Cheap extraction fan for spray booth

[Saltfever]

You are probably talking about a pressure differential of only 25mm or 30mm of water. Here is a link to a Dwyer differential Magnehelic on eBay. But you probably don't need it. No matter how you run your system you will always have a differential pressure related to the ambient conditions in the shop. If you are blowing you will have a positive pressure and if you are sucking it will be a lower pressure. Wherever you locate your probe will determine the + or -.

A small, clear tube (partially filled with water), bent in a U-shape about 1-2 meters long should work. One end is free to the atmosphere and the other end is inserted into the area you want to measure. Take a reading when the filter is new and the system is running. Mark that spot (the water height) on your clear tubing. As the filter gets laden with debris the pressure will change and the water height will move either up or down. It is up to you to determine what is satisfactory filter life. Its a simple device and costs about $2 but always gives you a visual on the status of our system. All the best.

PS: Found this link that shows a simple u-tube. Since you are dealing with such a low pressure water is all that is needed. You can make this a science project (as you have most likely seen in your research) or it can be a simple visual indicator!