Sciencemadness Discussion Board - Need a new vacuum gauge... suggestions?

Need a new vacuum gauge... suggestions?

evil_lurker - 16-1-2008 at 23:12

I need new digital vacuum gauge... and am looking for something that will not break the bank... anyone have any experience with A$hcroft?

I'm thinking of getting one, but damn those things are expensive @$350 a pop from Grainger, but at least they have 316 stainless wetted guts which should hopefully hold up for a little while.

bio2 - 17-1-2008 at 10:05

A digital unit is less reliable, they drift, than say a Bennert type closed end manometer
which can easily be made with a piece of glass tubing and a couple CC's of Hg. These
have a relatively short range (1-75 torr or so) but are fine for distillation and require no calibration just filling into a hard vacuum and they never drift.

If you are set on an electronic meter get one that is easily calibrated and for that you need another accurate guage anyway so it's sort of pointless IMHO.

Digital thermometers, vacuum guages, etc. tend to lull me into a sense of false security so that's why I don't like them. It's easy to watch the readout trusting it implicitly only to find out 6 months later it's drifted and been wrong all along.

I had a $350 (20 years ago price) thermocouple type, refrigeration application,
unit that was full range and very accurate when working properly. These type
are notorious for any minuscule contamination screwing up the sensing head.
Even the pump oil fumes would throw it way off in only a short time.

If you can find one with a diaphragm isolated sensor this is what is needed but
nothing beats a column of mercury in my experience.

BTW; Ashcroft was always known for high quality instruments
in years past but maybe it's chinese junk these days but the name was highly respected in industry at one time.

[Edited on by bio2]

evil_lurker - 18-1-2008 at 19:23

http://www.chemglass.com/images_product_1/4402.jpg

Is this what your talking about bio?

And is it only good for measuring vacuum 120mm below atmospheric, or does it only start to read once the absolute pressure goes below 120mm?

not_important - 18-1-2008 at 23:18

That gage is pegged until you get down to 120 mmHg absolute, at which point the mercury column begins to drop.

Vacuum Gauge

MadHatter - 18-1-2008 at 23:20

Evil_Lurker, it's a manometer. Pricey son-of-a-bitch in their catalog ! That one has 2 connections
like my Remax, analog, dial vacuum gauge. I noticed that their 1 connection version is $235.
The real question would be how much vacuum precision do you need ? They want $780 for their
digital version.

I found the Remax to be more than accurate enough for my distillation needs. Water, running
through my hydroaspirator at 59 F, shows 13 torr on this gauge. I forget exactly what I paid
for it on eBay($60 ?), but it's still cheaper than any gauge I saw in that catalog.

bio2 - 19-1-2008 at 13:11

Even a Bennert manometer is way overpriced from any source I've ever seen considering that if you have a tiny bit of mercury the whole thing can be put together with a few cents worth of 3mm OD glass tubing and the price of a simple isolation valve in just a few minutes. Even the valve is not essential as the Hg is held against the closed end when not in use by atmospheric pressure.

A constriction is highly recommended to prevent the mercury slamming into the closed end if the vacuum is released too fast. Place the constriction about half way up the closed end.

To fill, simply pull a hard vacuum with your pump then use an attached piece of rubber tubing containing the correct amount of Hg. When the vacuum is pulled heat the glass a little to remove any moisture then tilt the rubber tube allowing the mercury to flow in.

These are easily read to about 1mm Hg which is more than adequate for any normal distillation use.

I built one times ago that serves well to this day.

A teflon coated isolating diaphragm for an electronic sensor is likely to cost more than
the digital guage and the usual SS diaphragms will quickly destroy with say HCl or
even formic acid fumes which will also wipe out the sensing head neverminding
the affect on the accuracy until it finally gives up the ghost.

evil_lurker - 19-1-2008 at 14:42

Ok now I think I understand.. when a vacuum is pulled on the apparatus, the mercury is allowed to flow freely since there is no pressure to stop the flow... vacuum the shit out of it, tilt the entire thing so that the mercury flows towards the end and your good to go.

