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Author: Subject: Did I ruin my digital vacuum gauge?
Chemist1357
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sad.gif posted on 11-4-2023 at 03:59
Did I ruin my digital vacuum gauge?


I have a CPS VG200. It was quite pricey and is said to be one of the most accurate in its class.

At first, I was having fun with it distilling methyl salicylate and it seemed to be working. I would clean it with methylated spirits after every run. The operator's manual says to clean it with denatured alcohol.

I have a strong feeling I ruined it however when I thoroughly cleaned it once with acetone. As some of the acetone dribbled down the sides of the gauge housing it began melting the plastic and making it all sticky. Dumbass. A thought crosses my head thinking that it's probably melting the insides too.

My next distillations confused the hell out of me. I pull a vacuum to 10 torr and nothing boils or wants to come over.. So I cleaned everything, changed the flasks and boiled some water. After multiple tries, the water would boil at 54c and would stay there steady even blasting through the heat. This would correspond to a vacuum of 120 torr while my gauge was saying 7 torr!

Completely wrong and before you ask I did test multiple thermometers and they all read the same. My heating mantle has no problem heating up to very high temperatures so it is the gauge...

So did I ruin it?

[Edited on 11-4-2023 by Chemist1357]

[Edited on 11-4-2023 by Chemist1357]
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Cathoderay
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[*] posted on 11-4-2023 at 07:08


Did you try replacing the battery?
Can you see any damage when looking around the battery?
The internal parts connected to the inlet should all be metal, I don't know if they are. Anything electronic is often more sensitive to damage if it is not well designed for what it could be exposed to.
Some of the reviews I've seen might suggest this meter isn't the most reliable available.

Acetone was obviously the wrong thing to use around just about any plastic.
Denatured alcohol isn't the same thing as methylated spirits of course, the case might be compatible with one and not the other.

Why are you cleaning it so often?
Put a plastic bag over it when using to keep splashes off. Use a hose and perhaps a trap using vacuum oil to separate the gauge side from the reactant vapor.
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Chemist1357
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[*] posted on 11-4-2023 at 07:46


Quote: Originally posted by Cathoderay  
Did you try replacing the battery?
Can you see any damage when looking around the battery?
The internal parts connected to the inlet should all be metal, I don't know if they are. Anything electronic is often more sensitive to damage if it is not well designed for what it could be exposed to.
Some of the reviews I've seen might suggest this meter isn't the most reliable available.

Acetone was obviously the wrong thing to use around just about any plastic.
Denatured alcohol isn't the same thing as methylated spirits of course, the case might be compatible with one and not the other.

Why are you cleaning it so often?
Put a plastic bag over it when using to keep splashes off. Use a hose and perhaps a trap using vacuum oil to separate the gauge side from the reactant vapor.


I did try replacing the battery but nothing changed and you can't see inside if there is any damage or not with my gauge. I clean it every time after use in case any oil vapour gets on the sensor. What digital gauge would you recommend then?
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[*] posted on 11-4-2023 at 13:44


Obviously the material of construction for the case doesn't have to be resistant to the chemicals but sometimes the wetted parts do have limited compatibility.

The manual says to inspect the gasket when you're doing the cleaning so there is some serviceability to it. I'd remove whatever nut lets me get to the gasket, and see if I can peek deeper into the instrument. Usually though it's some kind of metal or PTFE diaphragm that's fairly resistant but I can't say for sure. Then again maybe the gasket swelled and now it's blocking the orifice.




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Cathoderay
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[*] posted on 11-4-2023 at 19:18


I found an online manual for the VG200.
It doesn't seem to be all that accurate. It is common for people to think that a digital readout showing many digits must be more accurate than an mechanical needle display but that isn't always true.
The manual seems to be typical of one translated from Chinese, odd phrasing and confusing statements.
The actual accuracy may be 10%.
The sensor is described as "self heating thermistor bridge", rather than a diaphragm. In other words a current is sent through it and the amount of heat loss (due to cooling from the gas) is measured. The pressure is calculated from that.
It is temperature compensated between 0 and 50 C. Accuracy is worse outside that temperature range.
That method of vacuum measurement depends on many design variable like coating thermal resistance, thermal mass, electrical resistance vs temperature, etc. It is true there are no moving parts. My guess is that the sensor is not as chemical resistant as a diaphragm, so it could have been damaged.
It may be that this meter is intended more for HVAC use than a chemistry setup. In HVAC use it would be exposed to Freon type gas and refrigerator and vacuum oils.

