Why cut of nipple on a manometer? 1

[DirtyHarry84]

Hello guys,

this is more of a curiosity. For years we have been using manometers (standard with fluid in its cavity) in my workshop for tests, but a new guy pointed out a sticker attached to the manometer reading "Please cut off nipple before operation" and asked why we hadn`t cut off ours. I couldn`t come up with an answer, the best thing I can come up with is so the manometer pressure is leveled out with the atmospheric pressure.

Do any of you guys know why one must cut the nipple off before operation on a manometer?

Thank you!

 

[DRWeig]

What nipple? Could you post a picture, diagram, or give us a make and model number? Also, the term "nipple" to me means a short length of pipe, which is open on both ends. There would be no need to cut one of those off.

My manometer has nothing that needs to be cut off...

In general, though, if you're measuring a static pressure relative to the atmosphere (only one connection to the manometer), the other port on the manometer needs to be open to atmosphere. If not, your reading will be affected by the fact that the pressure under test is compressing the air in the space on the other end of the column.

Best to you,

Goober Dave

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[MikeHalloran]

All of your test data is inaccurate. You may be able to back-correlate it to a properly vented manometer and apply the correction to your historic records, but don't expect miracles.

The same thing happens with oil-filled Bourdon tube gages. The shell is sealed, so that the oil won't leak out in shipping. The attached sticker says to cut off or pierce the rubber nipple that ends up at the top once the gage is properly installed. If you don't cut off the nipple, then you are measuring your test pressure against the sum of the Bourdon tube's bending resistance and the nipple's resistance to expansion by pressure, instead of just the Bourdon tube, so your gages' response will be suppressed in an uncontrolled way.

The same thing happens with Magnehelic gages, which have four ports, two high and two low, for end use packaging convenience. In non-differential operation, two of the unused ports should be plugged, but not three.

Similarly, if a manometer is shipped already charged with liquid, it must be sealed to keep the liquid in, and when it's placed in operation the side opposite the test port must be pierced, cut off, or otherwise vented, once the manometer is properly mounted.





Mike Halloran
Pembroke Pines, FL, USA

 

[KiwiMace]

You have had a shocker, so just cut it off and carry on. Fire the new guy, he knows too much.

 

[rtronics]

I have noticed the message on gauges that I have been testing for accuracy, gauges in the range from 100 kPa up.
Always there is a bubble in the filling liquid, which is glycerine in the gauges that I test.
Anyway, I have not seen any significant difference in the readings whether or not the plug is installed.
So, because the gauge is likely to leak all over the place if there is a hole in it, I never open the stopper.
Ray.

 

[MikeHalloran]

If someone is paying you to test and certify liquid filled gages, and you are not removing the stopper to do so, your test report should say that, and you probably should put a sticker of your own on the gage.



Mike Halloran
Pembroke Pines, FL, USA