Yeah, Chemglass is the cheapest I've found... scibay has one from kontes for $250, chemglass is a hair more expensive for the dual outlet version @$295 (I get a nice $70 discount off list). Even at that price, its still cheaper than a digital version and much more accurate.

bio2 - 20-1-2008 at 11:38

..........its still cheaper than a digital version and much more accurate. .........

Yea, you got it regarding the filling.

What is the resolution & range on those low end (price) digitals?

Do they come with the isolating diaphragm or maybe is it built into the sensor?

Without one, IMHO, they are pretty useless unless only used with common solvents or unhalogenated substances. Any chlorides even NaCl will attack even 316L SS.
Some of the exotic SS alloys stand up pretty good. Normally in chem plants the teflon coated diaphragms are utilized and they can be a PITA to fill.

The Bennerts range can be increased to about 200mm IIRC by simply making the
thing taller. The one I made was for 75mm because I liked the more compact height
and that range is more than adequate for my application. About a thermometer bulbs
amount of mercury is enough using thin bore tubing. Get the heavy wall stuff to be on the safe side. Regular wall will break the end out without the constriction if the
Hg slams into the closed end due to sudden pressure release.

Intergalactic_Captain - 3-3-2008 at 09:37

Didn't think this required a new thread, so I'll ask here. I've got 3 high vacuum pumps (salvaged from a varian mass-spec/leak tester some years ago) that need to be tested next week. The set consists of an oil diffusion pump and two rotary-vane roughing pumps. They came from a manufacturing plant about 5 years ago - Apparently the machine was donated to them by Ford for a contract job, after which the company had no more use for it...The machine was supposedly wheeled out to a loading dock for disposal, at which point the pumps were removed, transported to a barn, and placed in a box where they've been well-protected from the elements ever since.

So, assuming they're in decent shape, I'd like to get them in working condition and sell them on ebay - I've still got to get the model numbers, but looking around it appears that I could net around 6-8,000 USD for the set. I'd use them myself, but I'm guessing just the first roughing pump would implode most of my glassware...

Anyhow, I need a gauge that will read down to the range that a mass-spec would operate at - I believe the operating range between them is 10^-1 to 10^-4 millitorr...The two absolute pressure gauges I have would have virtually zero resolution in this range. Since I'm most likely going to unload these, I'd prefer something cheap and analog. Could I get away with a diy mercury manometer, or am I getting into the highly-specialized (expensive) gauge range here?

bio2 - 3-3-2008 at 10:32

........I'd use them myself, but I'm guessing just the first roughing pump would implode most of my glassware......

The difference percentage wise between say 50mm Hg and
millitorrs is not very much in absolute pressure terms so
this premise is erroneous. 50mm is already like 93.5% of a
full vacuum (50/760).

........I believe the operating range between them is 10^-1 to 10^-4 millitorr..........

Thermocouple type gauges among others are suitable for this range. Manometers ain't gonna cut it except a McCleod.

Do a little basic research, bubbha!

http://en.wikipedia.org/wiki/Vacuum

Intergalactic_Captain - 3-3-2008 at 12:46

Hmm...Maybe I'll try out the first pump then, see if my glassware will hold up. What concerns me is the fact that the walls aren't too thick - This is 19/22 ware from DPGlassShop on ebay if anyone can comment on their products...

As for the gauges, I figured I'd ask the experts before I spend my money on anything. The biggest issue is that since I'm probably never going to need equipment like this, I'll probably only use a gauge once - So, cheap is the operative term. I checked out the McLeod gauge, interesting concept...

So a thermocouple gauge would be my best bet? Wonder how much that's gonna set me back...

chemrox - 4-3-2008 at 01:33

I got a Brennert from American Scientific for around $175. Is there a reason you want digital over glass U?

