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Pressure Gauges Testing/Accuracy

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Stewart Hart27/11/2016 08:35:26
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I Hope this ? doesn't set to many hares running.

Does any one have any information/opinion on testing/accuracy of the small pressure gauges we use on model locomotives.

So far my on line searches have turned up.

1:-None of the suppliers seem to quote/guarantee an accuracy for there gauges.

2:-The smaller the gauge the worse the accuracy, the smallest gauge I can find for industrial/domestic use is 1 1/2" Dia and they have an accuracy at its operating limits of 3% and at its middle range 2%.

If say a 1/2" gauge has an accuracy of +-10% (assuming a gross error) a loco with a working pressure of 90 PSI could be blowing of at a true pressure of 99 PSI or 81 PSI.

Boilers are tested with a large factor of safety so I don't believe this is any great issue.

But I would be interested on other views

Stew

John Haine27/11/2016 08:39:35
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Interesting also to know the temperature at which they are calibrated (assuming they are!) and the temperature coefficient of the reading.

Neil Wyatt27/11/2016 08:54:08
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I think many people calibrate them by adding a red line when the boiler is tested against an accurate gauge.

Neil

Martin Connelly27/11/2016 09:25:30
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Bourdon tube pressure gauges come with either stop pins near the zero point or without the pin. For most critical uses the ones without pins must be used because the pin can cause the gauge to give erroneous readings. Also when these gauges are used for things like pressure testing often the instructions will state that gauges must only be used with pressures in the range of 10% to 90% of full scale. Where I work we hydrostaticly test pipes and we replaced a range of Bourdon tube gauges with one electronic process gauge which has a high maximum pressure well over the maximum indicated pressure so is not put out of calibration by overpressuring it. Calibration is now yearly instead of three monthly. They are also far more linear and have a higher accuracy through all their range. If you had access to a process gauge of this type you could use it to check your gauges to find out if they are giving a reading that is acceptable to you.

I have in the past used dead weight pressure testing equipment to check gauges but it was over 30 years ago.

Martin

colin hawes27/11/2016 09:49:30
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A club boiler inspector can test the accuracy of these small pressure gauges against the club's calibrated gauge. You need to be a club member to use this facility. Colin

John Rudd27/11/2016 09:51:46
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Plus1 for a dwt.....

We had a Budenberg dwt in the workshop for cal'ing gauges....

IanT27/11/2016 10:32:10
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The test rigs used by boiler testers have gauges that have to be calibrated regularly Stewart and one of the checks made when a boiler is tested is to make sure that the engines gauge is reading correctly. So one solution would be to build a simple test rig and test your gauges against a 'master' gauge that you could have calibrated from time to time (although a commercial calibration service is quite expensive). As also mentioned, a dead weight tester could be used (essentially a defined weight that pushes down on a hydraulic tube connected to the gauge) and a design for such a device was published in ME some years ago (although I cannot find the reference off hand).

As Neil says, the usual route is to mark (red-line) the gauge at the working pressure of the boiler using a test rig/gauge and then check this as part of the regular boiler inspection. So whilst a gauge may not be 'marked' 100% correctly to begin with (as purchased), the issue really then becomes of whether it can hold it's accuracy once 'calibrated'.

Finding someone with a boiler test rig (or building one yourself) is probably the simplest route if this is a concern for you and it would also enable you to set the safety valves accurately off the engine if required, as well as test other fittings individually (for seal etc.) something that can be very useful.

Regards,

IanT

Andrew Johnston27/11/2016 10:49:20
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Once I've finished mucking about with helical and internal gears I intend to have a go at designing and making the pressure gauges for my traction engines. I have the basic design and manufacturing methods sorted out in my head. Of course the key component is the Bourdon tube. In order to play with these, in a quantative way, I bought a secondhand dead weight pressure tester; 0-8000psi, which is a tad more than I need:

dead_weight_tester.jpg

Andrew

David Jupp27/11/2016 11:43:00
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Surely the relief valve(s) would be set against something a bit better than (and independent of) the miniature gauge on the model? In which case the gauge on the model is manly a guide for the operator, and doesn't have any direct safety function.

BCPROF27/11/2016 13:17:29
137 forum posts

If the minature gauge is correctly tested against a callibrated gauge at the annual steam test with the

"Red Line " marked the scale ( not the glass that can rotated out of position ) ,and the safety valve

set against this, then is this not the same as the defined standard and then callibrated sub standard system

that holds true for our other measuring devices?

Brian C

Swarf, Mostly!27/11/2016 14:42:02
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Andrew,

Do you 'spin' the turntable carrying the weights when using the DWT?

Best regards,

Swarf, Mostly!

Martin Connelly27/11/2016 17:36:47
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The pictured tester looks like the one I used all those years ago and yes, the instructions were to rotate the weight table to avoid sticking of the piston. We also had what we called the bomb that was used with oxygen gauges to keep the hydraulic oil away from oxygen systems. It had an internal bellows and had oil on one side and a solvent on the other. After testing the oxygen gauges went into a low temperature oven for a couple of days to remove the solvent.

