Re: Steam gauge question-experts please.

Hi John. No doubt your gauge is a standard Bourdon tube type pressure gauge. These gauges are generally designed to be most accurate in the mid-scale range. As you may recall, the steam gauges on the K-28's, 36's and 37's were 0-400 psi gauges, so that 200 psi was mid-scale, at the straight-up 12 o'clock position. I don't know what pressure the 315 ran, but I would guess that the normal working pressure was pretty close to the mid-range on your gauge. If your gauge is only 2.5 to 3.5 psi off at its mid-scale reading, I'd say it's OK, as that represents an error of about 1% on a 0-300 psi range. Given the age of the gauge, the number of trips over the road, the hostile environment, there is no doubt some wear in the small gear and sector as well as the linkage, all of which can result in inaccuracy. I'd be surprised if the original specs on the gauge were any tighter than 1% anyway. A little WD-40 probably wouldn't hurt anything, or better yet, one of the newer Teflon-based dry film aerosol lubricants would be good.
To get the gauge to be as accurate as possible at the normal working pressure, you'd need to set it up on a dead weight pressure gauge tester. Both the D&S and the C&TS have them in their shops. Set up the tester with the appropriate weights to give normal boiler working pressure. If the gauge is not right on, pull the needle using a gauge hand puller and reset the needle on the stem so that it is reading exactly the pressure indicated by the weights. Remove some weights, then add them back on and see if the gauge repeats. If it does, I'd say you're ready to hook it up the the 315 and get under way. If you can't get access to a dead weight gauge tester, or you don't want to risk messing up the gauge, you should probably do as Dan suggested and send it to an instrument shop. Given all the gas/oil field suppliers and the Four Corners steam plant, you might find an instrument shop in the Farmington area.