henrybot

I'm thinking about building an automated homebrew system for my final project (due at the end of spring term, 2009). I'm not worried about anything except a way to take a specific gravity reading a few times throughout the process. Any ideas?

So far I've considered buying a pre-built digital hydrometer and gutting out the transducer (too expensive). One design I read about uses a weight in the inside of a PVC tube that is displaced relative to the density of the liquid that into which it's submerged. The description said that it uses a hall effect array to measure the displacement.

I was thinking about building my own, and scrapping the hall effect sensors.
I've attached a very crude mspaint sketch showing what I mean. I'm thinking of having a hunk of material with a 1 g/cc density that suspends into the liquid, and two plates on the top of that material with an LED between them. I could have phototransistors built onto the wall of the pipe around the LED. Whatever level the LED rises to is then sent to the processor which logs it and displays the number.

I've never tried anything like this before, and am hoping that someone here might have some great ideas.

Thanks,

Mike

Attached Images

hydrometer.JPG (14.7 KB, 19 views)

3v0

Place a triangle on the float and have the LED shine through a slot in a mask. As the float position changes you will get a linear change in light. You many want to ensure that the led produces a uniform intensity light across the slot. Maybe a diffused LED.

Attached Images

indicator.jpg (4.7 KB, 3 views)

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jpanhalt

For a give fluid of varying composition, like urine, refractive index and specific gravity are closely correlated. Clinical labs, for example, usually measure the refractive index of urine, but report the result as the specific gravity.

Automated refractive index instruments are readily available, handheld, and probably cheaper because of their widespread use. You might even get an older unit from a clinical lab for free.

Have you considered refractive index instead of specific gravity? Will it work for beer?

As for specific gravity per se, I would consider a capacitive sensor to detect the float level instead of light.

John

henrybot

I didn't until I read that! I think you're onto something with that. I bounced some ideas off of one of my teachers, and this is what we came up with.

(NOTE: This idea isn't original, it's stolen from a piece of test equipment we had lying around)

A laser is aimed at a mirror that is attached to two solenoids. The solenoids are charged 180 degrees apart, 100 cycles per second. This makes a very wide "line" that is calibrated for 1.000 SG. The calibration is done by taking distilled water and filling the tank. (Distilled water should be darn close to 1.000, right??) Wherever the edge of the "line" is, a photocell is fastened. As the specific gravity increases, the refraction draws the line away from the photocell, decreasing the voltage into the ADC.

Pretty cool if you ask me. Any forseeable problems with it? I just need to find a slick way to make the tiny solenoids.

-Mike

jpanhalt

You clearly have the right idea about how to measure RI. My suggestion would be to avoid moving solenoids. You might consider a CCD type of linear detector to detect the refractive change.

RI detectors are very common in liquid chromatography (HPLC). Search on "HPLC refractive index." Attached is a design from Waters, a well established company in HPLC.

My advice would be to scavenge something from an HPLC or clinical instrument. Are you near the Oregon Health Sciences Center? I know some people there who also like beer and wine and might know of some unused instruments.

John

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henrybot

I'm about 3 hours south of OHSU. Not a bad drive at all for something that would make life so simple. I think you're right about the moving solenoids. We have two of that type of system that were donated when they quit working... The solenoid was the problem on both instruments.

Are university hospitals allowed to just give away old equipment? It seems like since it's a public funded entity there would be a lot of red tape. Worth a shot though. What department do you think I should contact? Thanks a lot for the advice. My classmates think I'm crazy for starting this early, and the teachers think I'm the only sane one. All the second year students put it off until the last minute, now they're scrambling to cut holes in coffee cans for whatever project they decided on that they could do quickly.

Mike

jpanhalt

I am sending you a PM with some contact information.

As for the matter of giving away old equipment, the places I have been were not directly state funded, and we often gave old equipment away to responsible individuals who would use it, and not try to re-sale it on ebay. The other option was the trash bin. John

Pommie

If I understand this concept correctly, could a mirror mounted on a servo be used. A laser going through the sample/reference and then reflected by the mirror would only hit a receiver at a particular servo position, the position would change for differing refractions. If the change in refraction is too small to measure then a convex mirror would serve to amplify the angle.

Mike.