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Invented by someone French.
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Normally for any parameter measured in field & indicated in some control room, separate instrumentation is required for indication & alarm.
For example consider oil level measurement in tank located 100 metre away from control room by means of ultrasonic transducer, signal conveyed to control room
& displayed by means of analog scale-pointer arrangement. Now for alarm, separate instrumentation would be required (cabling, signal conditioner/ distributor etc.etc.).
The proposed idea is to put an inexpensive electric bell (or similar sort of inexpensive buzzer) next to above analog indicator. When pointer moves beyond predetermined limits, a suitably thin wire (that would practically not resist pointer movement ) would touch a strip to close the circuit & actuate buzzer/ electric bell to prompt operator action.
analog meter with adjustable relay contacts [csea, Nov 15 2004]
||Baked, see [link]. Interestingly, searching for "meter relay" on Google brings up a bunch of hits about race running...
||If you have ultasonic technology, then I don't understand the need for indicators with alarm contacts. Why don't you install a DCS or PLC system?
||[ling]the whole idea is based on cost reduction. if you put analog indicator in control room & proposed alarm based on pointer movement, the involved cost would be nothing compared to PLC or DCS.