Possible realtime SG sensor idea

OK, this is not really my idea but saw it in another post but it seemed like it was passed over and noone really replied to the idea.
What someone suggested is using 2 pressure sensors with one takeoff mounted 10 mm above the other. Then using difference in pressure between the 2 and some simple maths you can find out the gravity ( delta_P = rho * 9.81 * h (with h = 0.01 m))
Well instead of 2 pressure sensors you could use 1 deferential pressure sensor, any reason this would not work?

This is interesting....I wish that I had any friggin idea! Maybe i can pick the cousin's brain since he's an electrical engineeeeeeeer....

mattd2 said:

1 deferential pressure sensor, any reason this would not work?

Click to expand...

This is an awesome idea! Ive very often wondered how to measure SG without a friggin sample. But anywho, a "differential sensor" is a system of two pressure sensors, and then the differential is processed in some way to find the difference in pressure. So, yes, using a differential senor would work, just as you described in the beginning of the post. It would cost about 6 for one, and maybe half dozen for the other.

This idea is genus though. You would need some pretty darn accurate sensors if you did it digitally. Is there a way to fashion a way to measure inches of water in a tube and correlate that to the gravity of the brew? I suppose that would only work for a single length of tube that keeps the water and beer separate.... Wait.... if the tube came from the beer vessel, below the level of beer, and formed a loop that goes above the level, back down and back above, when water is poured into the tube, the water would be elevated x amount of inches above the beer level in the beer vessel. The difference in elevation would get you the pressure and weight difference in the fluid, and thus the specific gravity.


*Engineered* <----this is my engineering stamp.


I'm falling asleep as we speak, but i think the differential would be 23 points per inch..... good night folks!

i wish i could think a way to do things like this...very interesting

I think your best way to do this would be to pump wort through a standpipe overflowing the top back to your kettle. You would use 1 pressure sensor to measure the head pressure in the standpipe and calibrate it for a certain SG. If you had say 12 inches calibrated at 12" of H2O then as the SG increases the effective head the transmitter sees increases and with a little math you can work back to SG. This method is used in industry as a simple inexpensive way of measuring density.

Another option may be to re-calibrate a digital pH probe. They work by measuring the conductance of the liquid (basically) with some trial to gather data for a base line, you may be able to find a trend in the conductance of gravities. For that matter, you could possible just use an ammeter and a spreadsheet of values. (I found my next study!)

The largest obstacle will be the lack of resolution in the pressure sensors and measuring circuits, you would need a lot of vertical separation to get a large enough signal to work with. The common method used in the commercial SG instruments is the vibrating fork or tube method, could be copied by using piezo transducers, oscillator and amplitude measuring circuits. The higher the SG the lower the output of the oscillating component through increased drag on moving component. A relatively simple bridge circuit that compares drive to measured signal would make for a relatively stable measurement if drive level shifts due to voltage or temperature fluctuations.