Re: HV Measurement - Back to Basics

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "by way of Terry Fritz <teslalist-at-qwest-dot-net>" <Mddeming-at-aol-dot-com>

In a message dated 1/4/03 11:16:16 AM Eastern Standard Time, 
tesla-at-pupman-dot-com writes:


>Original poster: "Matthew Smith by way of Terry Fritz 
><teslalist-at-qwest-dot-net>" <matt-at-kbc-dot-net.au>
>
>Hi All
>
>Could some kind soul give me a hand with this little problem?
>
>I bought (blind) a 15kV voltmeter, which I planned to sit on the end of my
>MOT-based power supply.  When I first saw it and discovered that the
>terminals are about 8mm apart, I decided that this is just a meter with a
>15kV *scale*, not a meter than can be connected to and measure up to
>15kV...  Never fear, I thought, it's just the question of sizing an
>appropriate resistor/resistor network.


Hi Matt,
        Many older Xray units used a cheap DC meter and rectifier on the 
primary side to measure input voltage but "show" output voltage. It was 
called a "pre-reading kilovolt meter". These were calibrated and re-scaled 
at the factory to show the corresponding output voltage.[1][2] This way, 
only the factory and service techs needed to have a true HV AC voltmeter 
for annual re-calibration tests and repair. If this is the case, you might 
be able to do likewise by building your supply with meter on the input 
side, and making friends with an Xray service tech who could calibrate your 
power supply for you (ask your dentist).
        The alternative is to build a divider network as you suggested.
Caveat Emptor: Many (most) people on eBay, etc., have little or no 
understanding of what they are selling. To them, if it says "kilovolts" on 
it, IS a kilovolt meter, if it says "0-500 Amps" then to them, it IS a 500A 
fsd meter.
Reference [1] also contains a lot of good info on vintage HV control and 
safety systems.
Hope this helps,

Matt D.
[1] "Fundamentals of X-ray and Radium Physics" J. Selman 4th ed. Charles C. 
Thomas, publ. USA  1965 pp. 229-230
[2] "Roentgenographic Technique" D. A. Rhinehart 3rd ed. Lea & Febiger, 
publ. Phila. USA 1943 pp.28-29