Pressure Gauge Dead Weight Tester

[Nigel BennettParticipant @nigelbennett69913]

Be aware that the boiler testing regulations require calibrated test equipment. Yes, you can calculate the dimensions you need for a dead weight tester, but you then need to get that calibrated when you've made it. If a boiler goes bang, how are you going to persuade an investigator from HSE or wherever that your testing regime is correct? Have you calibrated your test weights? Have you calibrated the measuring equipment that you used to check the dimensions of your tester?

Your Testing Equipment ought to be traceable to National Standards. Far simpler to get an accredited body to test and calibrate your pressure gauge every couple of years; and it's often as cheap to buy a calibrated gauge new.

[DMBParticipant @dmb]

Clubs have managed boiler testing for many decades, often relying upon large pressure gauges. I doubt if many possess a dead weight tester. Such concern over extreme scientific accuracy doesn't seem entirely necessary or even desirable, given the enormous safety margin built into the design calculations. So would it really matter if the test pressure was measured to read say 200lb when the actual pressure was say 195 or 205? As long as experienced club testers weed out dodgy looking amateurish boiler making then I think that we can continue to be as safe as we have been for umpteen decades up to the present. I suggest that the majority of boilers are probably professionally made to a good satisfactory standard. One major problem affecting all boilers is that of being choked up with chalk deposits causing overheating of the plates and a shortage of inspection holes as used in full size locomotive boilers. I know of one club that uses modern electronic inspection equipment to view the inside.

[duncan webster 1Participant @duncanwebster1]

Unfortunately the regulations require:

The hydraulic pressure test indicated in Section 10 shall be carried out using a test gauge which has, within the previous two years, been checked and calibrated to within ± 2% either against a currently validated dead weight test apparatus or against other traceable equipment.

Perhaps NAME and SFED should invest in a tester and keep it up to date as a service to clubs

[Paul KempParticipant @paulkemp46892]

Posted by Nigel Bennett on 29/08/2022 16:35:39:

Be aware that the boiler testing regulations require calibrated test equipment. Yes, you can calculate the dimensions you need for a dead weight tester, but you then need to get that calibrated when you've made it. If a boiler goes bang, how are you going to persuade an investigator from HSE or wherever that your testing regime is correct? Have you calibrated your test weights? Have you calibrated the measuring equipment that you used to check the dimensions of your tester?

Your Testing Equipment ought to be traceable to National Standards. Far simpler to get an accredited body to test and calibrate your pressure gauge every couple of years; and it's often as cheap to buy a calibrated gauge new.

Nigel, in pedantic terms what you say is correct but in practical terms are you not stretching the point a little, certainly considering the ‘guidance’ as quoted by Duncan from the code? Do you think the national standards laboratory would even consider calibrating a 3/4” dia 0 – 150 psi gauge to even 2% when the width of the needle would equal that? If the proposed deadweight tester is checked against a calibrated gauge every 2 years then the spirit and intent of the rules will be met.

[HopperParticipant @hopper]

Posted by Paul Kemp on 29/08/2022 23:31:56:

Posted by Nigel Bennett on 29/08/2022 16:35:39:

Be aware that the boiler testing regulations require calibrated test equipment. Yes, you can calculate the dimensions you need for a dead weight tester, but you then need to get that calibrated when you've made it. If a boiler goes bang, how are you going to persuade an investigator from HSE or wherever that your testing regime is correct? Have you calibrated your test weights? Have you calibrated the measuring equipment that you used to check the dimensions of your tester?

Your Testing Equipment ought to be traceable to National Standards. Far simpler to get an accredited body to test and calibrate your pressure gauge every couple of years; and it's often as cheap to buy a calibrated gauge new.

Nigel, in pedantic terms what you say is correct but in practical terms are you not stretching the point a little, certainly considering the ‘guidance’ as quoted by Duncan from the code? Do you think the national standards laboratory would even consider calibrating a 3/4” dia 0 – 150 psi gauge to even 2% when the width of the needle would equal that? If the proposed deadweight tester is checked against a calibrated gauge every 2 years then the spirit and intent of the rules will be met.

But don't they use larger diameter, about 3 or 4 inches, gauges for hydraulic tests, for this very reason?

[NealebParticipant @nealeb]

I can imagine the response from club members on boiler test day when they hear that boiler testing has been cancelled "because our lawyer is off sick."

Or am I just getting cynical these days?

