I'm helping a friend check his VRB. Luckily he has the original standards lab folders on the unit that cover the first 2 decades of its life, and make for an interesting read.
It started life in 1964 as a HW Sullivan Type T 2100, with 200 ohm/volt sections and ratios of 2, 5, 10, 20, 50, 100, 200, 500 and 1000V to 1V and a 0.001% accuracy at rated voltage and 20degC. Original British NPL calibration report indicated low ppm error and uncertainty, up to 7ppm error for 500V and a 5ppm uncertainty for 200 to 1kV ranges. It had repeated cross comparisons over the next 2 decades with two other volt boxes of the same type in a defence standards lab, and showed at worst possible low ppm drifts. Testing also showed at most 2ppm change from 1/10 to rated voltage (ie. 5W at 1kV), and 200,006.3 total. Local test equipment used was typically a Guildline DCC and a 100 and 10k references cal'd to 0.1ppm, but not much detail of that in the notes.
There is no provenance after 1985, but it then likely spent a few decades in a test section at an electronics company, and then literally a farm shed.
I've only quickly confirmed that the section resistances are nominal with a handheld dmm, but first I need to clean the external surfaces, and check the oil for moisture and IR. When it landed in Oz it was filled with high voltage transformer oil by the local electricity supply lab, and as far as the lab notes go the oil is likely original. The oil shows only a slight colouration if any and is to a nominal fill level by the dipstick. Apart from a crackle test, what in-situ IR testing has anyone done, and is that appropriate? The original oil spec was 2x 10^12 ohm, and a test after about a decade measured it at about 1.1x 10^12 at 500V. I can IR test from the 0V tap to enclosure screen although my handheld only goes to a 2x 10^9 reading, so if it is more than that then I'll have to break out the vintage BPL 175LZ Mk2 megohmmeter which goes to 5x 10^12 at 500V. I'm not keen on replacing the oil at all, and certainly not initially, and would prefer to leave that decision to later.
The two thermometers each have 0.2degC minor gradations, with a 15 to 25C span. I'll let them settle, but they both were showing the same value so that is a good start too.
The step values of 200, 200, 600, 1k, 2k, 6k, 10k, 20k, 60k, 100kohm should make it relatively easy to bridge compare groups of steps against some balanced resistor pairs I have using a 6-digit meter for null measurement and using polarity swapping, so that is likely how I will start. If all goes well then it may not end up being used as a volt ratio box per se, but more for absolute resistance cross-comparisons and also for 1, 10, 100 and 1000 ratio checking of Wheatstone bridges.
Tim