Martin

Brian H27/11/2016 19:46:10
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Presumably the weights used on a dead weight pressure tested also have to be calibrated?

Michael Gilligan27/11/2016 20:08:03
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**LINK**

http://www.npl.co.uk/science-technology/mass-and-force/services/calibration-of-weights-all-oiml-classes

julian atkins27/11/2016 20:44:23
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fsd in workshop.jpg

Here is the master. The late Freddie Dinnis with his dead weight tester with one of his miniature gauges being tested (can be seen below his right shoulder).

All Freddie's gauges had proper drawn phos bronze tube, and each gauge was individually calibrated. They are now much sought after.

All the current commercial small scale gauges use brass bourdon tube which due to it's thinness is apt to corrode over time. None are individually calibrated, relying on printed discs.

As all my locos have FSD gauges I cannot comment on the current commercial examples except for the above corrosion comment and lack of calibration. However I would add that the 'red line' is merely to highlight the working pressure of the boiler. If a commercial pressure gauge was not accurate, it would fail a boiler test regardless of the red line.

Cheers.

Julian

Nigel Bennett27/11/2016 21:11:19
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To test a pressure gauge you need to have your testing apparatus tested to a standard traceable to National Standards. Unless you can prove this, your Deadweight Test or your nice big Club Test Pressure Gauge is invalid. If a boiler goes bang and you can't prove you've got this traceable certification, you're going to be in Very Deep Trouble.

A Calibrated test gauge isn't that expensive - and it's valid for two years. Ours was about £50 from RS last year.

All you need to know on a miniature pressure gauge is the position on the scale where the maximum working pressure is - the Red Line. The position of the needle anywhere else is just a guide and no more.

B Terry Aspin (Chuck) described making his own gauges - and he never bothered putting figures on his scale - just a red line at the appropriate position.

A 1/2" diameter pressure gauge is never going to give you "accuracy". The level of precision needed to achieve say 3% accuracy on a 1/2" gauge would be well beyond any amateur manufacture. And yes, I've made my own.

When I'm doing the annual steam test on boilers at our club, I always check the loco's gauge against our own calibrated gauge - Julian's point about corrosion is most pertinent.

Andrew Johnston27/11/2016 21:19:05
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Posted by Swarf, Mostly! on 27/11/2016 14:42:02:

Do you 'spin' the turntable carrying the weights when using the DWT?

Yes, although I must have read about it on the internet, as I don't have a manual for my DWT.

Of course one could 'calibrate' the weights, but would need to know what the spigot diameter, and hence area, is. My weight that gives 20psi actually weighs 1lb 9oz, on the kitchen scales. That's because the area of the spigot is significantly less than 1 square inch.

Andrew

Neil Wyatt27/11/2016 21:42:48
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Posted by Nigel Bennett on 27/11/2016 21:11:19:

A 1/2" diameter pressure gauge is never going to give you "accuracy". The level of precision needed to achieve say 3% accuracy on a 1/2" gauge would be well beyond any amateur manufacture. And yes, I've made my own.

Common sense agrees - a scale 3/8" long reading up to 120 psi is only 30-thou between 10psi steps... more or less impossible to read to better than 5psi no matter how well made & tested.

Neil

julian atkins27/11/2016 22:06:51
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I have to disagree.

Both Roy Amesbury and Freddie Dinnis (in Freddie's case commercially) made their own pressure gauges of scale size for 3.5"g and 5"g locos etc and all were perfectly accurate. Freddie's were so accurate they were used by Rolls Royce.

You can read the accuracy very well same as you use your eyesight to read a vernier caliper.

Roy used sifbronze rod drilled and reamed to fit onto a mandrel via loctite to turn the finished size for the bourdon tube before flattening and curving. I have used the same method. Roy described making his gauges on a number of occasions in ME.

Alan Dinnis has published a book on Freddie's life, his father, which includes a chapter on his gauges made when in 'retirement'.

I remember being shown how most of the parts were made in the mid 1980s by Freddie including how the phos bronze bourdon tube was drawn. I have a number of lengths if anyone wants to have a go at making their own.

Any purchaser of a gauge from Freddie was often subject to a trial of the calibrated gauge on the dead weight tester before money was handed over. Some of mine are 40 years old and all yet are as clear in reading and accuracy as when first made in Cowes Isle of Wight.

I bought a 1" dia vacuum gauge from Reeves in 1990. The vacuum gauges require thinner bourdon tubes. (Freddie made these as well). The Reeves vacuum gauge with its brass bourdon tube lasted but 5 years before the bourdon tube corroded. I have a box full of old commercial miniature gauges with brass corroded bourdon tubes from the days of Bassett Lowke and Bonds O' Euston Road.

Cheers,

Julian

julian atkins27/11/2016 22:38:34
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dsc00927.jpg

Here are top row 3 FSD (Freddie Dinnis) miniature gauges of 3/4" dia. The first is an early example with curved glass and dates from circa 1970.

Below 3 knackered old gauges with corroded brass bourdon tubes.

Cheers,

Julian

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