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Author: Subject: Cheap chinese diaphragm pumps?
Download
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[*] posted on 6-12-2020 at 20:08
Cheap chinese diaphragm pumps?


Looking on Aliexpress there seems to be two types that are commonly sold.

A smaller and cheaper one and a slightly more advanced looking one.

Has anyone given these a shot? I was hoping to know how good a vacuum they can get. Some of the listings give a value for "suction" in metres but it's a bit inconsistent. Some say they'll do 0.8MPa (~80m head), will work with gasses and they're self priming, so I'd think they should be able to do an okay vacuum. Might need to grease the valves though.

I'm mostly looking for something that will do 1-10kPa absolute. So not groundbreaking stuff, just something for distillations.
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B(a)P
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[*] posted on 7-12-2020 at 00:57


I have recently installed something similar to the 'slightly more advanced' looking one in a camper trailer to feed the tap over the kitchen sink. The outlet of the tap is approximately 200 mm above the top of the supply tank. When the tank is full the pump will only just self prime. When the tank is half empty (~400 mm of head) the pump will not self prime.
There is no way my pump would pull or push 80 m of head.
The pump is also quite loud.
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[*] posted on 7-12-2020 at 03:53


No pump could draw a head of 80 feet (or draw a vacuum of minus seven bar absolute- which is essentially the real problem)
If a pump is quoting that figure then it's either wrong (deliberately or accidentally) or it's a pressure, not a vacuum.

On the other hand I used to use a diaphragm pump for vacuum filtration in the lab and it was quite capable of boiling acetonitrile or methanol. A good one is certainly capable of drawing a vacuum much better than 400 mm H2O.

Essentially, B(a)P has been sold a lemon.
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[*] posted on 7-12-2020 at 12:56


Quote: Originally posted by Download  
Looking on Aliexpress there seems to be two types that are commonly sold.

A smaller and cheaper one and a slightly more advanced looking one.

Has anyone given these a shot? I was hoping to know how good a vacuum they can get. Some of the listings give a value for "suction" in metres but it's a bit inconsistent. Some say they'll do 0.8MPa (~80m head), will work with gasses and they're self priming, so I'd think they should be able to do an okay vacuum. Might need to grease the valves though.

I'm mostly looking for something that will do 1-10kPa absolute. So not groundbreaking stuff, just something for distillations.


https://www.youtube.com/watch?v=tYLlkTDstmo
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Dr.Bob
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[*] posted on 7-12-2020 at 16:33


Those pumps are designed for water pumping, so they typically will not pull much of a vacuum. Ones designed for vacuum will do much better at pulling a vacuum than those designed for water. And trhose are small pumps, the 12V one by definition will be weak, as the amperage needed for even a small vacuum pump won;t work well at 12V.

I would look for a used vacuum pump on Ebay; laboport, KNF, Welch, and VWR all make diaphram pumps for rotovaps, they are on Ebay often, som e for reasonable prices.
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[*] posted on 8-12-2020 at 06:48


To add to what Dr. Bob said, I bought a used KNF pump identical to this for $175 shipped a few years ago. https://www.ebay.com/itm/KNF-Neuberger-UN726-3-FTP-Diaphragm...

As long as the diaphragms aren't torn, they're pretty much bulletproof with PTFE/FFKM pumping parts. Highly recommend this model if you want to invest money in a good pump as it's rated at 10 torr when configured in serial pumping mode (I've measured 12-15 torr.)
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[*] posted on 16-12-2020 at 14:40


If you want a cheap setup for vacuum then get a water pump, the ones looking like your "slightly more advanced" pump and use it to pump water from an ice bucket into a aspirator so the water circulates from and into the bucket with ice water.
That will give a decent vacuum from the aspirator side port that can be used for distillations.
And the aspirator isnt sensitive for corrosive chemicals and gases but remember they will end up in the circulating water.
Good diaphragm pumps are somewhat expensive and the cheap small ones as the one in your first example can only be used for filtrations.
I have not tried use the small ones for any distillations but i think they are way to weak.
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RustyShackleford
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[*] posted on 17-12-2020 at 05:13


Aspirator builds are hit or miss. The one i built (according to nurdrages video) can barely pull to 0.8atm, absolute shit.i even got the same chinese "brand" and specced pump. pump cost like 25 bucks and the aspirator like 10, at that point its better to just throw in an extra 20 and get a cheapo mechanical one.
I know of 2 other people who had the same experience building their own aspirator setup.

[Edited on 17-12-2020 by RustyShackleford]
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[*] posted on 18-12-2020 at 15:21


Quote: Originally posted by RustyShackleford  
Aspirator builds are hit or miss. The one i built (according to nurdrages video) can barely pull to 0.8atm, absolute shit.i even got the same chinese "brand" and specced pump. pump cost like 25 bucks and the aspirator like 10, at that point its better to just throw in an extra 20 and get a cheapo mechanical one.
I know of 2 other people who had the same experience building their own aspirator setup.

[Edited on 17-12-2020 by RustyShackleford]


I wonder why people have varied success with those types of builds. I briefly owned a lab grade recirculating aspirator pump (Buchi B169) and it pulled very good vacuum, definitely close to its 20 mbar spec. They're built in a similar fashion to the DIY types -- maybe the type of pump has something to do with it? I think the Buchi had a centrifugal pump feeding the venturi aspirator.
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[*] posted on 18-12-2020 at 15:32


Quote: Originally posted by monolithic  
maybe the type of pump has something to do with it? I think the Buchi had a centrifugal pump feeding the venturi aspirator.

Could be. most homebrew setups including mine use membrane pumps.
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[*] posted on 19-12-2020 at 01:37


https://www.ebay.com/itm/Black-12V-DC-Brushless-Small-Water-...

From the US, uses a common power supply and relatively easy to work with for distillation and reflux
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