How long to run a vacuum pump for

[Mr-Pooman]

In my quest to achieve a 1Pa to 5 Pa vacuum (see earlier posts) a question of how long to run the pump for. The pump reaches an optimum pressure quickly and then the reduction becomes almost insignificant but there appears to be very small reduction over 15 to 30 minutes is there any point in running it longer or significantly longer. Background information I've recently upgraded to second-hand Edwards 1.5 pump extracting from a 0.032 m3 vacuum chamber. Not sure if the pump is close to its 'as new' performance (but it is better than the previous pump) and questions about the accuracy of the sensor. Limited budget so unable calibrate. Your comments please.

 

[Mr-Pooman]

A subsequent question the sensor shows the pressure bouncing about by say +/- 5 Pa and occasionally +/-10 Pa. Is this the sensor or would this be a result of pressure waves?

 

[mk3223]

It looks you are dealing with several variables, i.e. unsure pump performance, gauge/sensor accuracy issue, etc. One possible way to answer the question could be to solve these variables one at a time.
Regarding the pressure bouncing, is it because a leaking of the system somewhere, so the vacuum couldn't be maintained while the vacuum pump keeps on and off?

 

[LittleInch]

I would look at the resolution / span of the pressure transmitter.

If it's set up to measure 100,000 Pa (1 bara) down to 5Pa, that's a big span. Also many transmitters are actually diff pressure transmitters with a sealed off very low vacuum on the other side. But this may not be s low as 1 Pa

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Mr-Pooman]

Thanks, mk3223 and LittleInch for your responses

LittleInch -The transmitter assembly manufacturer claimed 0 to 2.5 bar, but now checking the details on the actual sensor, a Honeywell piezoresistive silicon pressure sensor, they only claim a range of 60mbar to 2.5 bar which explains much. Does a piezoresistive sensor fall under diff transmitter you refer to?

mk3223 -Yes, I am aware that there are two or three variables. I took the sensors claimed range to be accurate and made changes accordingly to the chamber and pump but as I get closer to my target, I now understand I am at the limits of the sensor's reliability. I'm now trying to work out how to proceed, as I say, on a limited budget. For all I know I may be close to the 1 Pa and hence my question about how long to run the pump i.e. how long it takes a vacuum pump, new or used, to reach its ultimate pressure. There again I could be a long way off. As for leaks yes there is some leakage but the rate of pressure loss when the pump is off is a lot less than the fluctuations when it's on.

 

[LittleInch]

If you're measuring absolute pressure then yes it seems there's an inbuilt near zero sealed chamber in the sensor.

Also the accuracy is probably no better than 0.1% of full span. Full span is 250000 Pa. So 0.1% is 250 Pa. So fluctuation of 5 or 10 Pa seems reasonable.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[EdStainless]

Is your pump a rotary vane vacuum pump? If so then it will likely be able to pull 0.1Pa or better.
It looks like all of the "pressure" that you are reading is actually your gage.
This is a tricky pressure to measure, too high for some real vacuum gages and too low for most full atmosphere gage.
A capacitance (capacitive) gage may be your best bet.
They measure the flex of a SS diaphragm and are very robust.
I have seen many of them for sale in the $500-$1,000 range.
You should be able to pump for hours and get a nice stable vacuum.
Then when you close the pump valve your chamber should stay at the same pressure for many hours.
If it doesn't then you need to start fixing leaks.
Just to make my life easier; 1Pa = 0.01mbar = 7.5mTorr (microns of Hg)


= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

 

[LittleInch]

Also are those pressures you're quoting bara or bar?

You need to buy transmitters etc for very low vacuum as a defined item like Ed says.
They are not cheap.

The edwards pump can go lo lower as Ed says.

https://www.akribis.co.uk/edwards-1-5-e2m-vacuum-pump

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Mr-Pooman]

Thanks EdStainless and Littleinch

As I said the pump is second-hand and I wasn't sure how they wear but your answer suggests this shouldn't be a problem which is what I wanted to hear. I have run it for up to 12 hours and it is very quiet and smooth and I have recorded a small but progressive drop in pressure where it plateaued* off which again with your comments suggests it close to my target pressure.

Littleinch- its bara

* Floodplained may be analogy. Ha Ha!