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.Dead Weight Testers
1
25/08/2023
I had been on the lookout for a reasonably priced dead weight tester for calibrating pressure gauges for some years now but they seem to go for silly prices even secondhand. They are not cheap new though so perhaps not surprising. Eventually in March 2019 one came up for sale at a very good price complete with weights. I actually made the seller an offer and got it a bit cheaper than advertised. I don't think that they are the sort of thing that many people would be interested in so the seller was probably glad to sell it.
The tester is made by Barnet Industries, now long gone, and the nameplate says that it was made in 1962. It's a little dirty but in excellent condition and so are the weights. These are quite soft metal (lead based?) so easily damaged if someone is careless in handling them. Any dings or bits missing will affect the accuracy of the results. There are 19 weights for 20psi, 1 weight for 10psi, two for 4psi and 1 for 1psi. That makes a total of 395psi plus the platform on the tester itself which gives another 10psi.
For anyone who doesn't know what a dead weight tester is, it's a unit for testing and calibrating pressure gauges to a very high accuracy. This particular tester uses hydraulic oil as the test medium but some use water or air.
Basically, it consists of a cylinder with a very closely fitting piston on the top of which is a platform which takes the weights. The cylinder is connected to a pump operated by a screw thread (the bit with the four handles in this case) and also an oil resevoir and a fitting to connect to the gauge under test. When carrying out a test the gauge is connected, the required weights put on the platform and the pump ram screwed in until the piston lifts the platform until it is free to spin. The gauge should then read the pressure equal to the number of weights on the platform. It's usual to test the gauge at five or more different pressures throughout it's range. Any errors are noted down on a test form.
Using oil for the test medium can be a problem and the gauges need to be washed out after testing. Probably not much of a problem for a steam gauge but a big problem if the gauge is used for oxygen! Some manufacturers such as Budenberg make an isolating cylinder which goes between the gauge and the tester. This uses another medium such as water to pressurise the gauge itself thus keeping it oil free. I am looking for one of those but they are £300 or more new and very rarely come up secondhand. I don't think it would be too difficult to make one as they just seem to use a rubber membrane between the oil and the water to keep them separate.
I haven't used the tester yet even though I have had it for four years! I want to clean it out and fill with fresh oil (which I have ready) and then I will have to make some adapters to connect the gauges.
You may wonder if the tester will still be accurate after all this time but there is really nothing to go wrong with them. So long as the tester and the weights are undamaged then it will still be accurate, certainly more than accurate enough for testing steam gauges and those used for hydraulic testing of boilers. The Federation of Model Engineering Societies issued a note about DWTs which said that they were acceptable for the calibration of test gauges so long as the tester is undamaged. The testers themselves do not need to be calibrated as they are classed as a 'Primary Standard'.
In 2021 another pair of testers came up for sale very cheaply so I bought those as well. One looks to be complete and the second one has one of the pistons missing. Both of these testers have two pistons rather than one. I'm not sure why though. Neither of them came with any weights.