Published on: November 9, 2015
By: Valcor Nuclear
Troubleshooting Reed Switches Used In Valve Position Indication
A common method to provide remote position indication in solenoid operated valves (SOV) is to use non-contact reed switches in conjunction with a permanent magnet. This article describes some common issues experienced with non-contact reed switches when used in these systems, and how to diagnose and fix the problems.
As part of a position indicating system in SOV’s, reed switches are normally mounted in parallel to a moving armature that has a magnet mounted to one end. In SOV’s, this armature can sometime be referred to as the “plunger”. When the valve is operated, the plunger and the magnet move in parallel to the stationary reed switch, and as the magnet travels it will move “in” and “out” of the sensing area (or range) of the reed switch. When it is in the sensing range, the magnetic force from the magnet will cause the clapper in the reed to make contact or “close”. When the magnet it is out of the range, the clapper in the reed switch will move back to the “open” position.
Standard non-contact reed switches used in position indication systems have Tungsten contact surfaces, good for 120 DC/AC voltages, and can handle up to 100 watts DC maximum. The standard pull-in range (the value that initiates the movement of the clapper to the closed position) is approximately 40-110 Ampere Turns. The lower the number of Ampere Turns for a switch would indicate the higher the sensitivity for that switch.
In measuring the resistance of a switch, the switch manufacturer first determines the minimum Ampere turns to pull-in for the switch, then adds 50% margin to that minimum pull-in. For example, if the pull-in for a switch is 80 Ampere Turns, a value of 120 Ampere Turns is used to close the switch. At this point, an electrical circuit is used to measure the resistance of the switch from the ends of the switch leads. This electrical circuit is powered by a minimum of 10 volts. A typical acceptable Initial Contact Resistance is 0.5 Ohms measured at room temperature between the leads. The manufacturer of the switch recommends that devices like a multi-meter should not be used to measure the resistance of a reed switch, as this can introduce an oxidation or film on the surfaces of the tungsten contacts. This film can raise the resistance of the switch dramatically, upwards to 200 ohms. If a multi-meter is used, the switch should be powered in a circuit at the time of the measurement.
Generally, the resistance of a reed switch will increase with temperature. The magnetic force of a permanent magnet decreases with increased temperature. With decreased magnetic force, the contacts of the switch will have less contact and further increase the resistance of the switch. Along with these factors, soldering of pig tails to the switch leads will also increase the resistance (if resistance is measured at the end of the pigtails). Many factors can change the resistance of a switch.
Some of the problems one can experience with using non-contact reed switches in a position indicating system are switch set point drifting, reed switch sensitivity, and erratic indication. These problems are common in applications involving high temperatures, position indication in SOV’s with relatively short strokes or installations where the SOV’s are located nearby structures that can cause magnetic interference, e.g. metal beams, etc.
Troubleshooting and Solutions
So what to do when the reed switches are not providing the correct position of the SOV?
First, recognize the failure mode. The failure modes include no indication (the lamp from the reed switch does not come on when the SOV is stroked), constant indication (lamp from reed switch does not go off when the SOV is stroked), flickering indication, and dual indication (both on and off lamps are on at the same time). Once the failure mode is established, one can begin to fix the failure. In the case of no indication, move the switch into the sensing range or changing to a more sensitive switch. In the case of constant indication, move the switch out of the sensing range or change to a less sensitive switch. For flickering indication, most likely the switch is just on the “edge” of the sensing range. Try moving the switch into the sensing range. For dual indication, find out which switch is incorrect and fix only that switch. Lastly, check and double check your work by stroking the valve several times and verify correct indication from the reed switches.
Remember that multi-meters are not recommended for use with reed switches. Any testing device should provide power of a minimum of 3 watts of power through the switch. There are SOV design improvements that make setting switches an easier task such as Vernier switch setting knobs. These Verniers are used to move the switch up or down in small increments without any tools, simply turn the Vernier knob. Another tool that can be helpful in setting switches is a test box. The test box allows for insitu diagnosis and operation of the SOV/switches without the need to communicate with the control room. Poor indication from reed switches can be a headache. These tools can help to cut down on maintenance time and ultimately cut down on personal radioactive dose.
* This is for informational purpose only. The values given in this paper are for reference only and not to be used as recommendations for field applications.