MagicJigPipe - 4-3-2008 at 08:32

I often wonder what it would take to cause standard thin-walled glassware to implode. In theory, at 15 torr a pressure of ~14.4 psi would be exserted on the outside of the glass. That seems pretty substantial but surely it wouldn't cause implosion. I'm not sure because I have never used anything other than a hand pump or aspirator. It's almost hard to imagine that thin glass would be able to withstand nearly 15 pounds of pressure pushing on every square inch (2.6 kilograms of mass at Earth gravity pushing on every square centimeter. That's just a quick guess, correct me if I'm wrong).

By the way, Intergalactic, why did you buy all 19/22 glassware? From my experiences, there is WAY more glass available in 24/40 and it tends to be cheaper because it is more "standard". Is 19/22 the "standard" in Europe?

bio2 - 4-3-2008 at 10:21

Standard round bottom flasks have no problem standing up to full vacuum.

Intergalactic_Captain - 4-3-2008 at 10:53

19/22 was simply due to the fact that the setup I was assembling at the time was cheaper in 19/22 than in 24/40 - especially condensers, though I'm having a tough time finding a decent fractioning column. ...And I'm in the US if that's what you were wondering.

So nobody's had any imploding glassware? I can understand the concept of a sphere (round bottom flask) being able to withstand extreme pressures, but it's the knowledge of how easy it is to break glass that frightens me. Oh well, worst comes to worst, I found another supplier that's got dirt cheap RBF's incase anything happens. I suppose the thing is that most people here doing vacuum distillations and such are using aspirators of one sort or another, not high-flow rotary vane pumps at 1mm Hg.

Thinking about it, though, I may just have the gauges... After looking at some pictures, I believe the diffusion pump has a thermocouple-type gauge mounted to it already, but as for the circuitry to actually use it...Not sure if that's there or not.

PainKilla - 4-3-2008 at 11:03

I've used glassware from that shop before, and it holds up quite well. It's actually very high quality glassware, just seemingly used. It holds up under vacuum and high heat (have tested >1mmHg), so I'd expect it to work quite well with whatever you throw at it.

19/22 over 24/40 any day IMHO. While it's a bit more expensive to get the more obscure glassware (or you need to use adapters) the size is really quite perfect for laboratory chemistry.

I absolutely hate using large oversized flasks to run reactions, especially when smaller flasks aren't around. That's why at home I use 19/22, with a custom 1L flask made just in case I ever need to distill something.

19/22 is good for 25-500ml IMHO. 24/40 is good for 100ml-2L.

As far as gauges go, I've never used an especially accurate gauge (especially not one that is $350), so I'm of no help in that regard.

[Edited on 4-3-2008 by PainKilla]

MagicJigPipe - 4-3-2008 at 15:51

I don't see what difference the joint size makes except for maximum flow rate. There are 25mL up to 50l flasks in 24/40 and I've never had a problem with the smaller sizes. I will only use 24/40 and 14/20 when I have a choice. I would have to be in a bind to buy 19/22.

Are there REALLY any significant advantages (other than size) between 24/40, 14/20 and 19/22.

One thing I hate is that most thermometer "ports" are 10/30. Just make them 24/40! 24/40 thermometer adapters are cheap and easy to find but 10/30 thermometer adapters are expensive and semi-rare. That has always annoyed me.

However, I used to just wrap a band of teflon tape around the thermo. and slid it into the 10/30 (before I had a 10/30 adapter). Most thermos. fit perfectly when done this way and I never experienced a leak. Another thing that always bothered me is the 10/30s are usually ground glass and it blocks you from viewing the temp on the thermometer at that point. Basically, the thermo. has to be the perfect length and size.

Annoying...

dann2 - 5-3-2008 at 15:51

Hello,

Less than 10 Torr transducer here if that is any good.
http://cgi.ebay.co.uk/MKS-Baratron-Pressure-Transducer-10-To...

