I use an npt gauge like this with an npt 3 way splitter and two barbs. The problem is that I've tried a few different of each piece and they're all machined quite poorly,
it doesn't seem to matter what you pay for it. This results in a weak seal and a slow leak. I've tried wrapping the threads in teflon tape and it
doesn't seem to help. I was thinking something like this would be ideal, but for that price I could just buy a whole new vacuum pump. I would like to reliably do vacuum distillations, but unless I
can fix this leak I can't do it, and I'm not running a vacuum w/o a gauge. Maroboduus - 22-11-2016 at 15:14
I didn't see any specification of accuracy on the second gauge you posted a link to, but the first one is only accurate to .9 inches of mercury at
below 20 inches vacuum. (+/- 3% of total range)
That's accurate to 23mm of mercury. That means if it reads 40mm it's somewhere between 17 and 63mm.
Not really all that helpful. Mechanical gauges tend to be very expensive or very inaccurate. And many get rapidly worse with wear.
As to installing it without leakage, most people put them on the trap between the pump and the glassware. A gauge with a single hose connection works
fine there. the hose fitting just goes in a hole in the rubber stopper, and should seal well in the right sized hole.
To seal the threads, plummer's tape should work. If not maybe you need to buy some better fittings. If you own taps and dies in that size you can
always go over the threads yourself. For the hose fittings you could just epoxy them on the threads. But if you epoxy them to the gauge you'll need
new ones when the gauge gets inaccurate or breaks.
Had another look at the second one you linked to: It's another Bourdon tube gauge. I suspect it's also just accurate to 3% (of the total range)at low
pressures, or they'd be bragging about if it was any better. This kind of gauge can be accurate to 1% or even less, but if that is the case for any
given gauge I think it's safe to assume the seller will point this out.
I would advise against buying ANY mechanical vacuum gauge if you need accurate readings. Either bite the bullet and buy one of the new electronic
ones, or get a mercury manometer.
BTW: did you notice the seller for the 2nd gauge:
1; doesn't know the difference between vacuum and pressure.
2: thinks 76cm=7600mm.
These are not good signs.
[Edited on 22-11-2016 by Maroboduus]
Oh, and I use a lousy mechanical gauge myself, but I test the vacuum by boiling water as well. I think it'd be a LOT better to use something that gave
accuracy to a few Torr, and have even been thinking of buying some vacuum pump oil and seeing if I can make a workable manometer with that. It'd have
to be 2-3 feet long to have a decent range, But I think it might be reasonably accurate down to a few mm if done correctly.
[Edited on 23-11-2016 by Maroboduus]Magpie - 22-11-2016 at 15:59
I recommend that you wrap the threads with teflon tape and teflon pipe dope then screw on a female NPT hose barb. Use thick wall rubber vacuum hose
to attach to the barb.
[Edited on 23-11-2016 by Magpie]Sulaiman - 22-11-2016 at 19:15
I have one of the cheap bourdon type vacuum gauges, not even oil filled,
with a ghetto connector - silicone greased silicone tubing over the thread and string over that
it seems to not leak at -75 kPa gauge pressure, 28 kPa absolute, over two hours which is the longest i've used it with the pump not running.
the gauge reading is (atmospheric pressure) - (port pressure) +/- errors
so when you get to the low pressure end of the scale interpreting absolute pressure is futile.
Having said that, so far, my gauge consistently goes to -1 atmosphere with my rotary pump.
Dr.Bob - 23-11-2016 at 07:25
Depends a lot on if you are looking at low vacuum, like for a rotovap, filtration or medium BP distillation, where a simple gauge works OK, or for a
high vacuum application, where you either need a mercury type gauge, or an electronic one to work well. But I can say from much experience, that
none of them is near as accurate or precise as they claim in real use. All of them work fine unless exposed to chemicals, solvents, changes in temp,
or any other real life conditions, and then they all start failing quickly. For high vac, the electronic ones are nice, you can find used ones
cheap, but the sensors for them don't last a long time, unless kept in clean and protected form. One example is below.
It's a manifold/fuel pump gauge bought at an automotive shop, shouldn't be hard to find something similar. aga - 23-11-2016 at 14:12
I just took the guage off my vac pump, sealed the hole, then used fairly-rigid gas pipe to move it via a T-piece to somewhere i could see it (pump
sits on the floor).
Pointless really, but it looks nice and the needle moves when i throw the switch to turn the pump on.
Jubilee clips were used to cramp the pipe onto the brass fittings.
[Edited on 23-11-2016 by aga]XeonTheMGPony - 24-11-2016 at 04:46
I use a V46 from supco, it is a vacuum thermister style sensor accurate to 1millitorr (Tested) and uses SAE 45 degree male flare connections
I want to get a mercury manomiter style vacuum guage for chemical side of reactions to monitor internal vacuumsCouchHatter - 27-6-2019 at 18:19
And here I was thinking I had discovered some hack to insanely low pressure. Negative NEGATIVE vacuum! https://www.youtube.com/watch?v=XjkaBQW2SDohappyfooddance - 27-6-2019 at 20:16
Lol! I've had cheap vacuum guages that do that, they're at least good for letting you know that the system is pulling a decent vacuum...
So for example, if it shows that your aspirator is pulling 5 torr (aspirator lol), you know it's really leaky and is only at like 150! Ubya - 28-6-2019 at 03:29
at the moment i'm not using a vacuum gauge, simply because as it's already been said, good ones are expensive, and cheap ones are nearly useless, but
i'm working on something.
i found this cheap absolute pressure sensor, the MD-PS002, for really cheap, less that 1 euro for a pack of 2 (prices can vary), i'll hook it to my
arduino and translate the voltage (it needs an amplifier, the HX711 from a load cell project i did should work) to a pressure value. even if this gets
damaged it is cheap to changeS.C. Wack - 28-6-2019 at 13:42
A decent micron gauge for HVAC techs is about $150, if you can get by without bluetooth. There will be no leaks because there will be compression;
they have 1/4" SAE flare threads. Brass male flare to male NPT adapters are cheap. A cold trap is probably necessary for this type of gauge, and they
foul easily from backstreamed oil too, but are easy to clean by soaking the end in IPA. For stronger vacuum than aspirator, AFAIK there is no ideal or
best value vac gauge.
