Sample Values - Emerson 375 User Manual

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 then the device has at least 250 of internal
resistance. If the voltage is less than the value
listed on the table for the given current flow, then
more resistance needs to be added to the loop.
If an oscilloscope is available for use then use it to
look for noise on the loop. It is necessary to use a
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.