Dann2

Magpie - 11-6-2011 at 15:03

Quote: Originally posted by bio2

(snip) Even a Bennert manometer is way overpriced from any source I've ever seen considering that if you have a tiny bit of mercury the whole thing can be put together with a few cents worth of 3mm OD glass tubing and the price of a simple isolation valve in just a few minutes. Even the valve is not essential as the Hg is held against the closed end when not in use by atmospheric pressure.

A constriction is highly recommended to prevent the mercury slamming into the closed end if the vacuum is released too fast. Place the constriction about half way up the closed end.

To fill, simply pull a hard vacuum with your pump then use an attached piece of rubber tubing containing the correct amount of Hg. When the vacuum is pulled heat the glass a little to remove any moisture then tilt the rubber tube allowing the mercury to flow in.

These are easily read to about 1mm Hg which is more than adequate for any normal distillation use.

I built one times ago that serves well to this day.

I had been wanting a Bennert vacuum manometer for some time so was watching eBay. Finally, 2 of them showed up at once! I bought the first one for a song - they are $250-450 new so I'm thrilled.

I retrieved Ace Glass' instruction for filling with Hg. This specifies a "hard vacuum" of at least 10^-2 torr (10 microns). I have no vacuum pump so I'm considering buying Harbor Freight's single stage $99 special which claims to pull 75 microns. Since 1 mmHg = 1000 microns I can't see why 75 microns would not provide sufficient accuracy for typical vacuum distillation work.

Let me know if you agree; or if not, please tell me why.

peach - 12-6-2011 at 00:09

Quote: Originally posted by bio2

Digital thermometers, vacuum guages, etc. tend to lull me into a sense of false security so that's why I don't like them. It's easy to watch the readout trusting it implicitly only to find out 6 months later it's drifted and been wrong all along.

I could not agree with this point any more.

It is truly remarkable the extent to which people are willing to trust a digital display over an analogue one, or worse, something entirely mechanical.

I can honestly say, at university I was the only person in the entire year group who bothered to check the pipettes were calibrated correctly; which took all of two minutes. This is one in a over a hundred people checking the thing works before trusting what it says on the display.

Quote:

One thing I hate is that most thermometer "ports" are 10/30.

I have seen prices for those ground thermometers vary from $15 (new from China) to £80 (new from QuickFit).

So naturally, I am inspired to think... "I wonder just how bad the Chinese one could be for it to be not worth it at that price difference?".

The only qualifying factors would be, is the joint ground okay and is the scale calibrated okay.

The joints... I don't know until I see it.

The scale, I can ask about the expensive ones because they will surely have some numbers on that. So I did. "Is the item (xxx) calibrated to any standard or external reference?"

No reply. On an £80 thermometer.

So, for all I know, the scale is no better than the $15 Chinese version.

My pH meter also functions as a datalogger, and can track temperatures from 0 to 100C to offset the pH probe.

I never really made use of that, and never bothered to install the software (since it needed Windows 98).

When I finally spotted an old laptop spare, I did. And I was really impressed with how much fun and use it is.

Rather than all this "it went up a bit slowly, then seemed to slow down, then go back up" or having to constantly get up and check things, I could now see a graph being plotted on the screen from across the room, along with the numbers in inch high font size.

This, combined with the ground thermometer problem, has had me wondering about making cheaper 10/30 and B14 drop in thermometers.

I can buy platinum resistance thermometers quite cheaply if I buy a box full of them. I could then encapsulate them in a PTFE probe with the taper on it, to stick in the top of a still. Turning a taper on PTFE is far easier than grinding glass.

These could be Class A band in terms of accuracy;

-200C
Error 0.55C

-100C
Error 0.35C

0
Error 0.15C

100C
Error 0.35C

200C
Error 0.55C

It's pretty tricky to read a normal 250C thermometer to less than 0.5C with any kind of accuracy. And that's assuming it is actually giving a reading accurate to 0.5C or better. A spirit or mercury thermometer can not log data, my face needs be right up to it and parallel to read it and they're very easy to break.

