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FM750 rate controller acting up
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tedbear
Posted 4/27/2023 06:26 (#10205062 - in reply to #10204604)
Subject: RE: FM750 rate controller acting up


Near Intersection of I-35 & I-90 Southern Mn.
Your flow meter is VERY old. It is the model RFM-55. They were used for several years but have been replaced by the RFM-60. The RFM-60 is a direct replacement and would plug right in to your system. The meter cal would likely be different.

You can easily disassemble your RFM-55 by removing the screws. It may be as simple as a piece of debris causing the turbine to hang up. Given its age it is certainly possible that the turbine shaft and associated ceramic bushing have worn considerably thus allowing the turbine to rub or stick against the turbine housing. It will be quite obvious when disassembled.

If you should find debris inside and the turbine and ceramic bushings seem OK, you may wish to try it again. The nut in the end of the turbine housing adjust the amount of free play. It should be adjusted so that if you shake the flow meter you should be able to hear a slight click. Using an air hose, give the flow meter a "shot" of air. The turbine should spin freely and coast to a nice gentle stop. If it stops abruptly, this is a sign of problems.

I doubt that parts are available and a switch to an RFM-60 may be your best plan of action.

Yet another possibility is the wiring leading to the flow meter. This flow meter like many others has a Hall Effect circuit inside. One of the wires in the cable is called the signal wire and when disconnected should show 5V. This is done due a Pull Up resistor in the controller or flow meter. When the turbine turns a magnet causes the Hall effect sensor to short that signal wire to ground dropping the voltage on the signal line to 0 V. As the turbine turns further the next magnet allows the circuit to unlatch and return to 5V. This creates a "pulse". The system counts these pulses and uses the meter cal to convert them to useful units such as gallons. This generally works fine but if the wires should become bare and the signal wire touches the ground wire then extra pulses may be created at times causing all kinds of unusual results.

The RFM-55's were known to have problems where the wire enters the flow meter. The wires are extremely small and fragile. Biting the bullet and getting a new RFM-60 would be my recommendation but look over the flow meter cable.

Edited by tedbear 4/27/2023 06:37
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