Public Lab Research note


Mini-Buck vs. the Bubbles

by mathew | June 11, 2015 00:14 | 263 views | 5 comments | #11969 | 263 views | 5 comments | #11969 11 Jun 00:14

mathew was awarded the Empiricism Barnstar by liz for their work in this research note.


What I want to do

Compare the $10 stopwatch & bubbles airflow monitoring technique @davidmack suggested to a $970 NIST-traceable pump calibrator called the mini-buck recommended to @NickShapiro.

My attempt and results

It was a bit sensational to call this research note "Mini-Buck vs. the Bubbles" since the Mini-Buck is an automated bubble measuring machine. A breakdown of how it works and is designed in the bottom section, "How the mini-buck works."

For this comparison I checked the flow of 3 used formaldehyde tubes and our lending library air pump prototype. I used the Mini-buck and the same graduated cylinder and bubbles method detailed here.

Tube#1

graduated cylinder flow rate: 329L/m

Times to travel 200ml: 36.67, 36.54, 36.36.

Mini-Buck flow rate: 335L/m

(three identical tests)

Tube #2

graduated cylinder flow rate: 309L/m

Times to travel 200ml: 38.90, 38.74, 38.78.

Mini-Buck flow rate: 313L/m

two tests at 313, one at 312

Tube #3

graduated cylinder flow rate: 302L/m

Times to travel 200ml: 39.78, 39.64, 39.57.

Mini-Buck flow rate: 305

(three identical tests)

Questions and next steps

My polypropylene graduated cylinder has a flow reading 1-2% lower than the mini-Buck. This could be the tolerances of the cylinder, or temperature dependent, as it is calibrated for 20 degrees and I did these tests at 27 (Celsius). I'm not going to apply a correction for now, I think more work is needed to prove that. Overall though, I'm impressed with the performance of the graduated cylinder and stopwatch.

how the Mini-Buck M-30 works

Mini-buck is a design in the public domain now, since its patent (4860590) expired several years ago.

IMG_6194.JPG

How it works: The measurement chamber is made out of a transparent acrylic tube*. On the bottom is a reservoir for bubble solution, and a spring-powered bubble wand with a wider diameter than the acrylic tube. When the button is depressed a bubble is made on the wand. as the wand moves back up to the acrylic tube, the bubble attaches itself evenly to the tube's bottom.

buttongif.gif

*I'm guessing the tube is acrylic since there are warnings about cleaning with acetone. It may be polycarbonate, as suggested in the patent.

IMG_6195.JPG

The bubbles always travel upwards, and to get the tube soaped up, one repeatedly presses the button to send bubbles up the acrylic tube until they start making it to the top (see above).

At the top of the chamber is a catchment for the bubbles to prevent them from entering the air line. the main acrylic tube is capped, and below the cap are four holes offset evenly around the circle. These holes exit into a wider tube surrounding the capped top of the main tube. From there a 1/4" air hose nipple draws air.

IMG_6218.JPG

Depending on whether one wants to pull or push air through the chamber, the air line is attached to the top or bottom, and the second air line connector left open to the atmosphere:

IMG_6216.JPG

The mini buck automates bubble detection with two matched sets of infrared emitters and sensors. When a bubble passes up the chamber an IR diode detects an interruption in the infrared light coming off an LED. A view of the two detectors:

IMG_6198.JPG

With the chamber taken out, the sensors can be seen oriented opposite each other. Note the IR LED's light is choked down by a pinhole in the case. That may be why I couldn't seem to get a reading on the LEDs with a Public Lab Spectrometer. I tried reflecting them off paper and other materials but to no avail. They could also be 960nm or another wavelength that the spectrometer isn't very sensitive in.

IMG_6215.JPG

Inside the MiniBuck is a PIC16F914, an 8-bit microcontroller running at 8mhz, with an internal real time clock, and a LMC6464 amplifier I'm assuming is used to either amplify the signal from the IR photodiodes or power the LEDS.

IMG_20150609_130047.jpg

Calibrating the Mini-Buck: The manual recommends calibration to a NIST-traceable stopwatch and a bubble traveling through a NIST-traceable 1000ml Buret at a flow rate of 1L/m.

