TX92A
TX92A
The TX92A is a compact industrial temperature transmitter by OMEGA Engineering, used to change raw temperature readings into a standard electrical signal computers/systems can understand.
It produces the standard 4-20 mA output.
Embedded Files
The relationship of a transmitter's calibration parameers, zero/offset and gain/span, can be depicted by the slope intercept form y = mx + b, where
y = output
x = input
m = gain/span
b = zero/offset
Terminals
Terminals
Terminals PS+ and PS- (power supply) connect to the power supply
Terminals IN+ and IN- (input) connect ot the
Adjustment screw zero/offset (Z) controls the lower endpoint of the transmitter calibration. It adjusts the output corresponding to the lowest input value (e.g., 0°F → 4 mA). It effectively adds/subtracts an offset to the output.
Adjustment screw span/gain (S) controls the upper endpoint of the transmitter calibration. It adjusts the output corresponding to the highest input value (e.g., 300°F → 20 mA). It changes the gain or slope of the input-to-output relationship line or the line's max value.
Terminal M (measurement/sense lead) or the compensation lead connects to a 3-wire RTD. An RTD sensor changes resistance via temperature. The wires to RTD also have a small resistance. If the transmitter measures only through 2 wires, it'd see RTD resistance with wire resistance, causing a temperature error. The M terminal connects to a transmitter's 3rd wire to estimate the wire resistance and subtract it out.
The left calibration circuit uses both IN terminal but not the M terminal because it uses a thermocouple, which generates a voltage, so calibration is done by via voltage across the IN terminals.
The right calibration circuit uses both IN and M terminals because it uses a calibration resistor, which isn't a voltage source, so the transmitter must create a voltage to send an excitation current from the M terminal to across the resistor to measure its voltage via Ohm's law.
In short, since the TC can generate its own voltage, the transmitter can directly measure the TC's measure, whereas an RTD cannot generate a voltage independently, so the transmitter's M terminal solves this.
[M1]
[M1]
Operation
Operation
As a 4-20 mA device, the TX92A transmitter has a LRV and URV calibration range, from 4 to 20 mA:
LRV (lower range value): corresponds to 4 mA and is controlled by the transmitter's zero parameter.
URV (upper rrange value): corresponds to 20 mA and is controlled by the transmitter's span parameter.
Note: There's no rough value that acts as the boundary separating the control ranges of the transmitter's zero and span adjustement parameters.
Calibration using a multifunction calibrator
Calibration using a multifunction calibrator
[M1]
Calibration The purpose of calibration is to force a value of 4 mA and 20 mA across the precision shunt resistor 250 Ω resistor when voltage is respectively set at 1 V and 5 V in the power supply.
Start by setting PS as whatever value it's required to (24 V in the diagram) and set the value of the calibrator as its lowest value, 0 °F, and is 4 mA is expected through the loop, but the actual current can be different so the zero adjustement screw must be turned (CW or CCW, which depends on parameter's internal wiring) to increase/decrease til the meter reads 4 mA.
Then set the calibrator to its max value, 300 °F, and 20 mA is expected on the meter. Turn the span adjustement screw to force the meter reading toward 20 mA.
Then set calibrator back to its min value, 0 °F and the meter reading should be off again from 4 mA, so the zero span must be turned again back to 4 mA.
And set the calibrator back to 20 mA and adjust the current loop to 20 mA, and so on.
Components The transmitter is a 2-wire loop-powered device - it gets its operating power from same wires used to transmit the 4–20 mA signal.
A precision/calibrator 250 Ω resistor, acting as a precision shunt resistor, is in series to the loop.
At 20 mA, it's at 20mA*250Ω = 5 V (min voltage)
At 4 mA, it's at 4mA*250Ω = 1 V (max voltage)
Instead of a thermocouple, a Prova 123 calibrator, Type K configured as a thermocouple simulator, replaces it to measure temperature.
If 250 Ω resistor is at its min voltage 1 V, 0 °F would be measured and if at 5 V, it measures 300 °F
Current path in the loop The red arrow shows electron flow, the opposite of conventional current, which starts +24 V
Into PS+ → through transmitter → out of PS− → through 250 Ω resistor → back to power supply negative. Thus forming a complete series loop.
Note: What differs a precision resistor from normal resistors is being manufactured at very high accuracy, with a specified tolerance determining how closely their actual resistance value matches the nominal value.
References
References
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