Instrument and automation technicians are constantly challenged to keep instrumentation loops and I/O working at peak efficiency, in the least possible time.
Technicians find that measuring loop current without breaking the circuit saves a lot of time. Loop calibrators are advanced tools which allow troubleshooting on the spot.
Tracing control loop problems
The first step is to measure the 4 – 20 mA signal, either by breaking the loop connecting in series with a digital multimeter (DMM), or by using a mA clamp meter and verifying the loop current value. There are three likely causes if the measured loop current is not as expected: broken/disconnected/shorted wires, a bad loop power supply, or faulty instrumentation.
If no problem is found in the wires, use a DMM or a clamp meter to check the loop power supply. If the power supply shows no output, use the 24 V loop power function of the meter to substitute for it. If the loop works properly, the source of the problem is obvious.
If the wiring and the power supply both check out, it’s time to check the transmitter. If you have a loop calibrator, process calibrator or multi-function clamp-on meter, use its mA simulate mode to substitute for the transmitter. If the loop performs as requested, the problem lies with the transmitter. If not, it is elsewhere.
If a final control element (valve positioner and others) is suspected, use the mA source/simulate mode on the clamp meter to feed a signal into it while watching the local indicator for a response.
If the problem is not a dead loop but an inaccurate one, likely possibilities include a bad I/O card on the PLC or DCS, or a bad final control element. It’s usually best to start by doing a field check of the transmitter, local or remote indicator or final control element.
For a final control element, use a clamp-on meter to measure loop current and compare the value to the local position indicator on the valve or other final control element. Relay that information to the operator to verify findings.
In the case of a measurement loop, use the clamp meter to measure loop current, then check with the operator to see how well the value indicated on the control panel agrees with the actual loop current. This will give a quick check on the PLC or DCS I/O card which handles the particular loop. It’s also possible to use the meter’s mA source or simulate mode to send a known signal to the control room. As before, compare the value as read by the operator to the actual current in the loop.
Some loops show random fluctuations or intermittent faults that tend not to happen while a technician is watching. The solution here is to use a clamp meter with a scaled mA output. In this mode, the meter measures the current in the loop without breaking the circuit, and produces an identical and isolated mA output. Feed that output to a DMM with a logging function. Any disturbance will be recorded by allowing the DMM to record over time.
Field checks and plant commissioning
Start by using a clamp-on loop current meter to check each loop for current in a matter of seconds, without disconnecting anything. A multifunction clamp meter can also make quick work of diagnostics if a loop is not working. If current is not present on some loops, go on to classic troubleshooting: check the wiring, the power supply and the control system’s I/O cards by using the meter to inject a signal into the I/O, and then by contacting the operator to determine what they see. If the operator agrees with what is being sent, then there may be something amiss with either the transmitter itself or, if this is a new installation, perhaps miswiring or the sensor’s input to the transmitter.
Checking DCS and PLC I/O cards
The mA process clamp meter can be used as an accurate signal source to check the operation of input/output cards on programmable logic controllers (PLCs) and distributed process control systems (DCSs). For 4 – 20 mA input cards, disconnect the process loop and use the meter’s mA source mode to feed in a known signal value (4 mA for zero; 12 mA for 50% using the meter’s 25% step function, and 20 mA for 100%) and compare it to the value shown on the operator’s readout.
Voltage input cards (1 – 5 V or 0 – 10 V) are checked in a similar way, using the meter’s voltage source function.
Checking a valve positioner
Milliamp clamp meters can be used for periodic in-field checks of electronic valve positioners as part of preventive maintenance programmes. Accounting for manufacturer-specific instructions perform quick operational checks using the clamp meter as a signal source while observing the valve stem position, mechanical position indicators, or flow indicators, as input changes are made.
The general method is to set the meter to the 4 – 20 mA source/simulate mode and to connect it to the input terminals of the valve positioner. Set the meter to output 4 mA and wait for the positioner to settle, then vary the current in small increments between 4 and ~3,9 mA while feeling the valve stem with your free hand to check for any sign of movement. Adjust for zero movement between these two current settings by using the zero-adjustment on the positioner.
Next, increase and decrease current from 4 to ~4,1 mA. Ensure that the valve stem just begins movement above the ~4,1 mA setting and closes fully at 4 mA.
Span can be checked similarly by setting the meter at 20; ~19,9 and ~20,1 mA. Linearity can be checked by using the meter’s 25% step function.
Checking loop isolators
To check a loop isolator, apply an mA input signal to the device and measure its 4 – 20 mA output using the clamp-on current measuring function. This two-channel simultaneous source/measurement function in the clamp meter can also be used for valves that report their position using 4 – 20 mA.
Variable frequency drives (VFDs) are used to power motors, blowers and fans in process applications as well as conveyor systems and machine tools. Control inputs are generally voltage (1 – 5 V or 0 – 10 V) or current (4 – 20 mA).
A mA process clamp meter can feed in a signal to simulate a normal input while the technician observes the result.
While not classified as loop calibrators, today’s mA process clamp meters have accuracies of 0,2% and can be used for quick calibration checks while cutting down on the number of instruments needed. For example, checking a process transmitter on the bench normally requires (apart from a pump and separate pressure standard) a loop power supply and an instrument for reading the transmitter’s 4 – 20 mA output. But, with today’s mA process clamp meters, it is possible to power the transmitter and read the output.
Today’s mA process clamp meters can save instrumentation and automation technicians a great deal of time in terms of troubleshooting because they replace a number of separate instruments. The technician no longer has to spend 15 minutes going back to the shop to get an instrument, because the one instrument he takes with him will perform all the necessary functions.
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