I would like to build or buy a device that can be submerged in rain water and read back the specific gravity of that water… Ideally it would return both the Temperature and the S.G., but if it had to have 2 probes, I guess I could live with it… Does anybody know of, or suggest any such device… or perhaps suggest how such a device could be built? 1-Wire??
Thanks in advance!!!
-Robert
So. let’s see… Specific gravity measures the density of the water. If you could accurately measure a given volume repeatedly, then weighing it would work. I originally thought of a constantly overflowing collector on a scale, but I don’t know if the volume would be steady enough.
Or, you could use a pressure transducer and measure the gauge (not absolute) pressure at the bottom of a column of water of known height.
Or, you could figure out how to measure the floating height of a hydrometer, maybe ultrasonically.
Since it’s rain water, you might be able to correlate conductivity to SG.
OTOH, rainwater is pretty darn pure. Measuring its SG is getting down to the picking the fly poop out of the pepper. Measuring the SG will likely also require knowing its temp very accurately.
So why the interest in this particular data?
EDIT: Do you want to measure it constantly, or every so often, and record it manually, or automatically?
If the floating hydrometer contained a rod of ferrous material for a weight, instead of lead shot, and it moved up and down inside of a coil of wire, the inductance of the coil would change depending on the position of the hydrometer.
Steve
I too came up with a lot of cute ideas, but then re-read the post and realized it’s rain water which as you point out is/should be very pure.
Stuntman: How much variation in SG are you expecting to see?
The water won’t be entirely rainwater as I believe pure water has a S.G. of 1.0… what I need to determine is the amount of additives I have in that water… it will be sitting in a collection container (a 6.5 gallon glass container)… I want to be able to drop a probe in it and chart temp and S.G. changes as the water is altered by minute additives… looking for a range of about 1.0 to 1.2 at the most…
1.2 8O Whoa, seawater is only about 1.025 IIRC from my aquarium days.
Yup… I’ll be adding sugars and other rather heavy elements into that captured water… so I was hoping to be able to monitor the temp and gravity changes as I went… figured some electrical wiz here might have a suggestion as to how to do that… 1-wire does so much, but I see nothing that does Gravity…
Sounds like beer to me :lol:
You could look into optical refraction. That’s a good way to monitor salinity, but in that case you are only concerned with one additive and can generate a table of values and make the measurement by comparison. Still not easy to automate though 
Use an ordinary glass hydrometer, fix some kind of linear binary scale to it that can be read by an array of LED/photosensors. I know what I mean by this, but I’ve probably not described it very well. Too early in the morning to be thinking totally straight!
You could buy one of these: http://www.mobrey.com/products/density/7828.php (a bit of advertising for the company I work for :D), mind you, I think that it will come at a price!!
“competitively priced” as long as you’re building a
Ian,
I buy many products from your company! 
I work for a huge Worldwide Engineering Company
http://www.niroinc.com
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“competitively priced” as long as you’re building a
Just a few times more expensive than the competitively priced VP2 weather station then 
Now that’d be nice, a hydrometer for my marine aquarium, which I can monitor through WD… cool !! 
Exactly!! I can’t believe nobody has created a submersible device that can be monitored for temp and gravity… and charted…
And yes, now that it has been mentioned… It COULD be used for monitoring homebrewed beer!
Sounds good for my homemade wine, also.
Cheers
MikeyM
This might give you some ideas… http://www.waterandwastewater.com/www_services/ask_tom_archive/density_measurement_theory_and_practice.htm
One final thought before I go to bed…
Use a normal hydrometer with a piece of dielectric attached to the top of it. Mount the dielectric between two plates long enough to cover the full extent of the travel of the dielectric for all values of interest on the hydrometer. As the SG of the fluid changes, the dielectric will move up and down between the capacitor plates giving you capacitance readings proportional to the SG of the fluid. You’d need to calibrate your hydrometer because the extra weight of the dielectric would probably make quite a difference. I hope this makes sense!
The measurement is relative to the surface of the liquid, so either you would have to measure that separately or make the whole thing float.