Emerson 375 User Manual page 87

Troubleshooting 5-3
Sample values
Loop Current Flow Minimum voltage for 250 ohms.
4 mA 1 Volt
8 mA 2 Volts
12 mA 3 Volts
16 mA 4 Volts
20 mA 5 Volts
If the voltage across the process-indicating device is
greater than the value listed for a given current flow, the
device has at least 250 ohms of internal resistance. If
the voltage is less than the value listed in the table for
the given current flow, more resistance needs to be
added to the loop.
If an oscilloscope is available, use it to look for noise on
the loop. It is necessary to use a scope with differential
mode capability or a battery powered/isolated scope to
avoid grounding one side of the loop. Noise with a
frequency of 500 Hz to 10,000 Hz is of particular
interest, as this represents frequencies near the HART
frequencies of 1200 and 2200 Hz.