Emerson 375 User Manual page 89

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. 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.