You can find lots of pressure switch suppliers on globalspec.com here :

http://sensors-transducers.globalspec.com/SpecSearch/Suppliers/Sensors_Transducers_Detectors/Pressure_Sensing/Pressure_Switches?RegEvent=login

There are a few options, depending on how in depth you need to be.

If the compressed air system has a large capacity, then a pressure switch may not detect a leak unless it is considerable, as the compressor will simply keep up supply based on its delivery pressure set point.

You may be interested in looking a flow meter option on the outlet of you compressor/s to detect an increase in air volume through your compressed air system.

You may be able to find an adjustable flow switch suitable for your system yourself or the manufacturer of your compressor may be able to recommend a suitable switch device, or alternative system for leak detection.

Knowing that more air (including wastage) means $$$ a flow meter option may be a better way to go.

Orion Instruments make all sorts of pressure switches in India. Choose one suited to your needs.

http://www.orion-instruments.net/functions.htm

Depending on the pressure itself, you could get by with a normally closed oil pressure transducer/switch from a car, wire it up to a piezo buzzer and a 12dc power source. Rock bottom budget kind of stuff, else as others have already suggested.

Dwyer Instruments makes a meter called Magnehelic that can be mounted in a panel to read air pressure or differential pressure. They can be purchased with a set of adjusable limits to trigger an alarm and are frequently used for this type of application.

Dear Ragotham,

I suggest you install flow limit switch or pressure limit switch to loading and unloading the compressed air.

Previously, we had facing same problem with some minor leakage cause plant compressed air pressure drop and production stopped. This problem solve after we install it the pressure limit switch.

Currently, we use only one unit of pressure limit switch to cut off the compressed air line to the high potential of air leakage. To reactivate or compressed air loading (or disactivate the pressure limit switch) we decided to manually activated in order to make sure our technicians inspected and rectified the defect / abnormal prior to reset the system.

For you case, I would like to suggest you to install two units of pressure limit switch.

1. One unit for signal to compressed air laoding and alternatively activated alarm when any pressure dropped. You may choose the pressure limit switch which sensitivity (can be 0.01 bargs, 0.1 bargs and etc depend on supplier specification) that suitable for your system application.
2. The second unit is for compressed air cut off to avoid over-loaded.
3. Eg. If you required system pressure minimum is 6.0 bargs and compressed air supply can loaded until 7.5 bargs. You can set the lower limit to 6.5 bargs and upper limit with 7.2 bargs. It means when pressure dropped till 6.5 bargs, the compressed air will start loading and alarm activated. Therefore, you technical team must be attend on site for inspection and disactivated the alarm. Meantime, the production still can on going.