The calibration of Baratrons

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Finn Hammer
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The calibration of Baratrons

Post by Finn Hammer » Thu Apr 08, 2021 11:08 am

All,
Since I found out my Baratrons were at best malcalibrated, but probably broken, I have bought a calibrated unit, waiting arrival here.

Obviously, I have had a gleane at the calibration reports offered, and even the ones issued by MKS themselves, show what to me looks like a gross neglect of the majority of the transducers full range output. Before I write MKS about this, I thought I would run the question by you guys, It goes like this:

A Baratron is promoted as being accurate to 0.5% of the reading over the full range of the transducer.
The calibration report only covers the top decade from 10V to 1V, whereas the pressures covered from 1V to 5mV are neglected in the report. Is it possible to get access to data concerning the calibration these missing decades of pressure?

An example:

The output of this Baratron (E28D.1TDF5B 0.1 Torr) relates to pressure in the following fasion:

1st decade : 100 mtorr to 10m torr -> 10V to 1V
2nd decade: 10 mtorr to 1 mtorr -> 1V to 100mV
3rd decade: 1 mtorr to 0.1 mtorr -> 100mV to 10mV
4th 1/2 decade: 0.1 mtorr to 0.05 mtorr -> 10 mV to 5 mV

Comparing this table with the calibration report reveals, that the accuracy of the first decade is well documented, but there is no data of how the instrument tracks a pressure in the following 2 1/2 decades. (since the zero point is set at a pressure of 1x10-6 it doesn't count)

Here are a couple of calibration reports, not of the part I describe, all of them focussing solely on the top decade of pressure reading.
test1.JPG
MKS has fewest test points, but probably more accurate
test2.JPG
One other lab report
test3.JPG
Yet another test report, baratron

Cheers, Finn Hammer

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Richard Hull
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Re: The calibration of Baratrons

Post by Richard Hull » Thu Apr 08, 2021 6:44 pm

Finn,

I have 10 Baratrons here! I use a 0.1 torr "heated" Baratron on fusor V. The expert to ask is Steve Hansen of the Bell Jar. He worked for MKS for years. If he doesn't know, no one on earth knows.

The reason the 0.1 torr baratron is heated is to retain precision in the stabilized electronics and sensor down those warranted 4.5 decades. One of the +/- 15 volt supplies needs a bit more current capability as one also heats the baratron. The padded sock, (seen in my photo below, keeps drafts from causing thermal heater oscillations. Check the manufacturer for current requirements.

I found all 10 of my baratrons at a single hamfest years ago. I got them all for $100! The guy was asking $20 each I made the offer of $100 for all 10 and he took it. Most all were 10 torr with 2 being 1 torr and the single 0.1 torr unit. You just don't stumble on deals like that!

You are well heeled and very serious if you spent the $1200-$1500 for a 0.1 torr baratron!! Most new baratrons in the 100 to 1 torr range are about a kilobuck each. The chart you supply shows a .2% error at 10 microns and you might depend on 1-2% and the bottom of the fourth decade, I would imagine. All provided you achieve equal regulation of the supplies and precision of you voltage readout.

I supply images of my setup below. As usual, click on images to enlarge

Richard Hull
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Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
Retired now...Doing only what I want and not what I should...every day is a saturday.

Chris Seyfert
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Re: The calibration of Baratrons

Post by Chris Seyfert » Thu Apr 08, 2021 6:54 pm

Hi Finn,

Particular models may vary, but MKS typically rates their baratrons for a measurement range of 2000:1 (versus full scale). So a typical 10 torr baratron would be good for measurement down to 5 mTorr. I am not sure why the calibration report only covers one decade. Possibly if it's linear in the first decade there's no physical reason why it would be non-linear lower down? That's just a guess on my part, but I have used baratrons extensively at work and never had one be "partially linear".

You probably know this already, but two particular things to be careful of with your 0.1 Torr model when checking calibration - first, some 0.1 Torr baratrons must be mounted with their inlets pointed downward. If they are mounted in other orientations, the linearity gets messed up. You would have to check a manual for your model to see what mounting orientations are approved. Second, your baratron has a 4 hour warmup time, so be patient before twiddling that zero knob . . .

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Finn Hammer
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Re: The calibration of Baratrons

Post by Finn Hammer » Thu Apr 08, 2021 8:24 pm

Thanks for your repliess, as always.
I had to turn to South Korea for a freshly calibrated 0.1 Torr unit, and it was expensive too, though well below brand new price.

