Special-purpose electronic test instruments called loop calibrators are manufactured for the express purpose of 4-20 mA current loop circuit troubleshooting. These versatile instruments are generally capable of not only measuring current, but also sourcing current to unpowered devices in a loop, and also simulating the operation of loop-powered 4-20 mA transmitters.
A very popular loop calibrator unit is the Altek model 334A, a battery-powered, hand-held unit with a rotary knob for current adjustment and toggle switches for mode setting. The following illustration shows how this calibrator would be used to measure current in a functioning input signal loop:
Here, the loop wiring is broken at the negative terminal of the loop-powered transmitter, and the calibrator connected in series to measure current. If this loop had a test diode installed, the calibrator could be connected in parallel with the diode to achieve the same function. Note the polarity of the calibrator’s test leads in relation to the circuit being tested: the calibrator is acting as an unpowered device (a load rather than a source), with the more positive loop terminal connected to the calibrator’s red test lead and the more negative terminal connected to the black test lead.
The same loop calibrator may be used to source (or drive) a 4-20 mA signal into an indicating instrument to test the function of that instrument independently. Here, we see the Altek calibrator used as a current source to send a 16.00 mA signal to the PV (process variable) input of the controller:
No transmitter need be included in this illustration, because the calibrator takes its place. Note how the calibrator is used here as an active source of current rather than a passive load as it was in the last example. The calibrator’s red test lead connects to the controller’s positive input terminal, while the black test lead connects to the negative terminal. The DC power source inside the controller is not used for loop power, because the calibrator in “source” mode provides the necessary power to drive current through the 250 ohm resistor.
An alternative method of sourcing a known current signal into an indicating instrument that provides loop power is to set the loop calibrator to a mode where it mimics the electrical behavior of a loop-powered 2-wire transmitter. In this mode, the calibrator serves to regulate loop current at a user-determined value, but it provides no motivating voltage to drive this current. Instead, it passively relies on some external voltage source in the loop circuit to provide the necessary electromotive force:
Note the polarity of the calibrator’s test leads in relation to the controller: the red test lead connects to the positive loop power terminal while the black lead connects to the positive input terminal. Here, the calibrator acts as a load, just as a loop-powered transmitter acts as an electrical load. The only source of electrical power in this test circuit is the 24 VDC source inside the controller: the same one normally providing energy to the circuit when a loop-powered transmitter is connected.
This simulate transmitter mode is especially useful for testing a 4-20 mA loop at the end of the cable where the transmitter is physically located. After disconnecting the cable wires from the transmitter and re-connecting them to the loop calibrator (set to “simulate” mode), the calibrator may be used to simulate a transmitter measuring any value within its calibrated range.
A legacy loop calibrator still familiar to many instrument technicians at the time of this writing is the classic Transmation model 1040:
Other examples of vintage loop calibrator technology include the Nassau model 8060 (left) and the Biddle Versa-Cal (right):
A modern loop calibrator manufactured by Fluke is the model 705:
With this calibrator, the measure, source, and simulate modes are accessed by repeatedly pushing a button, with the current mode displayed on the screen: