mA, 4-20mA, etc. At present, the most widely used one is to use 4-20mA current to transmit analog quantity.The most widely used in industry is to use 4-20mA current to transmit analog quantity. The reason for using the current signal is that it is not easily disturbed. And the internal resistance of the current source is infinite, and the resistance of the wire in series in the loop does not affect the accuracy. It can transmit hundreds of meters on the ordinary twisted pair.
The reason why the signal maximum current chooses 20MA:
The choice of the maximum current of 20MA is based on: safety, practicality, power consumption, and cost considerations.
Safety spark meters can only use low voltage and low current.
Consider the connection distance between the instruments on the production site, the load and other factors.
There are also factors such as power consumption and cost issues, requirements for electronic components, and power supply requirements.
The reason why the signal starting current chooses 4mA:
4-20MA pressure transmitters are mostly two-wire systems. The two-wire system means that the power supply and the load are connected in series, and there is a common point. The signal communication and power supply between the field transmitter and the control room instrument only use two wires.
Why is the starting signal not 0MA?
This is based on two points:
1. The transmitter circuit will not work without the static working current, and the signal starting current is 4mA. DC is the static working current of the transmitter.
2. At the same time, the electrical zero point of the instrument is 4mA.DC, which does not coincide with the mechanical zero point. This "live zero point" is conducive to identifying faults such as power failure and disconnection.
The signal current of general instruments is 4-20mA, which means that the minimum current is 4mA and the maximum current is 20mA.
So use the current signal as the standard transmission of the pressure transmitter!
The characteristic impedance of the twisted pair is about 50 ohms, and the characteristic impedance of a 0.2 square millimeter wire separated by 1 cm in width is about 300 ohms.
Therefore, it is ideal to choose a load resistance of 50-300 ohms. For the convenience of AD conversion, the maximum range of the signal on the load resistance is generally 5-10V.
Then weigh all, the load resistance is 250 ohms, the current is 20 mA, and the load voltage drop is 5V.
So why choose 4-20mA instead of 0-20mA?
In order to reduce the complexity of wiring, it is much simpler to choose 2-wire than multi-wire for the sensor. 2-wire not only transmits signal, but also supplies power to the sensor.
Therefore, the designer supplies power to the sensor amplifier circuit from the 4mA current, so the 4-20mA standard is determined.
