I've been meaning to make one for some time and recent issues with the safety valve setting on one of my locos. turned up the necessary 'Round Tuit'.
Deadweight testers are available commercially for calibrating pressure gauges by manufacturers and testing labs. Kozo designed this one for small steam pressure gauges and the plunger is 1/8" dia., so a mass of 112g on the plunger applies a pressure of 20psi to the working fluid (oil). The top hat mass used to carry the other masses is a little lighter, as the mass of the plunger must be added to obtain an accurate pressure reading.
This picture shows the various bits machined and awaiting assembly. The deadweights were weighed on a little electronic balance. I happen to have a set of standard laboratory masses that I used to check the balance, so I know the counterweight masses are accurate and the pressure readings will be accurate to 0.5 psi. The masses shown allow the calibration of a pressure gauge to be checked at 20, 40, 60 and 80 psi, by adding masses one at a time.
This photo. shows the assembled tester in use to check a Roundhouse gauge. There is some parallax error in the photo., but the gauge is reading low by 1-2 psi at 40 psi which is near enough for our purposes. Another gauge read exactly 40 psi when tested. These little gauges are more accurate than one might expect from a mechanical instrument this small.
Regards,
Graeme