[Nigel BennettParticipant @nigelbennett69913]

Paul, I am not for one moment suggesting that you send your 3/4" diameter Tich pressure gauge to the National Physical Laboratory for testing. What you do need is that your Club's test equipment is calibrated every two years by somebody who has had their test equipment verified to National Standards. Then you use the Club's gauge to test your boilers and check your own pressure gauge.

If there is a Boiler Incident, there should be no doubt about the testing of the boiler in question. If you haven't complied with the regulations, you can get taken to the cleaners by the injured or bereaved. If you've not complied, then the Insurance Company will happily walk away and leave you to pick up the pieces.

Do you AND the club's Officers & Committee want to be bankrupted and lose your homes (and workshops!) because you've taken a silly short-cut? I don't!

[noel shelleyParticipant @noelshelley55608]

In principle the dead weight tester is so simple that it is almost infalable when properly designed and operated by a person understanding how it works. With the piston diameter checked by a micrometer calibrated to a set of slip gauges, weights checked agaist accurate scales, and a weight to pressure ratio of say 5 being used to set/check a 4" gauge will give an accurate gauge capable of safely testing boilers. The safety factors used in boiler design and construction along with the boiler testing regime have resulted in good safety. That a 3/4" dia gauge is capable of repeatably reading to an accuracy of even 5% I would consider good. Even a 4" gauge set to 2% would require VERY careful reading to reapeatably read to this accuracy !

I fear that perceived precision has overtaken practicallity ! In practical terms I would rather have a test gauge that has been calibrated on a home made dead weight tester than not calibrated at all. The likelyhood of a boiler failing due to scale build up is far more likely yet I see no insistance on an internal examination, YET ! The laws of physics don't change, but hairs can be split ! Noel.

[duncan webster 1Participant @duncanwebster1]

I didn't write the rules, but the rules are there. If you choose to ignore them and have an incident, someone could get hurt, the insurance can refuse to cough up, and in the extreme someone could go to jail. Much easier to just follow the rules. If the rules are thought impractical then get them changed. As I said before, all this would go away if NAME and or SFED had a master gauge or dead weight tester which they got certified every 2 years. This could then be transported round the country from club to club to enable everyone to comply

[SillyOldDufferModerator @sillyoldduffer]

Posted by Nealeb on 30/08/2022 08:14:50:

I can imagine the response from club members on boiler test day when they hear that boiler testing has been cancelled "because our lawyer is off sick."

Or am I just getting cynical these days?

A point often missed is the actual accident and removal of the victim is usually just the start of a long, stressful and expensive process. Lots of awkward questions get asked: who was in charge, is there a work order, are you insured, can you show all the legal requirements were met, where's the risk assessment and Accident Book, can I see your Heath and Safety Policy, what training was done, and what PPE was provided? Who is your legal advisor in case there is a prosecution or civil claim. Etc. etc. etc.

Any foolish bystander can offer slapdash advice as a way of speeding up work because he won't carry the can if it goes wrong. No problem! People in a position of responsibility have to be much more careful.

Dave

[Paul KempParticipant @paulkemp46892]

Posted by Nigel Bennett on 30/08/2022 13:40:31:

Paul, I am not for one moment suggesting that you send your 3/4" diameter Tich pressure gauge to the National Physical Laboratory for testing. What you do need is that your Club's test equipment is calibrated every two years by somebody who has had their test equipment verified to National Standards. Then you use the Club's gauge to test your boilers and check your own pressure gauge.

If there is a Boiler Incident, there should be no doubt about the testing of the boiler in question. If you haven't complied with the regulations, you can get taken to the cleaners by the injured or bereaved. If you've not complied, then the Insurance Company will happily walk away and leave you to pick up the pieces.

Do you AND the club's Officers & Committee want to be bankrupted and lose your homes (and workshops!) because you've taken a silly short-cut? I don't!

Nigel, I have absolutely zero concerns about our club compliance with the quoted regs. We use an 8” gauge that is checked or calibrated against a master gauge at least every two years by a commercial organisation which issues a cert and a calibration certificate so it is a traceable and auditable process. Prior to this it was checked against a dead weight tester owned by a member of a neighbouring club along with gauges from other local clubs. Again a record of the test was provided for our files. So no shortcuts taken. My understanding was the OP required a method of checking his own model gauges for his own use, there was no mention of issuing his own certification for boiler safety?

My point on the 3/4” gauge was you are testing it against a 2% standard but it is incapable of being guaranteed to be much closer to 10% accuracy itself throughout its range. Model gauges are at best indicators.

Paul.

