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Low pressure air flow of vacuum pump

Low pressure air flow of vacuum pump

My science club is planning on releasing a high-altitude balloon with a vacuum pump on board in order to filter air and collect microbiological samples.
It will fly to an altitude of around 30 km.
The sampling system will be similar to the one described here (see chapter 2.1):
We will probably use Boxer 7KQ vacuum pump which technical data can be found here:
My questions are:
The air pressure in at 30 km is really low (around 10 mbar). How will it affect the performace of this pump?
Which point at the flow rate vs. differential pressure should I look at?
How can I calculate the volume of filtered air throughout the experiment?

RE: Low pressure air flow of vacuum pump

Interesting problem.
I would think that the flow rate will decline with altitude and that there will be an altitude where that flow will be zero. After all it is atmospheric pressure that forces air into the pump chamber and as that pressure diminishes less air will flow in.

I would suggest that you talk to the vacuum pump manufacturer about its performance at altitude.

"Any water can be made potable if you filter it through enough money"

RE: Low pressure air flow of vacuum pump

what are you trying to do with this air flow from the pump? Just flow some air over a filter or pressurise something?

I would be very wary of any vacuum pump which is not certified to draw a vacuum of < 10mbara in your application. Anything which uses a reciprocating action and needs valves to open may well not work as you don't have sufficient pressure to flow air into the pump past non return valves etc.

the pump you reference I don't think is suitable.

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

RE: Low pressure air flow of vacuum pump

Not least because 60 liters/min. flow rate seems a bit high for air sampling through millipore filters.

Another thing, from a science point of view: what was the control sample in the study you cited? If there is no control sample, is it really science?

RE: Low pressure air flow of vacuum pump

That vacuum pump would not be suitable. The valves will not operate at such a low differential pressure. The other factor to consider is if the drive motor can work in a vacuum where there is no cooling air, and lubricants evaporate.

" After all it is atmospheric pressure that forces air into the pump chamber and as that pressure diminishes less air will flow in." That is true for all vacuum pumps. The sampling pump we are discussing is not a high vacuum pump, but pumps designed for high vacuum generally have a flat pump curve where the actual liters of gas pumped per minute is constant with variable inlet pressure. This is because the pump displacement per revolution is constant. In units of standard liters per minute the flow rate decreases linearly with pressure.

RE: Low pressure air flow of vacuum pump

Thanks everyone for your replies!

We tried consulting it with the manufacturer but they said they do not have any data about their pumps' performance in stratospheric conditions so they simply can not guarantee that it will work.

The reason we chose Boxer 7KQ is that an older version of this pump (Boxer 7502) was used in another, similar experiment which documentation can be found here:

We just want to flow some air through a filter. Valves will be controlled electronically by servos, they do not need any pressure to open.

Do you know any air vacuum pumps that can create high vacuum and are not too heavy for flying it on-board of high-altitude baloon? I've searched for one for a while and it seems that finding anything under 5 kilograms is impossible.

RE: Low pressure air flow of vacuum pump

Interesting reading but the paper contains little technical detail about the actual performance of the pump. Whilst it would appear that there must have been some flow through the filters ie: the bead that was stuck on the filter proves that there was some airflow its not actually apparent how much.

Have you tried contacting the people involved in the original work to see if they have any data?

The valves that are being referred to are internal valves in the air pump that allow air in on the suction strike but close on the compression stroke with others opening on the compression stroke but close on the intake. These are often not much more than a simple hinged flap and require a certain airflow to ensure they open or close.

"Any water can be made potable if you filter it through enough money"

RE: Low pressure air flow of vacuum pump

Oh, I totally forgot about the internal valves in the pump. I thought LittleInch and btrueblood were reffering to the valves connecting the tubes that force air to go through particular filters.

Contacting the people involved sounds like a great idea and I will definitely give it a try as soon as possible.

RE: Low pressure air flow of vacuum pump

The issue of minimum cracking pressure for the check valves wasn't brought up by me, but it is a good one. Good mechanical vacuum pumps don't have check valves, they use mechanically actuated valves.

I could envision a pump made from a stepper motor, drive screw, syringe, and two solenoid valves, with a suitable controller to drive it.

Whatever pump you choose, test it. Put it in a vacuum bell jar with a pipette (with a "bubble" of dyed vacuum oil in the near end), syringe or even a balloon attached to the outlet. Pump the chamber down to 10 mbar and then run the pump for a fixed period of time and observe the displacement of the syringe/balloon.

RE: Low pressure air flow of vacuum pump

Oh, and I like your second cited source much better. Note the amount of time and care they took not just devising a decontamination procedure, but actually testing that procedure, and including a control sample on the craft. And achieved a rather important result - that in spite of such effort, they had a clear indication that the craft itself was a source of contamination of the experiment.

RE: Low pressure air flow of vacuum pump

You need to use a pump design that does not use internal valves to operate. These would include vane and scroll-type pumps. Considering the the low pressure that you are operating at a small blower would work but I think you want something with a known flow rate. A blower would only create a differential pressure and the flow would depend on the resistance of your sampling system.

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