Emerson 475, User Manual

Page: 72 / 114

Troubleshooting 5-2
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.