[Edited on 4/12/2023 by Cathoderay]
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[*] posted on 12-4-2023 at 02:17
Tricky Vacuum Trouble


Starting a new post so I can show pictures.
So the problem starts off that I think my gauge is giving the wrong reading. I would pull a vacuum and then boil water at the corresponding temperature that my gauge was showing me.

After getting completely the wrong temperature reading for what my gauge was showing I took my gauge to a HVAC shop and they put my gauge along with another gauge on a vacuum it proved my gauge was giving the same reading as other gauges so they told me the gauge is fine.

Ok, so I thought maybe it was just moisture contamination so I ran the test again. I blast through the heat and using a needle valve I had the vacuum sit at 4 torr which should make water boil at 1c. Instead, the temperature on the probe and the distillation head was at 31.6c which would roughly be a vacuum of 32 torr. The trouble isn't the power of the vacuum it is more than strong enough. My issue is the gauge showing 4 torr while water is boiling at a temperature completely different to the gauge reading.

I'm baffled what to do at this point. Is it my fittings on the pump doing something? Is the gauge reading correctly at its position on the fittings but the vacuum is different in the glassware? You would think that in a closed system, the gauge is reading the pressure of the whole connected system. It's driving me crazy.
The photos are attached.

Thanks :(

Vacuum.JPG - 2.2MBTemp.JPG - 2.4MB
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[*] posted on 12-4-2023 at 04:23


What sort of sensor does the gauge use?
Is it calibrated for air or for water vapour?
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[*] posted on 12-4-2023 at 04:45


Quote: Originally posted by unionised  
What sort of sensor does the gauge use?
Is it calibrated for air or for water vapour?


I'm not sure. On CPS's website it says: "Sensor type: Self-heated thermistor bridge with integral temperature compensation from 0 to 50 °C"
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[*] posted on 12-4-2023 at 04:50
I'm pleased that your gauge is ok - but not as much as you must be ;)


As you are expecting near 0oC vapour
- What do you use for coolant?
- is the receiving flask temperature also below expected b.p.?
(otherwise it becomes a boiling flask with atmospheric heating)

Thermal conductivity of low pressure gasses is very low,
(that's how this vacuum gauge works!)
I suspect that your temperature probe may be heated by the glass tube exposed to the atmosphere.
Try measuring the liquid temperature instead.?
(or try warming (holding? ) the exposed glass stem and check for a change in temperature reading)

Where in your setup is the needle valve that controls pressure?

I guess quite a lot of water vapour has passed through your pump,
so, after you sort this problem out, an oil change is a good idea.

P.S.. Nice gauge,
designed for some moisture in air,
to match working pressure range of rotary pumps.
I'd considered diy-ing one, but calibration?

[Edited on 12-4-2023 by Sulaiman]




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[*] posted on 12-4-2023 at 07:04


Quote: Originally posted by Sulaiman  
As you are expecting near 0oC vapour
- What do you use for coolant?
- is the receiving flask temperature also below expected b.p.?
(otherwise it becomes a boiling flask with atmospheric heating)

Thermal conductivity of low pressure gasses is very low,
(that's how this vacuum gauge works!)
I suspect that your temperature probe may be heated by the glass tube exposed to the atmosphere.
Try measuring the liquid temperature instead.?

Where in your setup is the needle valve that controls pressure?

I guess quite a lot of water vapour has passed through your pump,
so, after you sort this problem out, an oil change is a good idea.

Nice gauge, designed for some moisture in air, very nice.

[Edited on 12-4-2023 by Sulaiman]


Water is the coolant, and yes you can't see but I had a thermometer immersed in the liquid that was reading 32c. So it's nowhere near 4 torr.

On the photo of the gauge, the needle valve is connected below the hose barb.

And there's no way of calibrating this gauge :/

It it possible that the vacuum at the gauge is stronger than the glass?