Here is a spirit filled 250C thermometer with a 10/30 taper. It's accuracy is stated as +/- 1C and it's price is $64.95

I think I will buy one of the Chinese ones for $15 and do a review for you.

I didn't think many people were interested in ground thermometers, but if a few people wanted the PTFE / platinum version I might look into them some more.

The beauty of the PTFE encapsulation would be that I could easily produce different lengths and joint sizes.

Magpie - 27-8-2011 at 19:03

I recently bought a used Bennert vacuum manometer for a very reasonable price off ebay. (See pic below.) It came without mercury so I am now getting ready to fill it with something < 7mL of Hg I estimate.

I plan on evacuating the unit with a vacuum pump which has shown to be capable of reaching a pressure of ~ 7mmHg. The pump may be capable of going lower if I can truly seal all the connections.

I have downloaded the instructions from Ace Glass. However, I still would appreciate any advice on how to proceed from anyone here who has actually done this.

chemrox - 31-8-2011 at 10:24

It's pretty straightforward. You need a small funnel and some tubing. Do get the writeup from Ace. I followed it exactly.
**************************************************
I have an extra digital vacuum gauge I'll sell.

[Edited on 31-8-2011 by chemrox]

Magpie - 31-8-2011 at 13:15

Quote: Originally posted by chemrox

You need a small funnel and some tubing.

I don't understand how you would use a funnel, ie, it would be open to the atmosphere. The picture below shows what I was planning to use.

Quote: Originally posted by chemrox

Do get the writeup from Ace. I followed it exactly.

That's what I would like to do too. The problem I have is that I can only get a vacuum down to about 50mmHg (50Torr). The Ace procedure calls for a vacuum of at least 10^-2 Torr. So, I can't fill the manometer with Hg until I find a better vacuum source, right?

watson.fawkes - 1-9-2011 at 06:10

Quote: Originally posted by Magpie

The problem I have is that I can only get a vacuum down to about 50mmHg (50Torr). The Ace procedure calls for a vacuum of at least 10^-2 Torr. So, I can't fill the manometer with Hg until I find a better vacuum source, right?

In the worst case, you'll have a column of air in the sealed end of the U-tube (left, in the photo) at the pressure of 50 torr. It will still work as a barometer, but it will be less accurate. On the right side, the force on the mercury will be the weight of the mercury plus the pressure on the line. On the right side the force on the mercury will be the weight of the mercury plus the pressure of the trapped gas. Suppose, even, that you could measure the pressure of this gas at some column height of the mercury. At other column heights, though, the pressure of the trapped gas will change because of a change in volume. You end up with a non-linear gauge. Whether this is tolerable depends on what you're doing with it. The worst case of 50 torr is pretty high, and your bias and non-linearity would be rather noticeable.

The procedure from Ace looks to be designed, in part, to minimize mercury vapor exposure. There are other ways to fill your manometer, though. The real goal is to eliminate all the gas in the sealed side of the U-tube. You can do this by filling the sealed side while upside down, so that all the gas is excluded by gravity. Then you tilt it carefully into an upright position so as not to incorporate air. I seem to recall old glass designs for these with a stopcock at the bottom of the U-tube for filling it. This was before the days of cheap plastic tubing. A length of thin tubing threaded through the U-tube should allow you to introduce mercury in the upside-down position directly. You may need to special-order a length of appropriately small tubing, if what's at the hardware store isn't small or flexible enough. Nevertheless, a couple feet of tube is certainly cheaper than a vacuum pump.

Magpie - 1-9-2011 at 07:24

Yes, watson, I have been thinking through those same arguments. However, I have come to the conclusion that the root of the problem is the small ~1mm ID restriction in the bottom of the lower "U" turn of the lumen. This restriction is 2-3" long and is there to prevent breakage of the closed end should the operator release vacuum too quickly. The remainder of the lumen has an ID more like 5-6mm. If this restriction wasn't there then the manometer could be filled by merely pouring in the mercury while manipulating as required.

