Hydrometer Accuracy

The surface of liquids do not change the density, the proof is that you will have the same reading putting your hydrometer directly the the primary fermenter, or in a thin cylindre.

It's all a question of weight by volume. One litre of pure water will weight one kg. The more sugar you put in the water, the more heavy it will get, but it will still have the volume of one litre. On the other hand, the hydrometer is balanced to float in pure water at the measure 1.000. It's weight by volume has been calibrated for that. It will sink in lighter liquids like oil, and will "FLOAT" on very dense liquids.

If the atmospheric pressure changes, the weight by volume of the liquide and the one of the hydrometer will remain unchanged. So the readings will be the same. If you change the temperature, the liquid may expand a little, reducing it's weight by volume (same weight, more volume). Solids like the hydrometer will not expand as much as liquids, so the readings may be different at higher or lower temperatures.

You know folks,

No offence intended, but it never ceases to amaze me the ignorance of humanity!

I am aware of the arguement about temperature, because temperature expands a liquid - but so does low atmospherice pressure,as does lower gravitational force.

If you lower the atmoshperic temperature then a liquid will boil at a lower temperature, hence its viscosity is increased at lower barometric pressures. As does a liquid subject to lowered gravitational force. It's simple physics! If one choses to ignore the environment then let it be, but the scientist must consider all factors, mustn't they?

Lets say that you are breathalized by a police officer and the legal limit is 0.05% alcohol volume. Now lets consider if the atmospheric pressure is abnormally low and the ambient temperature is relatively high. Shouldn't it stand to reason that your REAL blood alcohol reading be lower than actual at the time of reading? Should we just simply accept the fact that the reading of 0.05 is acurate and we are in the wrong if the reading is 0.051? NO! Not at all - there are other differential factors working at hand, even though we may not be not aware of them.

Then the same reasoning should apply to our Hydrometer readings because there may be other influences working at hand that we are not sure of.

The gravitational forces of the moon DO NOT work equivalent to the Hydrometer reading because gravitational force is relative to weight and distance. This is simple grade 8 science. So the weight of your Hydrometer is far less significant than the weight of the liquid being measured for Specific Gravity, isn't it?

This means that the liquid that one is measuring is being PULLED much more than the Hydrometer that we are measuring with, isn't it? The question remains, what is the significant difference of measurement? The same question applies to atmospheric pressure, doesn't it? The real question is, as to what degree or percentage is the actual difference?

This question is not requested for speculation, but for actual scientific reasoning, evidence, and calculation. Does anyone have a scientific answer?

Cheers, Steve!

wrote: > The surface of liquids do not change the density, the proof is that you > will have the same reading putting your hydrometer directly the the > primary fermenter, or in a thin cylinder.

I'm wondering if dropping the hydrometer into your fermenter is such a good idea. If you accidentally break it or you are using a small container and it hits the bottom and breaks won't you now have a container of fermenting wine contaminated by mercury? I think it's best to stick with the glass tube for testing.

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Posted via MooTalk

No, pressure will not significantly change the volume of the fluid so it will not effect hydrometer readings. In virtually all practical physics, even involving multiple atmospheres of pressure change, fluid is considered incompressible.

Ray

Although it's true that an increase in temperature expands the volume of a liquid, a decrease in atmospheric pressure does not. Neither does a decrease in gravitational force.

That's not quite correct. You must have meant "lower the atmospheric _pressure_" - right?

hence its viscosity is increased at lower

Huh? None of that sounds like good physics to me.

I'm sorry, but I don't see what all that means within the context of this thread. Actually, I can't make much out of it at all.

One can only imagine...

Perhaps, but so what? A small cylinder of sea water will measure at the same specific gravity as an entire _ocean_ of the same water. The difference in weights - or more correctly _masses_ - between the hydrometer and the ocean does not factor into the measurement.

It's clear to me that you have some notions of physics that I could only characterize as quaint.

Tom S

There's no mercury in my hydrometer! Or even in my thermometer come to think of it --but I wouldn't like to break the thermometer in my wine.

Hi Steve, Yes I have a scientific answer, but to which topic? I came into this late and this one is all of the place.

The effect of barometric pressure shifts in the range of +/- 30 mmHg from 760 mmHg are miniscule, as are air density and gravity corrections. If you read the S.G. on the hydrometer to only 3 places there is no need to correct for it as far as I am concerned.

The definitive text in the States on this subject is Polarimetry, Saccarimetry and the Sugars by Bates; it may be available in a good university library 'down under', but it's pretty old. It is NBS Circular C440 published in 1948, (I have a copy). You may be able to get access to some ISO or DIN hydrometer calibration documents, that is really what you need. (The book I mentioned has equations you can consider, they are pretty involved.)

I can post a hyrdometer calibration document I wrote up derived from this, but Clyde already added it to the FAQ he maintains.

All of this depends on the reference material, by the way. Weight and Mass are not the same in air, the density of the object in question must be considered. The standard reference material used to be a well-defined Brass, it is now a well-defined Stainless Steel. (Both are around 8/g per cc, air is around 0.0012 g/cc.) Only very precise measurement equipment can pick up the diference.

Again, at the precision most winemakers are satisfied with, temperature of the liquid is the only variable to correct. (The hydrometer and liquid are assumed to be at equilibrium; they should be at the same relative temperature after a few minutes if the difference is extreme.)

Best regards, Joe

Great post Joe. Thank you. lum