Discussion on the strange phenomena related to reed switches for remote transmission
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Instrument status: Magnetic flip plate with reed switch for remote transmission. Event sequence: Early in the morning, the control room called to say that the liquid level reading was above the specified range. The on-site measurement showed a signal of 34mA, so it was suspected that the zero point of the reed switch needed to be calibrated. The reed switch was removed and calibrated; after that, a magnet was used to test the signal, and it showed 4-20mA as expected. Once the reed switch was reinstalled, the current reading was still 34mA. As a solution, the reed switch was removed again and fixed from the side using tape, and the problem was resolved. Although the fault has been resolved, I’m not sure why it happened. It’s preliminarily estimated that there is a problem with some part of the magnetic flip switch. Has anyone encountered a similar issue? Feel free to share your insights!
The tie straps are made of stainless steel, so it shouldn’t cause any problems, unless they are made of carbon steel?
They are all made of stainless steel, so I was also puzzled; after replacing one of the remote transmission columns, things returned to normal
There might be a ground connection somewhere in the circuit, especially at the reed switch.
Magnetic flap level gauges in use on-site, whether they have remote transmission capabilities or not, generally come equipped with insulation measures; this involves using rock wool along with protective iron or aluminum enclosures. To prevent the rock wool from falling off during the insulation process, workers usually tie it down with wire. Since wire has magnetic conductivity and residual magnetism, it can develop magnetism over time, which may affect the remote transmission reed switches or the magnetic flaps on-site. This can cause the reed switches to engage in areas where there is no float, resulting in inaccurate level readings. Furthermore, since the transmitter for the remote transmission part of the reed switch is located at the top of the reed switch, this results in an unbalanced design. To secure the reed switch, tape is often used for fixation; when no tape is available on site, wire is used to hold the lower part of the gauge in place. Since this area is higher than the pressure-sensing valve tube on the upper side of the magnetic flip plate, it is assumed that it will not interfere with the operation of the reed switch. However, if there are multiple reed switches in this area and they are above the upper pressure-sensing tube, it can cause a short circuit in the reed switches, generating a signal of more than 20 milliamps. The upper part of the stainless steel column surrounding the reed switch is marked with the maximum measurement limit; when tested with a magnet, the signal does not exceed this limit, so the test appears normal. If the installation position of the remote sensing column equipped with a reed switch is too low, below the liquid pickup tube on the upper side of the magnetic flip board, an over-range condition will occur even when the float is in the upper position. Furthermore, if the reed switch does not return to its original position after engaging during operation, it can also cause inaccuracies.
Your answer was very professional and representative. Nowadays, many factories do not pay enough attention to instruments, which leads to chaotic usage of these instruments. In some construction projects, supervision is lacking, resulting in numerous interference factors for the newly installed instruments, making it difficult for them to function properly. Thank you for your answer! But this isn’t a problem related to the conditions on site; stainless steel pipe clamps were used for binding. The instrument suddenly showed an abnormal full-scale reading, while on-site measurements indicated 34mA. When the reed switch was removed, its zero and scale settings were found to be accurate. However, the full-scale reading appeared again once the switch was reinstalled. It was initially thought that there might be an issue with the magnetic flap on site that was affecting the reed switch; after draining and cleaning the area, it was confirmed that the magnetic float was functioning normally. In the end, replacing the reed switch resolved the problem. It’s frustrating; the problem must lie with the reed switch itself, but I haven’t figured out exactly where the issue is!
Is there any issue where burrs between the solder joints of the upper lead wires of the reed switch cause a slight short circuit between those two wires or a short circuit to the metal tank? Problems can also occur with the transmitter side; many older transmitters use analog circuits that include resistors for zeroing and full-scale adjustment. Poor contact or short circuits in these components can also lead to inaccurate readings. In addition, there are cases of damaged connectors for wires or broken insulation on the wires. Once, a reed switch was disassembled, and it was found that the two wires leading from the reed switch were twisted together like a twist tie; if the insulation on the wires is damaged, it can easily lead to short circuits.
Reed switches are usually soldered onto strip-shaped epoxy circuit boards and then inserted into metal tubes; they are prone to being damaged, accumulation of liquid may occur, the wires might touch the walls at certain points, or there could be a ground connection. It is possible to try installing an isolator at the 4–20mA output terminal in the control cabinet – if everything works properly, it means there is a ground connection somewhere in the external circuit.
Thank you, that’s a professional answer. I’ll go check the replaced reed switch again later