[Mark RandParticipant @markrand96270]

Having used both dead weight testers for calibrating gauges and pressure transducers and Budenberg dead weight gauges to accurately measure high pressures in the days before we and our customers trusted transducers sufficiently, I'll say that calibrating a dead weight tester every couple of years would be an excercise in stupidity.

The local acceleration of gravity doeasn't change (yes, you do need to correct for that, it isn't 9.80665m/S^2 everywhere), the temperature correction per degree doesn't change (also needed for precision work), the weights don't change over decades if they don't go rusty and the piston dimensions don't change over decades because it's in a perfect environment (precision fit, low load, oil lubricated, low rotational speed).

Added to that, the UKAS lab doing the calibration will be using a dead weight tester with a much longer calibration interval than a couple of years…

[duncan webster 1Participant @duncanwebster1]

This is where you've got to read the rules carefully, not just have a knee jerk reaction. The 2 years refers to the gauge the boiler tester is using, the requirement for dead weight tester is 'currently validated', no time limit.

[NealebParticipant @nealeb]

This kind of discussion always reminds me of the "four rules", applicable to almost any area of modern life.

1. The rules don't make any sense.

2. If you get it wrong, you go to jail.

3. Any questions – see rules 1 and 2.

4. The boys ain't got a sense of humour.

So, so, true.

[HopperParticipant @hopper]

Posted by Nigel Bennett on 30/08/2022 13:40:31:,,,

…

Do you AND the club's Officers & Committee want to be bankrupted and lose your homes (and workshops!) because you've taken a silly short-cut? I don't!

 

I think these days clubs are incorporated bodies so the incorporation and not individual members and committee members are liable in the case of lawsuits. I know that is the case in several clubs and community groups I frequent. Otherwise, it would be rather hard to get people to join I up, I should think.

I have to wonder though, have there been cases of people killed or even seriously injured by an exploding model boiler?

When I worked on industrial boilers years ago, small boilers (which were bigger than most models) were exempt from the regulations, inspections, licensed operators etc. It was considered they were not big enough to be dangerous. (Those were different times though, of course. Lawyers had not yet developed their picnics into fully catered banquets at the Ritz. )

Edited By Hopper on 31/08/2022 12:21:16

[Michael GilliganParticipant @michaelgilligan61133]

You may want to read this, Hopper:

**LINK**

https://www.hse.gov.uk/foi/internalops/ocs/100-199/130_8.htm

MichaelG.

[Howard LewisParticipant @howardlewis46836]

I used to get our club "Master gauge" checked on a Budenberg Deadweightb tester at work.

Eventually, this could no longer be done because the Budenberg tester was no longer in calibration.

How do you calibrate such a device?

Presumably:

Measure the plunger diameter (With a calibrated and traceable Micrometer, under standard conditions of temperature and humidity? )

Weight the dead weights on a set of calibrated and traceable scales, again, under standard conditions?

Which is why having any measuring instrument checked and calibrated against traceable conditions becomes expensive..

What is going to wear / deteriorate on a deadweight tester?

Plunger and barrel. probably identified by leakage. How difference will a decrease in diameter of the plunger, or increase in bore diameter by 0.001"?

Unless handled carelessly, abraded, or allowed to become dirty, by how much will the weight vary?

Getting towards counting angels pin heads, but the regulations have to be obeyed.

"Larger fleas have lesser fleas upon their back to bite 'em, and so on ad infinitum"

Howard

[John BaguleyParticipant @johnbaguley78655]

The subject of using Dead Weight Testers to check master gauges was addressed by the Federation of Model Engineering Socities some years ago by 'Information Sheet no. 21' for December 2015. Basically, they are considered to be a 'Primary' standard and as such do not require recalibration so long as they are not damaged etc. Any errors in their accuracy are not likely to change with use, wear or enviromental factors and considered to be insignificant for the use of calibrating master gauges used for amateur boiler testing.

Information Sheet No. 21 is available for download from the FMES website but it's only available if you are a member and logged in.

However, of course, this applies to commercially produced DWTs and not homemade ones. I would guess that homemade testers would initially require to be checked for accuracy.

I picked up a couple of Barnett DWT's for a very reasonable price a few years ago and also a third for spares so I can test and calibrate all our clubs master gauges if necessary.

John

[DMBParticipant @dmb]

Lets suppose that the small piston is .5" dia. and wears evenly around its circumference by .0005". That would mean an annular gap that amounts to the piston having worn by a thousandth of its original diameter. My guess is that would result in tears of fluid appearing similar to small leaks seen during actual boiler testing and would represent a tiny error. When a factor of safety of 8 is included within the design calculation, I don't think that there is any need for the foregoing hysteria about calibration,etc, etc. I am unaware of any serious incidents resulting from "inaccurate" boiler pressure tests. Failure of overheated plates due to lack of water circulation caused by deposit buildup is far more likely but I don't think that's occurred so far.