What the vacuum pump does is remove all the air so that the mercury can be poured through that restriction without having to fight air trying to escape countercurrently.

I've attached what I hope is a better picture of the manometer. Hopefully you can see this restriction. I'm very doubtful that a small tube could be inserted through this restriction that would still pass mercury.

watson.fawkes - 2-9-2011 at 05:12

Quote: Originally posted by Magpie

What the vacuum pump does is remove all the air so that the mercury can be poured through that restriction without having to fight air trying to escape countercurrently.

You might be able to get it filled in stages. Your pump will evacuate to about 1/15 of atmospheric, which is to stay that it will evacuate 14 parts out of 15 of whatever gas is there. So instead of fighting counterflows, just get most of the air out and compress the rest with the mercury column. After that, invert the column and the remaining air will bubble up, but this time through wide tubing in the close end. Now evacuate again and repeat. Two or three rounds of this should get you enough fill. You should be able to get about 3/4 fill, say, in the first round.

Another idea. I'm not sure you want to try this one. If you seal off the system while evacuated, a heat gun should decrease the viscosity of the mercury, while not generating significant overpressure to the glass. You'd really have to heat it to get back to 1 atm of internal gas pressure; there looks to be a gasket that would fail first. Let it cool while sealed in a position so that the bits of condensing mercury vapor coalesce toward the reservoir.

Magpie - 2-9-2011 at 08:46

Quote: Originally posted by watson.fawkes

That seems like it should work. If my current efforts to acquire use of a hard vacuum through a friend at a university do not pan out I will try it. Thanks.

Magpie - 21-11-2011 at 19:29

As stated earlier in this thread I acquired a Bennert manometer but it came unfilled with mercury. I had the necessary mercury (~4mL) but also needed to be able to pull a deep vacuum (<1mmHg) for the filling process.

Earlier I had bought a Harbor Freight 2.5CFM single stage vacuum pump (now <$100) rated at 75 micron (0.075mmHg). The pump worked well for vacuum distillations but I was skeptical that it would actually pull a deep vacuum.

After numerous attempts to get others to help me (some sincerely tried, others blew me off) I got fed up and bought my own Pirani vacuum gauge capable of reading down to 1 micron (0.001mmHg).

Today I set up the system I will need to fill my Bennert manometer. I was delighted when the system pulled right down to 121 micron (0.121mmHg). So it looks like I'm good to go.

Below is a picture of the system at 121 micron absolute pressure. I'm sorry that I didn't use a flash for the close-up of the gauge reading. You'll have to trust me on that.

Questions and comments are welcomed. But please don't remind me that now that I have the Pirani I don't need the Bennert.

[Edited on 22-11-2011 by Magpie]

starman - 21-11-2011 at 23:07

Hey magpie I have the same pilot plus though directly attached to the pump on a tee.Make sure you always release the vacuum gradually,I did the sensor on mine and fortunately they replaced the unit on warranty.

Magpie - 22-11-2011 at 09:09

Quote: Originally posted by starman

Make sure you always release the vacuum gradually,I did the sensor on mine and fortunately they replaced the unit on warranty.

Thanks for the warning, starman. I will keep that in mind although a globe (or needle) valve would be better for that than my ball valve.

I installed the ball valve for isolation of the gauge prior to shutting off the pump. Hopefully this will help in keeping the gauge sensor from being contaminated with pump oil, etc.

jock88 - 30-10-2014 at 04:51

Can anyone advise me on controlling this pirani gauge.
http://www.ebay.co.uk/itm/3-three-x-Applied-Materials-Capaci...
Is it simply a matter of putting somewhere between 15 and thirty volts on the correct pins and then perhaps reading the output using a multimeter and converting voltage to pressure (using a chart perhaps)?
Or do I need a precise controller.
Would this elcheepo controler work.
Applied Materials 0190-27313 Capacitance Pirani Gaug

Thanks