Siemens SITRANS FMS500 - Fluctuating process values

Electrode resistance check
• Measure the electrode resistance between connections 82 and 0 with a moving coil meter.
With a sensor full of 󹪞uid the resistance should be between 5 kΩ and 50 kΩ.
If the sensor is empty the resistance will be in󹪝nite.
• Repeat the resistance measurements between connections 83 and 0.
The results should be the same.
If the resistance is low there may be a short on the electrodes or wiring (in the case of a
remote mounted transmitter). Alternatively there may be water ingress or moisture in the
terminal box.
If the resistance is high and the pipe is completely full of 󹪞uid check the following:
1. Fluid is electrically conductive.
2. Electrodes are not coated with grease or any deposit.
3. Electrode circuit is not open.
4. Remote mounted transmitter has a 3 core cable with an overall shield continuously from
sensor to transmitter, including junction boxes and terminal rails inside panels.
5. Shield is connected to 0 or to earth terminal (PE) on sensor.
Note
Sensors removed from line
For sensors removed from line with dry bore, use megger between terminal 82 and
compression plate, and 83 and compression plate to show any water ingress behind
electrodes or within enclosure.
7.2 Fluctuating process values
Question
Why do the displayed process values 󹪞uctuate when the electrode cable is moved?
Answer
There can be several causes for 󹪞uctuating process values:
• Deposits on electrodes
– Clean the electrodes.
• Defect electrode cable
– Replace the cable
• Incorrect cable connection
– Connect the electrode cable (82, 83, 0 and shield) according to the instructions
in Electrical connection (Page32)
Diagnostics and Troubleshooting
7.2Fluctuating process values
SITRANS FMS500
44 Operating Instructions, 01/2024, A5E52677520-AA