Reliable Hydrometer

[Dale Owen]

I just bought a new hydrometer, because I realized my last one was giving me false readings (I tested it in water and realized it was off).

The one I just bought to replace it is also off. In tap water at 60 degrees it reads 1.006. I live in a city where, although our water is from wells, it is treated. Not sure if that even means anything, but I'm sure water could be a blanket term for any number of H2O concoctions that come out of faucets.

Is my tap water "heavy"? Or, do I need to try harder to find a quality made hydrometer?

I guess if my hydrometer is "reliably" off, I could just get used to knocking off .006 from all my readings.

Wondering about using a refractometer. I see people point out you have to do calculations to get correct post-fermentation gravity readings with them, but, are they more reliable?

 

[RPh_Guy]

It would take an extreme amount of minerals to budge a s.g. reading by even 0.001. That's not a problem.

You can do a 2-point or 3-point calibration on your hydrometer and just adjust the reading. You don't know that it's off by 0.006 across the whole range.

A refractometer can be used, and it's reasonably accurate with the correction applied (for both OG and FG). They can be calibrated. However the refractometer needs to be at a consistent temperature for accuracy. It's a little more difficult to read vs a hydrometer, in my opinion.

 

[Dale Owen]

You don't know that it's off by 0.006 across the whole range.

Do you mean for different specific gravities the readings could be more skewed? Am I understanding correctly? If so, that might account for why the extract kit I brewed came in at 1.070 when it was supposed to be 1.050. And that was taken when the wort was at 80 F.

Really would like to get a reliable hang of this, as I'd like to start all grain brewing. I think having accurate gravity readings would be important, to judge efficiency - right?

 

[doug293cz]

Do you mean for different specific gravities the readings could be more skewed? Am I understanding correctly? If so, that might account for why the extract kit I brewed came in at 1.070 when it was supposed to be 1.050. And that was taken when the wort was at 80 F.

Really would like to get a reliable hang of this, as I'd like to start all grain brewing. I think having accurate gravity readings would be important, to judge efficiency - right?

Yes, the offset may not be constant at all SG's. You might have an offset of 0.006 at 0.000, and an offset of 0.010 at 1.050. In these cases you can make reasonable corrections by assuming the offset varies linearly with SG. You could also make up calibration solutions at various SG's, and curve fit a more accurate offset correction function. If you are interested in how to make up calibration solutions at different SG's, I can provide instructions.

You also need to be aware that SG measurements are only accurate at the calibration temperature of the hydrometer, and reading are in error on the low side at higher temps. Most hydrometers are calibrated for measuring at 60°F. There are on-line calculators available that will correct your SG measurement when measuring at higher temps, but for best accuracy you should measure as close to the calibration temp as possible. The calibration temp should be printed on the paper scale inside the hydrometer.

Yes, to get accurate efficiency calculations for all grain brewing you need accurate SG readings, as well as accurate volume measurements, and accurate grain weights.


Brew on

 

[cfrazier77]

I got a set from this place, American made. https://bradyinstruments.com/
I tested them against ro water and they were dead on. I am very happy with them.

 

[RM-MN]

Do you mean for different specific gravities the readings could be more skewed? Am I understanding correctly? If so, that might account for why the extract kit I brewed came in at 1.070 when it was supposed to be 1.050. And that was taken when the wort was at 80 F.

Really would like to get a reliable hang of this, as I'd like to start all grain brewing. I think having accurate gravity readings would be important, to judge efficiency - right?

When I take a final gravity reading I'm not overly concerned about the exact reading, I'm concerned about it being near the expected FG and stable so I don't have to deal with bottle bombs. For my readings that .006 error would not be of much concern and I might just make the correction.

For all grain brewing the gravity readings are useful but extreme accuracy isn't necessary unless you are a number freak that needs to have the OG to be exactly on. However, I find that using a hydrometer for all grain brewing to be a real pain. You need a sample big enough to get the hydrometer to float, then need to cool it to the hydrometer's proper temperature. Instead use a refractometer with temperature compensation. It only takes a few drops and they can be put right onto the refractometer prism without cooling. That gets you a reading in seconds without the large sample. My refractometer is calibrated in Brix and specific gravity. I find that for the gravity of the wort going into the fermenter it matches my hydrometer.

 

[Dale Owen]

Yes, the offset may not be constant at all SG's. You might have an offset of 0.006 at 0.000, and an offset of 0.010 at 1.050. In these cases you can make reasonable corrections by assuming the offset varies linearly with SG. You could also make up calibration solutions at various SG's, and curve fit a more accurate offset correction function. If you are interested in how to make up calibration solutions at different SG's, I can provide instructions.

Yes I am interested in how to make calibration solutions. Just curious though: the reason for the offset not being constant, is that because an hydrometer functions by the displacement of volume, so the heavier the liquid, the measurement changes geometrically? Not sure if I'm making sense, but that's my best use of my college math and science I can scrounge up.

 

[doug293cz]

Yes I am interested in how to make calibration solutions. Just curious though: the reason for the offset not being constant, is that because an hydrometer functions by the displacement of volume, so the heavier the liquid, the measurement changes geometrically? Not sure if I'm making sense, but that's my best use of my college math and science I can scrounge up.

I don't know if hydrometers ever actually have variable offset vs. true SG. I was just speaking from a general viewpoint of measuring instrument error characteristics. A two or three point calibration will determine the situation for any given hydrometer.

Calibration solutions are made with water and sucrose (ordinary table sugar, cane or beet), and made by weighing out precise amounts of both sugar and water. So, you need a scale with good precision, that can weigh up to about 5 lb or 2.2 kg. You can weigh the components at any temperature, but the resulting solution should be cooled to the calibration temp of the hydrometer when checking the calibration.

You determine how much sugar and water to use for a given calibration solution as follows:

1. Determine desired volume of solution in liters (Vol)
2. Determine desired SG of solution (SG)
3. Calculate weight of solution in kg (Sol_Wt): Sol_Wt = Vol * SG * 0.9982
4. Calculate °Plato of solution (°P): °P = (-1*616.868)+(1111.14*SG)-(630.272*SG^2)+(135.9975*SG^3)
5. Calculate required weight of sugar (Sugar_Wt): Sugar_Wt = °P * Sol_Wt / 100°
6. Calculate required weight of water (Water_Wt): Water_Wt = Sol_Wt - Sugar_Wt
7. Thoroughly mix the water and sugar

The 0.9982 factor in (3) is the density of water at 68°F (20°C), and is used because the equation in (4) is valid at 20°C.

You can easily put together a spreadsheet to do the above calculations. Here's the output of one I did for 1.0 liter solutions:

​

Brew on

 

[RPh_Guy]

I don't know if hydrometers ever actually have variable offset vs. true SG.

FWIW I've seen people report variable offsets.