In this space, visitors are invited to post any comments, questions, or skeptical observations about Philo T. Farnsworth's contributions to the field of Nuclear Fusion research.
Subject: Capacitive manometers, tips
Date: Jan 18, 9:27 am
Poster: Richard Hull
On Jan 18, 9:27 am, Richard Hull wrote:
Most of the following info was condensed in an old bell jar article a few years back. Richard Hester noted some of these earlier. I thought I would close that thread and start afresh.
All....Repeat all capacitive manometers are very position sensitive!! Also there is no "proper" position for mounting! They will mount and work 100% in any position you can think of.
Due to their nature, it is best to mount them vertically, (butt end or connector end up), to horizontally (180 degree span) However, hanging upside down or any where in the lower 180 degrees is acceptable though not advised as stuff falling from the system could fall into the instrument. All have modern capacitive manometers shields to avoid problems here, but why test their effectiveness with a $1000 trasducer.
The key point is that the transducer must be zero'd or cal'd in place and never moved 1 degree off axis after that!!!
Example: I obtained several such manometers by Tylan General in the 100torr, 10torr, 1 torr, and .1 torr variety. (found those puppies at a hamfest $10.00 each) I prefer the 10 torr for coarse work. I did a reality check with the unit by attaching it to my direct drive 8 CFM alcatel pump via a rubber hose and letting it pump to the point where the gauge indicated .010 volts, or 10 microns. The gauge was laying horizontally on the table. As I picked the gauge up and rotated it to the vertical, and through to the opposite upside down vertical, the same pressure in the hose went through 26 microns of continuous variation! If 10 microns was a critical measurement here, then based on the position of the gauge head, the error could be +\- 110% about that figure.
The up side is that if the gauge is rigidly fixed and accurately cal'd, in place, it will consistently return to the same pressure reading to a one-thousandths part of the span to within .02% every time! Whatsmore, it is totally gas independent it measures true pressure and nothing else. They can have minor temperature corrections required if the tranducer is in a wildly variable temperature environment, but for the most part it is a non-issue.
Richard Hester is correct in that the heated versions (.1 torr units are very temp sensitive) do require plus and minus 15 volt supplies with at least a half amp or so of solid regulated capacity, but this is easy with the 7815 and 7915 solid state regulator ICs which run about $0.89 each. Other non heater versions rarely require more than .05 amps.
Capacitive manometers for most vacuum enthusiasts are a distant dream,however, due to their cost. please, never ever pay for a controller. Any digital multimeter will work with them. Just supply +\- 15 volt supplies and a voltmeter and you are measuring vacuum.
On another note, I use a metal curved hose leaving my fusor to avoid a straight line plasma or beam path to my micromaze or pump. Go to one of those giant hardware mecca's (Loews, HQ, etc.) find a lower sink, corrugated copper, hydro-formed hose. These have odd ball connectors on each end. Cut them off and trim the metal hose to length. Next, silver solder your QF connectors on each end and bend carefully to suit. cost $8.00 plus the QFs. Maybe $24.00 out of pocket. Such a SS item will cost a minimum of $75.00 and be of a rigidly fixed length.
- Re: Capacitive manometers, tips - Richard Hester Jan 18, 3:16 pm