Surely the weights would need validation for accuracy before the dead weight tester could be regarded as a “primary standard”. Our Society has such a tester, but we get our boiler test gauge tested and calibrated by ani independent test house.
I think it only needs doing once in the lifetime of the equipment. The beginning I would hope!
When I was in Flight Test Instrumentation we had the standards lab in our building. Out of interest I got my tyre pressure gauge tested (an expensive Longacre one I used when I was racing) and to my suprise it very accurate all the way until almost full scale deflection where it started to drift slightly (but consistently). At the important points (between 20 and 40 psi for our stuff) it was virtually bang on. For mass produced kit I was very impressed.
_____________________________________________________________________________________ Nice, constructive feedback welcomed and ingested, if not always acted upon.
As Jon says, the whole equipment will be checked for accuracy after it is made and that should be good enough for life so long as there is no damage to the weights etc. You can get them recalibrated/checked but the cost would be more than the equipment is worth, certainly for me. You have to send the whole kit off i.e. the tester and the weights. Once I've got it sorted I will check it against a known gauge out of interest but I've no doubt that it will be spot on.
Dead weight testers are normally accurate to something like 0.02% or better so even if they do drift a bit over the years any errors are going to be negligible for our use so long as nothing is damaged. We are not calibrating laboratory equipment. Even if the accuracy falls to 1% the error is only 1psi in 100psi and you would be struggling to read most gauges to that accuracy.
At the end of the day, how accurate does a test gauge need to be for our use? Does it matter if a gauge reads 5psi high or low when testing a boiler to 200psi? The boiler test figures are only an arbitrary number that someone has come up with so if the test is 195psi or 205psi it's not going to matter in the slightest. It would be more critical when you are testing the small gauges we use on our boilers to show the steam pressure but most of those are notoriously innaccurate and you are struggling to read them to 10psi at the best of times. That's why you should never set your safety valves to them unless you have had the gauge tested recently.
Anyway, the Southern Fed have stated that their use is ok for calibrating test gauges so that is good enough for me
I've found out what the oil is that is used in the testers. It's just a normal mineral hydraulic oil of ISO 20 to ISO 40 so I've ordered a bottle of the 20. Apparently, the thinner oils are more accurate at lower pressures but the thicker oils are better for high pressures as there is less leakage around the piston.
We used deadweight testers a lot on the larger Chevron tankers which were steam ships and the steam ranges ranged from about 3.5bar to 60bar. I seem to recall using Castrol Hyspin AWH32 in the testers which fits in with your figures.
Although I understand why some might want an accurate test pump, I only need a leak tester. The boiler inspector will provide the kit when the time comes to certify a boiler or set the safety valves. However, while I still have the club's test kit, I took the opportunity to calibrate my own gauge with steps of 25 psi.
The gauge is quite insensitive at the lower end of the scale - the first mark is 50 psi - but it seems fairly linear after that. Mine held 150 psi for ten minutes with no problem but I agree with John, a glass marble in a plastic moulding doesn't make for the best clack valve.