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EHelp Tips: Signal Limiter and Alarm Trip

Q. On a 4-20mA gas detector, while we replace the cell, the output reverts to 3mA. This causes the device receiving the 4-20mA signal to go into a critical alarm condition and can shut down a critical process. What device do you have that will keep the output at no less then 4mA? It would also be convenient to have this instrument provide me with a relay alarm contact output when my input goes to 3mA. It can be 24Vdc or 120Vac. It should be in a DIN housing.

A. Moore Industries CPT PC-Programmable Temperature Transmitter and Signal Isolator/Converter CPT/HLPRG/0-20MA/117AC/-C [DIN] can be used for this application.The selected model allows you to limit the minimum and maximum output, even when you are replacing the cell. The alarm relay option allows you to set an alarm at whatever level below 4mA for maintenance notification. Figure 1 below shows a configuration that would work for your application. A configuration cable is required to utilize the PC programming capability. 

The CPT has a menu selection for upper and lower limits. You can set the low alarm limit there, such as 3.5mA.

Figure 1. CPT PC Configuration



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