Back in the early nineties when I was flying gliders we had a saying: "the instrumentation should cost as much as the airframe", and now I realise this saying goes for a well equipped fusor too.

What is the use of reporting a pressure if it comes off a dodgy transducer, how to compare results? Voltage and current is easy enough. Flow should come easy with my freshly calibrated flow controller, now pressure can also be measured with some confidence. Soon ;-) . --I hope...
I do not have the experience of you guys, so the build is evolving slowly, I want to be sure each step forward is done right, and verified.

Cheers, Finn Hammer

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Richard Hull
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Re: The calibration of Baratrons

Post by Richard Hull » Fri Apr 09, 2021 4:58 am

Sorry I forgot to mention the 2 hours warm up period on the 0.1 torr model. I found out the hard way back in 2004. I had to constantly readjust the zero, until I gave mine the demanded 2 hour warm up period. Once I really got it zero'd, if I tried to run it right off the bat, it was way off at the low end, but after a couple of hours it was right where I put it on zero. I typically light up the neutron detector and the Baratron about 2 hours before I turn on the pumps or get serious about fusion. My background on neutron counts is rock steady around 6-8 cpm unless the sun is doing something nasty and I have seen it go to 12 cpm only once. Therefore, I am mindful of the space weather at that site.

https://www.spaceweather.com/

With the new sunspot cycle underway, the cosmic rays will decline as is typical. I followed the recent unbelievable "quiet sun" of the last few years. Stunning quiet sun period of many years, it is believed, brought the intensification of the little ice age.(1303-1860) So continuously spotless was the sun that astronomers of that time, 1650-1720, thought that the drawings of Galileo of a sun with large spots were Galileo's imaginings. There has been much hoopla of late due to the recent intense spotless nature of the sun that some fear another mini ice age is upon us.

Similar observations and disappearances went for Robert Hooke's observations of the great red spot on Jupiter in the late 1600's that took over 150 years before it was first seen again by amateur astronomers in Australia in the 1870's and it was then called "the pink fish". I have, myself, observed it as an amateur astronomer in the 1970's as a brilliant traffic light signal bright red only to see it fade in the 1980's to where the small 2-3 inch amateur telescope was hard pressed to see it at all. Even a 4-5 inch telescope would be hard pressed to see it by an amateur who had not known of it prior brilliance.

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
Retired now...Doing only what I want and not what I should...every day is a saturday.

Chris Seyfert
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Re: The calibration of Baratrons

Post by Chris Seyfert » Fri Apr 09, 2021 6:17 pm

Edit - I misspoke earlier about the baratron orientation - it affects zero, not the linearity. Earlier baratrons had large diaphragms to sense low-range pressures and if mounted in the wrong orientation, the effect of gravity was too large for the zero adjust circuit to compensate. Most modern baratrons do not have this issue, as they leverage modern electronics to use smaller diaphragms that have smaller movements and are less affected by gravity.

Just as a heads-up, though, even for modern baratrons if the orientation is changed it will still require re-zeroing - I've seen a new 10 Torr baratron zero point move by ~50 mV when rotated upside down. Easy to re-zero, of course, but it's a little "gotcha" that can surprise you if things get rearranged.

For the electronic geeks among us, I know at least one manufacturer uses the amazing AD7745 capacitance-to-digital converter chip, boasting an astounding 4 fF accuracy and 4 aF resolution - yes, that's femto- and atto-farad, respectively! Something to keep in mind for skilled adventurers that want to try replacing bad electronics on an older unit. Digi-Key sells an evaluation board for $57, complete with USB interface (EVAL-AD7746EBZ-ND).

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Richard Hull
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Re: The calibration of Baratrons

Post by Richard Hull » Sat Apr 10, 2021 5:10 am

I'm not so adventurous as to mess with the baratron electrons. Tylan got it good enough for my purposes.

The orientation issue and recal issue was important when I moved the orientation on fusor V. I went overboard and let the thing warm in its new position for 3 hours before zeroing. With the Turbo, zeroing was a lot easier than with the diff pump. I did note that the TC gauge and the Baratron tracked real close until at 15 microns. Of course, the TC response lagged as the Baratron was virtually instantaneous.

Position sensitivity reminds me of early color TV's. Moving an old color TV, (circa 60-70), to another spot in the living room warranted a must do de-gaussing at minimum if not a complete session behind the set with the 20 pots using a bow, bar, dot and pattern generator. It really wasn't until masks in the tubes (Trinitron) that moving a color TV meant you might just have to de-degauss 1980's until the end of CRTs.

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
Retired now...Doing only what I want and not what I should...every day is a saturday.

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