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 Home Brew Forums > Can it even be done? Measuring Specific Gravity using Arduino or Raspberry Pi
02-01-2013, 02:41 AM   #11
FuzzeWuzze
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I had plans to do this...

Honestly due to krausen and CO2 bubbles, i dont see how any float type sensor would work.

This leaves your two options really that i was looking into

1) Mass measurements, easy to brute force but you obviously have to use a big enough scale thats accurate enough...
2) Pressure differential...i was looking at using a pair of pressure sensors with tubes of known distance seperating them(height wise) going into your wort. With that you can use the equation
pressure difference = density x gravitational acceleration x height difference
And calculate for density, from there you can calculate SG.

I have the differential pressure sensors and everything, just havent had time to put it all together. I am still struggling with a cheap method of "wrapping" my tubes and getting them in a fixed location where they cant ever move. I was thinking of some sort of food safe plastic tubing with the smaller tubes squished in, or maybe even heat shrink wrapping them together? And set it so that there is a big delta, something simple like 6 inches or a foot. Something like this would be only really usable for buckets, which is all i care about...it could be done with a carboy though.

02-01-2013, 07:09 AM   #12
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I have also put some thought into this... not an easy solution for sure.

I was thinking about creating a container in place of your airlock that has a blow-off valve. The blow-off valve would trip at a certain pressure and then equalize once opened. You know the volume of your container and the pressure inside the container when the blow-off valve trips. After counting the valve trips, you would have enough information at anytime to know how much CO2 has been produced.

This is certainly a non-trivial task, but seems like it would work.

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02-01-2013, 03:17 PM   #13
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Quote:
 Originally Posted by FuzzeWuzze 2) Pressure differential...i was looking at using a pair of pressure sensors with tubes of known distance seperating them(height wise) going into your wort. With that you can use the equation pressure difference = density x gravitational acceleration x height difference And calculate for density, from there you can calculate SG. I have the differential pressure sensors and everything, just havent had time to put it all together. I am still struggling with a cheap method of "wrapping" my tubes and getting them in a fixed location where they cant ever move. I was thinking of some sort of food safe plastic tubing with the smaller tubes squished in, or maybe even heat shrink wrapping them together? And set it so that there is a big delta, something simple like 6 inches or a foot. Something like this would be only really usable for buckets, which is all i care about...it could be done with a carboy though.
The last company I worked for made a custom fermentation controller for large 40 gallon wine fermenting vessels. There are 150 of them installed at UC Davis. They used the pressure differential method to measure specific gravity. The two tubes were spaced about a foot apart. For this to work correctly they have to use a small air pump to push air out so no liquid goes up into the tubes. I worked with the guy who developed it and he said it was quite a challenge to get it working as the readings get thrown off when the air bubbles break away from the end of the tube.
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02-01-2013, 03:43 PM   #14
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02-01-2013, 05:46 PM   #15
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Quote:
 Originally Posted by crane The last company I worked for made a custom fermentation controller for large 40 gallon wine fermenting vessels. There are 150 of them installed at UC Davis. They used the pressure differential method to measure specific gravity. The two tubes were spaced about a foot apart. For this to work correctly they have to use a small air pump to push air out so no liquid goes up into the tubes. I worked with the guy who developed it and he said it was quite a challenge to get it working as the readings get thrown off when the air bubbles break away from the end of the tube.
I heard this as well but never really thought about it happening in a 5 gallon batch, who cares if the beer goes up the tube slightly? The pressure wouldnt be high enough to push it into the sensor would it?

02-01-2013, 05:57 PM   #16
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I've been playing with this on and off for a few years now.

The most effective solution I've come across so far is to use a mass flow meter to track the total quantity of CO2 blown off. This is an imperfect (though quite good) correlate to gravity and a perfect correlate to ABV. I use a mass flow controller that I got off ebay for around \$65...works a treat and makes a great spunding valve for pressurized fermentations (if you're into that).

I could never get the floats and suspended stuff to work properly. Fermentation is just too gunky.

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02-01-2013, 06:18 PM   #17
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Wouldn't it be possible to simply measure the ethanol concentration in the vapor in the fermenter to arrive at the current gravity? Here's an example:

http://www.vernier.com/products/sensors/eth-bta/

They even describe experiments to follow the formation of ethanol by yeast using simple sugar. If you know the OG and the fermenter temperature, it is possible to calculate the current gravity.

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02-01-2013, 06:41 PM   #18
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Quote:
 Originally Posted by craigmw Wouldn't it be possible to simply measure the ethanol concentration in the vapor in the fermenter to arrive at the current gravity? Here's an example:http://www.vernier.com/products/sensors/eth-bta/ They even describe experiments to follow the formation of ethanol by yeast using simple sugar. If you know the OG and the fermenter temperature, it is possible to calculate the current gravity.
It's an interesting idea, but this sensor only goes up to 3%. I browsed around for one that went up higher but didn't see anything immediately. I wonder if headspace atmospheric vapor measurements are a reliable correlate to liquid ABV.
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02-02-2013, 05:58 PM   #19
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Measuring ethanol in the vapor in the headspace of the fermenter should give a reasonably accurate estimate of the alcohol concentration in the beer due to its partial pressure. Temperature would certainly impact this, but this could be controlled by using a separate sensor in a sealed chamber containing a known ethanol concentratio (e.g. 5%) near the measuring sensor. There is a cheap ethanol sensor called the MQ303A that is made for portable breathalyzers that might work under appropriate conditions. Still too low of a range though. Another option would be to measure CO2 output coming out of the airlock with a CO2 sensor. This is simply a way to measure fermentation activity though, not final gravity.

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02-02-2013, 06:14 PM   #20
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Quote:
 Originally Posted by craigmw Measuring ethanol in the vapor in the headspace of the fermenter should give a reasonably accurate estimate of the alcohol concentration in the beer due to its partial pressure. Temperature would certainly impact this, but this could be controlled by using a separate sensor in a sealed chamber containing a known ethanol concentratio (e.g. 5%) near the measuring sensor. There is a cheap ethanol sensor called the MQ303A that is made for portable breathalyzers that might work under appropriate conditions. Still too low of a range though. Another option would be to measure CO2 output coming out of the airlock with a CO2 sensor. This is simply a way to measure fermentation activity though, not final gravity.
Like I mentioned in my earlier post, though, I get quite reliable correlations to gravity by tracking total CO2 released.
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