Why I'm interested

We have to figure out how to measure the flow of both particle monitors and formaldehyde tubes. One of the central questions of the Open Air projects is how much air is going through devices. It is heartening to think that the difference in precision between a $10 and a $1000 flow meter is 1-2%. That said, the mini-buck is a lot less messy and much quicker.


5 Comments

Wow, very cool. I'm thinking of what kinds of vessels could be easily adapted to a DIY version of the sort of spring-loaded bubble making frame in the Mini Buck. Maybe like two nested cylinders , with a spring between... hang on, i'll doodle it.

OK, what about a pair of electric wires at the top which are connected by the bubble membrane before it's popped? And a rubber band based harness for dipping?

IMG_20150611_100905.jpg

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@warren How do you start the timer? I don't see a sensor for the start, only the end. Another set of electrodes appear necessary. Will that set of electrodes pop the bubble?

A bubble will have to be formed before air flow starts or you will blow bubbles thru the bubble juice which perhaps will be bad.. Unless the cylinder is kept very still (vertically) the bubble will not represent an accurate volume measurement.

This configuration only works for blow and not suck. An attachment for the tube would need to be fixed to the top where the hose would have to kept still during the measurement.

I like the bubble management of the Buck. Only a small rod to seal and the flow does not have to be interrupted to start a bubble. Your gasket may have a problem with leaks

Can you add salt to a bubble solution to make it more conductive and still have good bubble forming characteristics? Is it conductive enough (what ever that means) without adding salt?

The cylinder volume can be calibrated with water and a sensitive balance or accurate graduated cylinder.

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Yeah Dan, I later realized that a draw valve at the top would be better than a push valve at the bottom, and I think it'd mean the gasket would be unnecessary.

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@mathew Great work and the title is appropriate! Sometimes when I’d compare instruments head to head it would be referred to a “shoot out”—an instrument dual of sorts.

A few observations:

  1. I’d like to see multiple Buck tests per test flow rate—at least three to show how precise the Buck performs but take more since it's so easy to use. Any bubble meter, whether it’s a DIY cylinder or a commercial timed system, is prone to errant readings caused by oddly shaped bubbles.

  2. The last tube 2 cylinder time of 39.78 seconds looks like an error and could be transposed incorrectly from your notes since it’s the exact same time as the first time for tube 3.

  3. This analysis quickly becomes a question of “how close is close enough?” I wouldn’t assume the Buck is perfect and all the flaws are that of the cylinder method. Looking at Buck’s specifications for this model the flow ranges from 0.1 to 30 lpm with +/- 0.5% of reading accuracy. That is a huge range and extremely small error. I’m skeptical the Buck performs this well at the extreme low end of 300 ml per minute. Manufacturers often exaggerate their performance specifications.

I think the cylinder method could marginally be improved with a glass apparatus but you'd probably need to buy this type of glassware rather than make your own. I'm not sure how much it would improve the timed cylinder method but glass is easier to read, typically has finer markings, and the volumes are often more reliable if made by a reputable brand.

Looking at your data--and omitting the 39.78 time for tube 2—the DIY cylinder seems to be well within 1% precision—it’s very consistent.

Here's how I graphed the results, with the one time I question circled in red.

flowmeasure.jpg

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@DavidMack Good catch! My notebook measures test 2 at 38.78, right next to 39.78 for the next test! I edited the note. thanks!

You're right about the Buck not being perfect-- sometimes it gets bubbles in the cylinder and gives a good bit of trouble-- a "side bubble" will form on the film traveling up the cylinder and drag the film sideways. All my MiniBuck tests for this research note were in triplicate-- the only variaion I had was in test #2 where it reported 312 twice and 313 once.

I agree a glass apparatus would be better-- I worry about compressing or messing up the plastic. The problem is the price! A 1000ml Burette is used to calibrate the Mini-Buck, and its more than $200 with shipping! I did some tests with a $45, 50ml pyrex pipette-- the biggest I could buy in-town-- but the bubbles just travel too fast in that apparatus.

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