A pressure switch compares the pressure acting on the area of a piston or diaphragm against an opposing spring force. Sometimes the opposing force is the buckling force of a thin disk.
A differential pressure switch compares two different pressures acting on opposite sides of the same piston or diaphragm.
Pressure switches switch on or off, based on pressure were as a differential pressure switch is a device which utilizes a differential air pressure to actuate an electric switch at a pre-set actuation point. This may be the difference between two positive or two negative pressures, one of each, or a positive and atmospheric or a negative and atmospheric pressure. The electric switch may be used to start or stop motors or fans, open or close dampers or louvers, light a warning signal, sound alarms, etc.
I have to use these switches on the discharge side of the fans to know the status of fan. Switches are interfaced with platform control system to give an alarm incase discharge pressure from the fan drops below a set point
From the response I understand that for this purpose, pressure switch should be used.
I have heard that the pressure switches are generally not very reliable and since for our project the PDS is interfaced with the platform control system, there could be a possibility of getting false alarm on the control station.
Now my question is. Can the fan status be monitored by taking a feedback from the fan motor current?
For proving a fan is operating I would use a differential switch in preference to a simple pressure switch.
Yes, you can monitor the fan motor current, and this is commonly done, although it has some pitfalls. You must know the fan's power curve, and the motor NLA.
You could also use a sail switch, or any number of rotary encoders.
Are there any benefits of using Pressure Differential Switch over a Pressure Switch?
If I use PDS instead of PS, then one side should sense the fan discharge side pressure in the duct, but what should the other port sense. Room pressure or Suction side pressure?
I think you need to determine how critical the nature of what you are measuring is. If you actually want to know if you have flow, then the pressure switch is the way to go. In your application the same type of switch (pressure or differential) would work. If you put the probe on the discharge side of the fan then you would connect it to the High port of the switch. If the probe was on the suction side, then you would use the Low side of the probe (sometimes I have to do this on low static output and for exhaust fans).
One of the critical issues with these switches is setting them up. Generally I like to put a meter across the two terminals in question on the switch and check for continuity. Keep your meter on the switch and turn the set screw until you have continuity on your meter (of course the fan is running while you are doing this). Once the switch makes, then back it off so it kicks in at lower pressures. Check out the AFS-222 from Kele.
Differential generally comes in to play if measuring only one pressure point is not enough. By connecting the second probe on the second port of the switch you increase the range and can make the switch actuate better. Again, this works well in a low static application.
If you use the current switch, then generally you will know if only the fan motor is running. But check out the Veris website as they have a way to use a current switch or transducer in an air movement application so you know if the fan and motor are running or if just the motor is running. Don't forget that if it is for a fan with a VFD then you need to use a different type of switch and it needs to be placed on the the load (motor) side of the VFD.