 

Edited By DMB on 31/08/2022 21:24:44

[SillyOldDufferModerator @sillyoldduffer]

Posted by DMB on 31/08/2022 21:19:55:

Lets suppose that the small piston is .5" dia. and wears evenly around its circumference by .0005". That would mean an annular gap that amounts to the piston having worn by a thousandth of its original diameter. My guess is that would result in tears of fluid appearing similar to small leaks seen during actual boiler testing and would represent a tiny error….

Makes sense to me.

My understanding is these devices work by applying pressure to a vertical piston of known area that's supporting a known weight – pounds per square inch. When the pressure lifts the piston off it's seat, the pressure must be the weight on the piston divided by the area of the piston. There's not much to go wrong and a well-made instrument is capable of defining pressure with extreme accuracy.

How about a leaky instrument? The pressure needed to lift the piston is still the same, but to maintain the pressure, the operator has to pump in more fluid to replace the leaked volume. A leak makes the instrument harder to operate, less certain, and reduces the time in which the pressure is stays stable. But I think even an instrument with a moderately bad leak would work well enough to calibrate a Bourdon gauge provided the leaking fluid was replaced steadily.

Commercial gauges are designed for accuracy, fine adjustment, and ease of use ; they have to be kept in good order and leaks are bad news. I think a home-made dead weight gauge could be better for calibrating model engine gauges than a wonky commercial item because home-made can be designed to compensate for leaks whilst remaining more than accurate enough for the purpose.

To me the give away that commercial dead weight testers are over the top for model engineering purposes is the need to compensate them for temperature, density of the fluid, air-pressure and humidity, and the local gravitational field. This is proper metrology rather than engineering, far more accurate than the best Bourdon Gauge or Safety Valve.

That's the idea, but I'm not expert in this field! It would need to be proved by building a leak-compensating gauge, using it to calibrate many ordinary gauges, and then comparing them against a trustworthy standard. If the leak-compensating gauge calibrations a found to be consistently 'close enough', then the machine is acceptable. I'm reasonably confident it would be because much of the early work on pressure measurement was done with relatively crude equipment.

Dave

[Michael GilliganParticipant @michaelgilligan61133]

From the horse’s mouth :

**LINK**

https://www.npl.co.uk/products-services/force-pressure-mass-density/pressure-barometers

Pressure balances (deadweight standards) are used to generate accurate reference pressures for the calibration of pressure calibrators, transducers and gauges. NPL provides a UKAS accredited calibration service for both pneumatically (gas) – and hydraulically (oil) – operated pressure balances.

[…]

The calibration of a pressure balance determines its effective area and, where appropriate, a pressure dependent term (expressed as ppm area per MPa pressure) also known as a distortion coefficient. The masses of the piston and other floating elements can also be certified.

MichaelG.

[DMBParticipant @dmb]

I still maintain that the hobby doesn't need expensive super accurate pressure measurement. A much lower degree of accuracy is plenty good enough to give a reasonable ballpark idea of the test pressure having regard to the built-in factor of safety which is usually taken to be 8 times greater.

Asking as a large diameter test pressure gauge is used and is itself checked very occasionally against a DW tester and the equipment is kept under lock and key storage, that should be good enough Clubs of which i am a member, do this and have done for many years

[John BaguleyParticipant @johnbaguley78655]

I quite agree. If the gauge reads to within 10psi then that shouild be plenty good enough for 'our' use. The 2% called for in the boiler regs is still over the top really.

John

[Michael GilliganParticipant @michaelgilligan61133]

Posted by John Baguley on 01/09/2022 16:36:23:

I quite agree. If the gauge reads to within 10psi then that shouild be plenty good enough for 'our' use. The 2% called for in the boiler regs is still over the top really.

John

.

Just out of interest … I suspect that they mean 2% of FSD [full scale deflection]

Can you confirm, or refute, that ?

Thanks if you can, and a little concerned if it’s not explicitly stated.

MichaelG.

[duncan webster 1Participant @duncanwebster1]

Just to add fuel to the fire, my understanding of calibration is that you produce a table of errors so that even if the gauge reads incorrectly, you know what the applied pressure is. The gauge does not have to read 200psi when the DWT is applying 200 psi.

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