Using shunt resistors to measure Loop Current

A similar method for non-invasively measuring current in a 4-20 mA instrumentation circuit is to
install a precision resistor in series. If the resistance value is precisely known, the technician merely
needs to measure voltage across it with a voltmeter and use Ohm’s Law to calculate current:

In electronics, such a precision resistor used for measuring current is often referred to as a shunt
resistor. Shunt resistor values are commonly very small, for their purpose is to assist in current
measurement without imposing undue voltage drop within a circuit. It is rare to find a 250 ohm
resistor used strictly as a diagnostic shunt resistor, because the extra voltage drop (1 to 5 volts,
depending on the current signal level) may “starve” loop-powered instruments of voltage necessary
to operate. Shunt resistor values as low as 1 ohm may be found installed in 4-20 mA current loops
at strategic locations.