[Edited on 12-4-2023 by Chemist1357]
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[*] posted on 12-4-2023 at 07:17


I think your witnessing a pressure gradient.
With the configuration in the pictures, the gauge will have a "less restricted" path than the boiling flask

Does the pressure remain constant with the niddle valve and the red vacuum valve closed?
It will change a little, do to water evaporation that equalize the pressure/temperature for the system.
But it shouldnt be dramatic




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[*] posted on 12-4-2023 at 07:27
OK... out of my depth already


Already at the leading edge of my knowledge :)
I expect that you will resolve this problem.
(then, if like me... D'oh!.. obvious - in hindsight)




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[*] posted on 12-4-2023 at 07:31


Quote: Originally posted by Rainwater  
I think your witnessing a pressure gradient.
With the configuration in the pictures, the gauge will have a "less restricted" path than the boiling flask

Does the pressure remain constant with the niddle valve and the red vacuum valve closed?
It will change a little, do to water evaporation that equalize the pressure/temperature for the system.
But it shouldnt be dramatic


Yes I did that and there is a dramatic change, so it has to be a big pressure gradient. So the glass isn't sealed properly? I can't think of any other way...
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[*] posted on 12-4-2023 at 12:30


I think what Rainwater is saying is that you have a high flow rate of gas (steam) flowing into the pump. The boiling water is the source and the pump is the destination of the steam. The vac gauge is near the pump. As you move through the system (black hose, condenser, etc.) the pressure rises.
You would have to move the vac gauge to the boiling flask to read the pressure there (of course then you might have to deal with the heat effecting the gauge reading).
I guess it is good news your gauge is OK (because you had it tested).
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[*] posted on 12-4-2023 at 14:15


Cathoderay put it better than i could. My setup is configured the same as yours, and I have the same issue, under the assumption that there are no leaks, in order to get a proper pressure reading, the system has to be allowed to equalize.
Usually only takes. Few seconds.




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[*] posted on 13-4-2023 at 06:59


It is probably a good thing to add that you are not running the distillation normally.
Normally you would be cooling enough to cause most of the vapor to liquefy and end up in the receiver. Since you are not, the vapor ends up in the pump. While traveling through the pump at some point the pressure will increase to atmospheric and by that time almost all the steam will have condensed. Some of the liquid water will mix with the vacuum oil. The pump mechanism is designed to pump gas not liquid and could be damaged because liquid isn't compressible.
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[*] posted on 13-4-2023 at 12:13


So I found a fix, but it is not perfect for every distillation, I been trying to nail down azeotropeic ethonol during vacuum distillation, keep getting different numbers so today, I had a, im a dumbass moment, rigged up a new inlet valve to #5 rubber stopper (24/40).
Before it had a length of 1/16 id copper cappalary tubeing brazed to a valve and crimped closed. It leaked just enough to prevent bumping. Today I jammed in another cappalary tube and brazed some fittings to a vac gauge.
Stable readings, matches bubble points to 1c+/- literature, and shows that from my pump, to my boiling flask a very nice gradient.
So what i was doing before was closing off the bubbler, and vacuum inlet. Then recording the pressure. After trying this, to see if I could apply a correction to the data collected before the modification. These readings where 15~30% higher within just a few seconds of closing the system. My guess is the delay and nonlinear closing of the inlet and outlet valves combined with thermal exchange into the system.
Cold fractional distillation taste so much better




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[*] posted on 15-4-2023 at 08:21


Quote: Originally posted by Rainwater  
So I found a fix, but it is not perfect for every distillation, I been trying to nail down azeotropeic ethonol during vacuum distillation, keep getting different numbers so today, I had a, im a dumbass moment, rigged up a new inlet valve to #5 rubber stopper (24/40).
Before it had a length of 1/16 id copper cappalary tubeing brazed to a valve and crimped closed. It leaked just enough to prevent bumping. Today I jammed in another cappalary tube and brazed some fittings to a vac gauge.
Stable readings, matches bubble points to 1c+/- literature, and shows that from my pump, to my boiling flask a very nice gradient.
So what i was doing before was closing off the bubbler, and vacuum inlet. Then recording the pressure. After trying this, to see if I could apply a correction to the data collected before the modification. These readings where 15~30% higher within just a few seconds of closing the system. My guess is the delay and nonlinear closing of the inlet and outlet valves combined with thermal exchange into the system.
Cold fractional distillation taste so much better


That's interesting... Do you mind if you could take a photo of the fittings setup you have? and I wonder also if using bigger sized fitttings would make a difference for the better as small fittings make the air much more tighter at the vacuum gauge. The readings I get are so far off the actual vacuum i'm getting in my flasks that it makes vacuum distillation of mixtures such as essential oils impossible.

