# How to measure the air pressure of a pipe without having water resistant sensor

I'm having a question regarding the sensors and the techniques in fluid mechanics. Since my skills are amazing in this field, here I'm asking you questions xD

I want to measure the air pressure of this tube/pipe (P1,P2,P3). This pipe will be flushed with water and then with compressed air.

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In principle, it should be just pressure sensor in each pipe and problem solved!

• Perhaps you could use a small extension tube with a loop in it... As the would be no flow in that tube then water should not get to the pressure sensor - but you will have to make sure you will get the required accuracy. Commented May 18, 2017 at 8:36
• Thank you so much for replying. Much appreciated. Can you please be more specific? why there will be no flow? and by loop you mean circular tube? I'll try to drow your idea and post it so you can check if it's the same as you mean. Commented May 18, 2017 at 9:55
• if you block one end of the tube with the pressure sensor then there will be no moving fluid therefore no flow... Commented May 18, 2017 at 10:43
• Just to make sure, this is what you mean by the loop and the pressure sensor? 2.1m.yt/CHGisly.jpg Commented May 18, 2017 at 11:17
• I would suggest a complete loop would be better with the end continuing above so any fluid won't condense down onto the sensor. Commented May 18, 2017 at 11:18

Perhaps you could use a small extension tube with a loop in it... Mounted vertically not horizontally, so as to act as a liquid trap... As the would be no flow in that tube then water should not get to the pressure sensor - but you will have to make sure you will get the required accuracy. Fit the pressure sensor to the top end.

• Ok thank you but from my point of view, I don't think this will work. As there will be drops of water when the air is flushed inside the tube, so the water (drops) will reach the sensor. Or if the tube (P1) is blocked, the water will go back to the sesnor and damage it. Thank you for taking the time to reply thou Commented May 18, 2017 at 12:47
• if you connect this to your 4way junction so it is vertical then how will the drops reach the sensor? I don't think you have followed the idea. I did this with a 2m long pipe to attach a pressure sensor that was only rated to 100 degrees C to steam pipe with superheated steam in it (over 170 degrees C) and got the pressure with no damage to the sensor. Commented May 18, 2017 at 12:55
• Ok I think this is what you mean? I hope that I fully understood it now as I'm trying my best to do so. 4.1m.yt/2TGEk0x.jpg Commented May 18, 2017 at 13:27
• Dear Mike, can you please confirm? as it is really important to me. Commented May 19, 2017 at 5:46
• @Manaf if the loop is perpendicular to the other pipes then it should be fine. Commented May 19, 2017 at 6:14

There are many solutions for this. The simplest is to use a standard pressure gauge on a vertical pipe sticking out of your pipes. It will be exposed to minimal water vapour and unless you are using a fancy delicate gauge you will be fine. Below is something I found on how gauges work and why you will be fine if water does get into the gauge:

Pressure gauges commonly use a Bourdon tube. Bourdon tubes are a coiled metal tube and as the pressure increases, it forces the coil straight which then moves the needle. (think blowing a party horn, which causes it to unroll) The type of fluid (water/air) makes no difference to the tube, only the pressure exerted against the tube does. And since most of the tubes are made from bronze, copper or stainless water will not corrode it any faster than moist air from the compressor.

In all said, the air compressor gauge will most likely work without any issues.

But if you really want to get a water gauge, go to a pool supply store. They have pressure gauges for filters that are designed for water and use a 1/4 npt. They also have a nifty little ring on the outside where you can mark your normal pressure so it is easy to see if your pressure is low or high. LINK

Pipelines that are drained and filled often have surges of water that can damage piping and gauges by what is called 'water hammer' when a slug of water is propelled at high velocity by air behind it. If your pipe is designed to fill and drain it should be able to handle these surges but I would monitor the gauge for a few cycles to ensure excessive pressures are not observed.

• Thank you so much for your reply. Hmmm I found this a little bit confusing. As I wanted to change the one I have ( water resistant) to a cheaper one (not water resistant). According to your reply, I can use both. But this goes to the pressure gauge not to the pressure transimitter (electrical) " this is what I have " Commented May 18, 2017 at 13:41
• @Manaf There are many pressure transmitters that work with the bourdon tubes that will not be damaged by water. The electronics should be water resistant if they are located where they could be splashed, but they will not be damaged by water on the inside. The water resistance of the one you have is probably splash resistant on the outside, they tend to be more expensive due to the nema certification and manufacturing processes needed. If your pipes are in a safe splash free area and the gauge only exposed to water on the inside of the pipe you should be fine to use the cheaper one. Commented May 18, 2017 at 13:54
• I think you got my point wrong. I don't want to measure the water pressure. I want to measure the air pressure only that's why I don't want to buy a water resistant pressure sensor. Commented May 18, 2017 at 14:25
• It will measure both and should not be bothered by the water. Commented May 18, 2017 at 15:43