What I want to do
I want to see if @davidmack's suggestion that a graduated cylinder with a soap bubble traveling along it can be used to calibrate the flow rate of a pump.
My results
My goal is to set a DIY formaldeyhide testing kit to 0.3L/minute of airflow. I chose a 250ml graduated cylinder, with the intent of watching the bubble traverse 200ml of its height in 40 seconds. I drilled a 3/8" hole in the base of the graduated cylinder and screwed in a threaded 1/8" brass barbed fitting, first to tap some threads in the cylinder, and then a second time with some teflon tape around the threads to seal the threads. I didn't use aquarium sealant as @davidmack suggested because the graduated cylinder is polypropylene and glues won't stick to it readily. to test the airtightness I filled the cylinder with water and waited a few minutes to see if there were any leaks.
I then filled a tray with an unmeasured mixture of a lot of dish soap and water. This bubble solution was terrible and wouldn't hold a bubble. I then googled "bubble solution" and made a mixture of 1/4 cup soap, 1/2 cup water, and a teaspoon of sugar, and of course forgot to save where I got that recipe.
I found that it was important to evenly coat the inside of the cylinder with bubble solution, otherwise the bubble in the cylinder didn't move straight down, and would get hung up on the walls and become lop-sided. I mitigated this situation by dipping some newsprint in the bubble solution and putting it down the tube at a few different points. I then let the bubble solution settle to the bottom of the cylinder and poured it out. This process needed to be repeated every half hour or so.
Running a test
I opened a formaldehyde tube and put it into the system and drew air into the cylinder, through the tube, and into the pump, the configuration that @nshapiro laid out.
(these images credit:@warren)
To prepare the cylinder I dip the top of the cylinder in bubble solution, and then draw a little air into it until the bubble is into the cylindrical part of the graduated cylinder, turn the air pump off, and wait for the bubble to settle into a flat shape. This is an important step to get a flat bubble.
I always give the bubble some travel distance above the mark I'm measuring from so the pump has time to warm up. Then I start the pump and wait for the bubble to hit the mark I'm measuring from, start a stopwatch, and stop it when the bubble gets to the second mark. With the 250ml graduated cylinder, these two points are at 250ml and 50ml. I didn't put in a dropout, or a sealed chamber with a break in the line to prevent bubbles from entering the test line, as @davidmack suggested. I just turned the pump off before it got to the inlet and then popped the bubble with a stick.
Sources of error
My different measurements of the same flow setup have a 300ms (milliseconds) range. For instance, 39.98 seconds and 39.65 seconds. were the high and low of 5 different trials. Given that human response time is somewhere between 100-500ms, this 300ms range is inevitable. In a 40-second time trial, it introduces roughly 1% variability.
Other sources of variability are that the graduated cylinder is only marked to 2ml accuracy, and is rated for its current volume at 20 degrees Celsius. I conducted these tests at 22 degrees. Also, the extra fitting for the cylinder may cause some changes in airflow.
Conclusions
The bubble method seems to work pretty well! Next week I'll have a high-precision NIST-traceable pump to compare it to, and will therefore have a better understanding of its accuracy. While not the fastest means of measuring, I was able to tweak a small flow control valve, and over about 25 tests and an hour of testing, I set the pump to the 300ml/minute flow rate I sought, to within a few percentage points of accuracy.
](https://i.publiclab.org/system/images/photos/000/010/219/original/115_0154.JPG)
[@mathew](/profile/mathew) That heart shaped object is the abrasive chip. When [@nshapiro](/profile/nshapiro) and I did my first measurement Nick said the scoring chip should be in the box but I could not find it. Days later I was inspecting the box and out came the scoring device. It was apparently stuck in the box because of the little zip lock bag. So completely empty the box.
My friend found this needle valve for $10: http://www.mscdirect.com/product/details/03005089?src=pla&cid=PLA-Google-PLA+-+Test&CS_003=7867724&CS_010=03005089
[@DavidMack](/profile/DavidMack) I think the driving forces are cheap and accurate which are conflicting requirements. That makes the users proficiency important. Vertical orientation, reading at the meniscus/ball center, accurate timing, no leaks all will be important.
I think there is no need for a filter since the measurement tube will filter any particles.
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21 Comments
Looks like you had problems breaking the tip of the tube. What process did you use?
I am really curious to find out how much flow variation there is between formaldehyde tubes. Breaking the tube in different places on the taper will vary the restriction. I have only observed 2 measurements so far so my sample size is small. I believe that the flow rate changed over the first minute or so and then became stable. I could propose that the chemical reactions in the tube caused this variation or some settling of the packing material is occurring.
A pipette might provide a more accurate bubble measuring setup. It has an opening in both ends, is available in multiple volumes, and is available cheaply as a disposable item.
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Looks like a nice setup! For bubbles, the cheap ready made option would be any liquid bubble--the kid's toy to blow bubbles. @danbeavers I think a pipette is a great idea for this low flow measurement. This is a very old, proven method for measuring flow but like all primitive methods it takes some tinkering to perfect. I also prefer to use an actual stop watch--not a phone app, which can be slower.
RE: breaking the tip at the tube consistently shouldn't effect the flow if you have a strong enough pump. If you have a pump that normally runs at 5 lpm without a valve and then you add a valve to restrict it to 0.3 lpm I think you'll find the flow is fairly consistent even with moderate loading. The high differential pressure across the valve creates a critical flow situation. Not all pumps designs can be restricted though--some will over heat. What is important though, is to warm up the pump before you start testing or measuring the flow. Often the pumps need a few minutes to settle into a constant temperature. Some pumps never reach a constant flow and will ever so slightly cycle in flow like a sine wave.
A sampling protocol might look something like:
then sample volume = (start flow rate + end flow rate) / 2 x (end time - start time)
for flow field measurements I would use a glass rotameter that was calibrated in the lab with the graduated cylinder technique. the glass rotameters aren't cheap but they're less expensive than the automated methods and they're quite durable (and never require charging!). There are many makers of these and the better ones are sold by Matheson (order without the valve):
http://www.mathesongas.com/pdfs/products/Model-FM-1050-High-Accuracy-Flowmeters.pdf
They do make tube tip breakers though. I used to use the Drager version and while the $50 cost is shocking, if you're doing a lot of testing it makes it much easier and safer. It's basically a shallow hole with a ceramic edge that you stick the glass end in and twist the tube to score the glass. Then you stick the end of the tube in a deeper, crooked hole and snap off the end.
http://www.a1-cbiss.com/gastec-tube-tip-breaker.html
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