Glycerine filled manometer

On Wednesday 28 August 2013 19:49 asalcedo wrote in uk.d-i-y:

Do you know how a moanometer works?

Not being funny - but some of your questions are at odds with that :-o

As water pressure pushes the glycerine up the tube, the air space beyond it will - if you don't cut the tip - compress. I would expect the compressing air to push back against the glycerine column so it might not move as high as it should, thus giving a falsely low reading. Possibly in exceptional circumstances the tube might burst.

Only if you want the glycerine to be pushed out of the tube.

Also the water pressure pushes against (I assume) a known (calibrated?) weight of glycerine - if you let some of that glycerine out the water pressure will push the smaller amount of glycerine that's still there further next time, thus giving a misleadingly high reading.

No. You need the weight of the glycerine to oppose the water pressure.

It's an iphone app apparently

Not to be confused with the nagometer

HTH

A manometer in the normal sense will be impractical for 6bar (some 60m metres of height being needed. Having googled, it seems that people are selling rotary spring pressure gauges and calling them manometers. Some of these have the needle and mechanism immersed in glycerine to damp out rapid fluctuations. Have to admit that most of the questions asked by the OP are rather strange though.

Oops; good point. I was thinking of doctors' mercury-filled manometers...

Considering the OP mentioned pressures up to 6 bar, glycerine must be pretty dense stuff considering 1 bar will only support a 30 inch high column of mercury...

Now a glycerine filled pressure gauge is a different thing. Googling about there are loads of references to "glycerine filled manometers" that, IMHO, should be called "glycerine filled pressure gauges".

On Wednesday 28 August 2013 21:12 Richard wrote in uk.d-i-y:

Ah ha...

On Wednesday 28 August 2013 21:29 Bob Minchin wrote in uk.d-i-y:

Mercury would work :)

You would still need a 4m column of the stuff for 6 bar!

These answers do not reflect how the type of glycerine manometer that I am referring to works. And are not accurate as a consequence.

A glycerine manometer is just a normal pressure gauge that uses a Bourdon tube in a casing that is filled with glycerine after it has been calibrated

'Do you know how to make!! - Pressure Gauges -'

Since the glycerine is never in contact with the water the pressure change in it will be minimal. Thus, I think it may work just as well without cutting the plug and installing it in any direction.

Obviously, I am missing something

Well all the ones I saw when I worked in a lab had little notes saying please install in a vertical position, so you might find yours says this as well. Brian

A manometer is a transparent U-tube filled with a liquid (mercury, water, a lcohol, etc.,) that measures pressure differences from the difference in th e height of the liquid column.

I think the replies you got were not what you'd expected because your termi nology was inaccurate.

I suspect your pressure gauge has been filled with glycerine and the filli ng plug has to be cut to stop thermal expansion of the glycerine breaking t he glass. I can't think of any other reason. I don't know for sure, all the ones I've seen had a small air bubble and the plug wasn't meant to be cut.

A bourdon tube works against the surrounding atmospheric pressure, so if the gauge body is sealed it will misread. Presumably the sealed rubber bung is there to stop the glycerine escaping in transit. You then need to have a hole in it so the gauge body is at atmospheric pressure when in use.

That is a sphygmomanometer.

A manometer is a liquid column in tube device for measuring quite low pressures. ie millibars rather the bars.

Eg domestic natural gas pressures, Atmospheric pressure, blood pressure.

No good at all for mains water pressure.

Leaving aside terminology, which, is good to learn, I have now the answer, I think, to my question:

A Bourdon tube does measure the relative pressure between the fluids inside and outside.

'Pressure measurement - Wikipedia, the free encyclopedia'

If the casing was sealed in a place with very different atmospheric pressure than the one where it is going to be used, there will be a less accurate reading.

I doubt, though that this is relevant for my purposes. I am going to use it a sea level and I am confident that the gauge was sealed in an environment of only a few mbars difference. Versus the thousands of mbars reading

The reason why, non glycerine filled pressure gauges do not have the plug to be cut, is because they are not air tight.

As a consequence, I will install the gauge horizontally which will make the installation much simpler and will check with a second manometer the offset, if any.

On Wednesday 28 August 2013 22:18 Bob Minchin wrote in uk.d-i-y:

You could read it from an upstairs windows - I call "practical" :)

On Wednesday 28 August 2013 22:25 asalcedo wrote in uk.d-i-y:

Riiighht. So it is not actually a "manometer" then...

To me, a manometer is specifically a U-tube filled with a liquid.

Liquid Osmium.