Pressure Switch input

[nebiter]

In using pressure switches to monitor for pump failure, what would be the plusses/minuses to using Normally Open vs. Normally Closed contacts of the switches themselves? The only difference I can see would be the open cicuit fault where using open on rise connection would not sense an actual pressure loss and using close on rise would transfer to lag pump then (shut down?) alarm needlessly when in fact pressure is OK. Is it a toss of the coin or is there a real difference? My company has always used open on rise apparently just because thats the way we've always done it. Comments please?

[Smoke]

It seen to me that in the US programmers try to keep use a normal open contact for an on condition. Some times the input device doesn't allow a change of state (Like light to dark) I have noticed that in Europe there is a lot of negative logic used. We try to use normal open contacts for the rising state enough that we NOT the input to get a normal close to a normal open. I personaly like to use analog pressure switches/transducers. Then you can see the start of a failure. Edited October 10, 2006 by Smoke

[IO_Rack]

Use Normally Open for failsafe operation. Pressure up and OK = Signal ON. This way, if a circuit fails (wire break, bad connection, etc..) you will alarm. If you have acceptable lag in your pressure then use an appropriate time delay or set your pressure sensor to a lower acceptable setting. If monitoring your pressure too critical for the above solution then I would use and analog sensor.