Good valve for really small flow rates?

[jbeale]

I'm just a hobbyist. I am playing around with a long-time-constant microbarometer apparatus, for which I want a valve with a very small flow rate. The working fluid is just normal dry air at atmospheric pressure. It might be considered more a calibrated leak than a normal valve. I know can buy fixed "calibrated leaks" at enormous prices. I have what I think is Amazon's cheapest needle valve, which is almost good enough, but the setpoint I want is somewhere between "off" and "harder off" and I'd like finer adjustability. Better looking industrial valves and "micro" needle valves range into some hundreds of dollars. Is there anything I should know before wasting a lot of money here?

 

[Baluncore]

I know can buy fixed "calibrated leaks" at enormous prices. I have what I think is Amazon's cheapest needle valve, which is almost good enough, but the setpoint I want is somewhere between "off" and "harder off" and I'd like finer adjustability.

Maybe you could make/draw progressively finer glass capillary tubes. The internal diameter will be most important in limiting flow, but capillary length will also have some effect.

Flow rate through the capillary will be proportional to the speed of sound, (average molecular velocity), hence temperature. You might vary the flow rate by changing the temperature of the capillary tube.

 

[tech99]

Could you place two needle valves in tandem?

 

[jbeale]

Thank you baluncore for that note! Indeed I already have some ~0.2 mm ID glass capillary tubes, which are not small enough, but perhaps I could draw them out. Adjusting flow via temperature is an interesting idea that I had not considered.
Thanks tech99, I'm not sure of the details but I thought two constrictions of the same size in series would only divide the total flow rate roughly in half. That would help, although my goal might need a stack of several more such valves.

 

[Ivan Nikiforov]

Good afternoon! If you need a calibrated leak, you can try using a permeable membrane or an air filter as an option. In the simplest case, you can make an air filter yourself if you press the required amount of filter material, such as cotton wool, into a suitable tube. This way you can get the required amount of leakage.

 

[Dullard]

This may not apply to your situation, but:

The use of a needle valve implies (I infer, anyway) flow from 1 volume to another in order to adjust the pressure in 1 of the volumes. The 'volume of interest' might be increased to make the cheap needle valve's range more appropriate. I'm assuming that a barometer cares about only Pa and/or Pa/sec.

 

[jbeale]

Thanks Ivan, I wasn't sure if I could get some permeable material to restrict flow in a well-controlled way, but it is worth a try. Your point Dullard is well taken, right now I simply have a large-ish glass jar (formerly, a large peanut-butter jar) with a hose connection through the lid, for my fixed air volume. It must be well insulated, otherwise my barometer becomes a sensitive infrared detector as the jar warms up just from standing next to it, given pV=nRT. With all the insulation it is already a bit bulky. But you could be quite right, the simplest way forward may be simply to use my existing valve with a larger backing volume, to reach longer time constants.

 

[DaveE]

Gore-Tex, etc. ?

 

[Baluncore]

Another way to regulate flow would be with a low-cost peristaltic pump. By using a fine silicone tube, with a high drive reduction ratio, you could control the rate of gas flow. Peristaltic pumps are often used for chemical dosing of aquariums. They cost about $20 online. There are also stepper motor driven peristaltic pumps that cost more, but can be programmed.

A peristaltic pump is a positive displacement pump, so will have a volumetric flow rate. That is different to the flow rate through a capillary, or a restriction valve, where temperature and viscosity dominate.

 

[jbeale]

Thanks for the all ideas! It looks to me like the people making similar sensors have settled on a glass capillary as most practical. A paper by Marcillo et.al. (2012) finds a 25 micron diameter, 1.1 cm glass capillary tube with only 1 cubic cm volume backing chamber, when a differential pressure sensor compares the chamber pressure to ambient, gives a single-pole highpass filter response with rolloff at 0.01 Hz. The rolloff frequency scales linearly with chamber volume, and to the 4th power of capillary diameter, so that tells me everything I need. A search for "heavy wall capillary tubing" shows that suitable parts are available in borosilicate glass at reasonable prices. https://journals.ametsoc.org/view/journals/atot/29/9/jtech-d-11-00101_1.xml

 

[Baluncore]

A search for "heavy wall capillary tubing" shows that suitable parts are available in borosilicate glass at reasonable prices.

There is 25 um capillary tube available on eBay.
Cost of surplus tube is about $50 for 10".
New is significantly more expensive.
Search for "Waters Capillary Tubing Assembly W/Frit 25u"