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Differential 2

Differential pressure is a subject that is often misunderstood. During the COVID-19 crisis the most common use of differential pressure was for isolation rooms. The idea being to keep bacteria and viruses in. We also have pharmaceutical and aeronautical companies that use differential pressure to keep drugs in and dust out in their respective cases.  For example, in the case of the isolation ward, one has a low pressure in the room compared to the atmosphere.

The operative word is ‘differential’. One is comparing one pressure against another. Differential pressure is always by comparison.

Most commonly, in the example of the isolation room, one port of the differential device is open to the isolation room and the other either to outside, or to a corridor etc which is at atmospheric pressure.

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High and Low pressure Ports on a differential gauge  A typical isolation room example

Another use of a differential gauge, is to measure a very low non-differential pressure, by leaving one port open to the air and the other used for single ended pressure.  

kane 3500-1 Handheld manometers* are often used by gas-fitters to check for 'draft' or to check for leaks etc.  This is a great way to measure really small differential or single ended pressure.
The model shown (a Kane 3500-1), on it's lowest range, has an accuracy of +/- 20 pascals! (not Kilopascal; pascals!)

*We tend to think of the the term 'manometer' as referring to a differential instrument, but lately I've had customers who use 'manometer' to mean a pressure gauge

Another very common use for a differential pressure gauge is to use the instrument to see when an air (or water) filter is dirty.
This is because when the filter is clean, it will offer almost no resistance to flow and the pressure will be almost the same either side.  However when clogged, the pressure on the inlet side will be higher than the output, and this differential can indicate a dirty filter.  In HVAC the rule of thumb is that a clogged filter will have twice the differential of a brand new one

Here's a slightly unusual application of differential pressure.  Ever wondered how the brake booster in your car works?  

A vacuum is created in the manifold and this vacuum is applied to a sealed chamber in the brake booster, greatly increasing the force applied by the brake pedal.
By the way, if you're interested in automotive brakes, you'll love the IR image and measurement data from one of our Optris IR Cameras which you can view here
vacuum brake booster applied2

We've done a couple of articles on pressure that you may find useful
https://www.homershams.co.nz/Articles%20and%20Videos/Absolute?mv_pc=707 on Absolute Pressure

And here on general pressure measurement https://www.homershams.co.nz/Articles%20and%20Videos/pgs?mv_pc=730
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