monitoring a fan

Hi I have a fan that rotates at 2800rpm. I need to make sure that the fan is operating. I need to monitor the fan. It has an output that gives a signal 2*rotation so its 5600 pulses which is 933pulses per second. so I guess scan time will be a problem so I cant use normal inputs. The machine has a siemens s7-226. I have read the tutorial of this site so that is my level of expertise with plc programming. I have read the manual on the siemens 27-2xx and it tells me that there is a high speed counter HSC. with something like 30KHz so I guess this is what i need right?
Also do I just need to count 5600 every minute to know that my fan is working, reset the counter and then count again? it seems very easy.....too easy so what am I missing (please dont hurt me

You can't simply mount a flow-switch in the air path?

Hej drenno.

Ahem.. 5600 pulses per minute is 93.3 pulses per second, not 933 pulses per second.
The problem may not be the pulse-to-pulse time, but the the pulse width. Depending on the sensor it can be very short.

Anyway, a dedicated counter input HSC seems to be the proper solution, also for the pulse-width.
You dont have to wait a full minute to read the count value. Every second would be possible depending on the requirement of your application.

Doh! great tak skald du have Jesper

Direct measurement of the air flow / DP is probably safest.. I had to check a sewage pond aerator the other day which wasn't making as much splash as the others, motor speed was fine, a blade had fallen off...If the fan is belt driven or variable pitch etc, can be a bit to go wrong down stream of the motor..

If you don't want to get into the S7, a standalone tacho style box should do the trick to give you a clean set of contacts..