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Re: Gauges

Posted By: Romarrk
Date: Monday, 1 April 2013, at 3:37 a.m.

In Response To: Re: Gauges (SVP Neon Equipment)

> You specifically said “DV6”, which was the
> predecessor to the DV-6M. Split all the
> hairs you want: I doesn’t wash with me.

Well, that was my error at first time. Hastings tubes won't work with DC drive. I already admitted it. Older 531 style and the one I use will work well.

> Hastings VT-6 (with the VT-6A being the new
> generation) gauges, used with the
> corresponding DV-6M tube are ambient
> temperature and barometric pressure
> compensated. Your suggested arrangement is
> not. A change in either parameter will alter
> any calibration you *think* you have done.

> For example: You are preparing to set up
> your “vacuum gauge” to take a reading. When
> you do this, the ambient temperature is
> 70°F, and the BP is 30.0” Hg. (We won’t even
> get in to discussing dew point and water
> grains per lb. of air in the atmosphere).
> All of a sudden the weather changes. It
> doesn’t really matter *how* it changes, just
> the fact that it changed. You can throw your
> set up and so-called “calibration” out the
> window (or door if you don’t have any
> windows in your kitchen)!

All this fancy wording actually just relies to cold junction/case temperature compensation. May be relevant when you need to make instrumentation guaranteed over full -55/+125C mil range, but not for neon shop desktop! Even old style tube with single thermocouple will work very well here once calibrated for a single time. Until it start drifting because of getting dirty, with which all compensations won't help anyway. This single thermocouple measures temp DIFFERENCE INSIDE sealed tube envelope, which depends on gas density/type, and heater power applied. And the only critical parameters that may in principle slightly drift with external temp are gas constants (not much for neon shop range) and platinum heater resistance (not much too). So you will get 1 micron reading instead of may be 1.1 Any relevant for neon work? And mentioning atmospheric humidity is just pure BS here. Until we are not sending it to Mars ;-) This is not a transmission tube. No serious heating at all, may be a few milliwatts. Sorry Mark.

> Btw, in experienced hands a spark tester can
> determine if a neon unit has a leak in a
> matter of a couple seconds. (Finding the
> leak may be a different matter). That is not
> the sole intention of a high vacuum gauge.
> In fact, that would be down on my list of
> priorities for use of one.

Quick closure of stopcock will reveal it. In my experience with professional pumping station with secondary pump and all that fancy stuff all reasonable tube defects still notably dropped absolute vacuum.

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