[Edited on 15-4-2023 by Chemist1357]
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[*] posted on 16-4-2023 at 11:29


So I first have a gauge into a tee fitting
One side of the tee goes to a valve when I can let gas in.
Second side connects the cappalary tube with a bit of quick set rubber.
Once my parts come in Ill put together a proper looking apparatus

20230416_151853.jpg - 2.5MB




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[*] posted on 17-4-2023 at 08:04


Chemist1357, another way of explain it is that you are measuring the temperature at one spot (boiling flask) and the vacuum at another point (near the pump).
The conditions are different at the two locations because of the flow of vapor.
You could move the vacuum gauge to the boiling flask and get a better match with the measurements.
It would also help to cool the condenser and receiver flask more.
You are probably getting a lot of pressure difference at the glass-to-hose joint as well as along the hose.
If you are not condensing the water in the receiver flask it is being sent out through the pump.
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[*] posted on 17-4-2023 at 09:54


Quote: Originally posted by Cathoderay  
Chemist1357, another way of explain it is that you are measuring the temperature at one spot (boiling flask) and the vacuum at another point (near the pump).
The conditions are different at the two locations because of the flow of vapor.
You could move the vacuum gauge to the boiling flask and get a better match with the measurements.
It would also help to cool the condenser and receiver flask more.
You are probably getting a lot of pressure difference at the glass-to-hose joint as well as along the hose.
If you are not condensing the water in the receiver flask it is being sent out through the pump.


No I was definitely condensing the water. Also, I don't see a way you could use a digital micron gauge near the boiling flask, it has a 1/4 connection that is mean't to connect to the vacuum. Do you use a digital gauge too?
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[*] posted on 17-4-2023 at 14:46


No I don't have a digital gauge, but I have used dial mechanical gauges.

The problem would just be a matter of plumbing, finding the right adapters and hose. A length of hose would isolate the temperature from the gauge, silicon hose would be best. There would be no flow so there would be no pressure change along the hose. There are many types of 1/4' connectors. A couple of wrenches, some Teflon thread tape and go crazy!
If you are in the U.S., Home Depot has a selection of brass fittings. If the gauge has a HVAC connector, an adapter might be a bit harder to find but certainly available.

1/4" NPT (National Pipe Thread) is fairly standard on general purpose gauges.
I just checked the pictures that you posted. The "T" fitting probably has 1/4" NPT.
Instead of the "T" use a hose barb adapter, then a hose. You would need to have a glass fitting with hose barb on it at the boiling flask end, that may be a bit harder to figure out. A flask with more than one neck would be one way, I think you have that.


[Edited on 4/17/2023 by Cathoderay]
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[*] posted on 17-4-2023 at 15:34


Regardless of what gauge being used, protect it from heat.
Even more if it is a precision instrument.

Gauges should be protected from reagents, just like pumps, when possible. And reguardless of price, are a consumable item. If properly protected, they can last years




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[*] posted on 18-4-2023 at 16:30


There's nothing wrong with both your gauge and your temperature reading.

You have to differentiate between a static system, where only one phase is present at a given temperature and the whole apparatus is under the same pressure everywhere (except there are leaks) and a system where a fluid flows, vaporizes, condenses, and especially temperature gradients and, like already mentioned, pressure gradients occur.

Just imagine the following: What happens if you put a liquid under reduced pressure in a sealed vessel? Some of the liquid will evaporate till equilibrium is reached at the given temperature and of course the pressure must rise.

The fluid always expands from the higher pressure to the lower. Actually if you would not have any pressure gradient there won't be any flow at all!
Now if you continuously introduce heat and remove and condense the vapor a certain equilibrium, dependent on the substance properties, the heat flux, the pressures, cooling temperature and especially also the geometry of the apparatus will establish. Everywhere where the flow has bottlenecks a pressure gradient will be significant. The more heat you introduce the more fluid evaporates and the higher the gradients become.
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