Using a clamp-on milliammeter to measure loop current

One better way to measure a 4-20 mA signal without interrupting it is to do so magnetically, using
a clamp-on milliammeter. Modern Hall-effect sensors are sensitive and accurate enough to monitor
the weak magnetic fields created by the passage of small DC currents in wires. Ammeters using
Hall-effect sensors have are completely non-intrusive because they merely clamp around the wire,
with no need to “break” the circuit. An example of a such a clamp-on current meter is the Fluke
model 771, shown in this photograph:

Note how this milliammeter not only registers loop current (3.98 mA as shown in the photograph), but it also converts the milliamp value into a percentage of range, following the 4 to 20 mA signal standard. One disadvantage to be aware of for clamp-on milliammeters is the susceptibility to error from strong external magnetic fields. Steady magnetic fields (from permanent magnets or DC-powered electromagnets) may be compensated for by performing a “zero” adjustment with the instrument held in a similar orientation prior to measuring loop current through a wire.