Hydrometer/Refractometer accuracy/calibration

[jasonaustin]

I've been spending some time trying to understand the different readings I get from my 2 hydrometers and refractometer. I must admit I'm rather confused by the results:-

Code: Select all

Solution                    Brix                             Theoretical SG    Hydrometer 1    Hydrometer 2
                            Theoretical  Measured  As SG
Distilled water             0            0         1000      1000              1000            1006
180g water + 20g glucose    10           8.9       1036      1040              1036            1042
160g water + 40g glucose    20           17.4      1072      1084              1076            1082

On the face of it hydrometer 1 appears to agree with my refractometer (although it is slightly out at the higher end). And hydrometer 2 appears to be consistently 6 points higher than hydrometer 1 (that one's for the bin ).
What I'm puzzled by is the difference between the theoretical and actual measurements. 20g sugar in 180g water should give a 10% sugar solution and therefore 10 Plato (and SG 1040). But it doesn't. Was glucose the correct 'sugar' to use for this test? (It's actually labelled 'Glucose Powder - Brewing Sugar'). I'm guessing that my 'glucose' is not 100% glucose. Be interesting to learn what else it adds to my brew on the odd occasion I use it

At the end of the day it doesn't matter too much, as the purpose of this experiment was to convince myself that my refractometer readings that I tend to use 90% of the time these days were accurate (when compared to my hydrometer). But I'd be interested to know why my sugar solutions were so below what I expected.

[Vossy1]

Hi JA,

Was the glucose sealed prior to the test, ie, free of water

If so, very interesting results

[jasonaustin]

Well, it was sealed in a bag. Is this what you mean?
Would you expect it to absorb atmospheric water to the extent shown here?

BTW Can anybody see the results table? It's disappeared from my view - just get an empty 'Code' section, although all the data is there if I edit the post.

[Seveneer]

Yes the table is visible.

[Andy]

Still see it.

[Jim]

Two things occur to me;

One, sugar is a natural product, and therefore it's unlikely to be absolutely pure glucose. You would expect some variation in the purity of any natural product, in fact.

Two, are you using calibrated scales and measuring cyclinders (I'm sure you are, but just to eliminate potential errors ).

[Andy]

And....is it a 10% sugar solution ? Does 20g of sugar by weight in 180g of water give a 10% solution ? Or should you add 20ml of a 100% sugar solution to a 180ml volume of water to achieve that ?

[jasonaustin]

I'd welcome correction, but I'm pretty sure that my 10% solution is correct. 10g sugar plus 90g water produces 100g of solution, of which 10% is sugar.
Jim, you're right, all of this is only as accurate as the most inaccurate equipment. I'm only using scales and using the fact that 1ml of water weighs 1g. (Although I'm not at sea level and haven't got my kitchen at 4 degrees, but I think it's accurate enough for this). The scales are pretty accurate - I certainly wouldn't expect the large differences between theoretical and measured to be accounted for by my equipment
I'm going to repeat the experiment with household sugar (sucrose?)

[Kev.]

100% sugar solution

Surely if it's 100% sugar it can't be in solution?

[Andy]

From t'web - instructions to prepare a %age sugar solution (5% in this example).

To prepare a 5.00% sugar solution, weigh out 5.00g of table sugar.
In a beaker, combine 5.00 g table sugar with approximately 75mL deionized H2O and stir until completely dissolved.
Pour the solution into a 100 mL graduated cylinder and add deionized H2O up to a total volume of 100.0mL.

[Kev.]

From t'web - instructions to prepare a %age sugar solution (5% in this example).

To prepare a 5.00% sugar solution, weigh out 5.00g of table sugar.
In a beaker, combine 5.00 g table sugar with approximately 75mL deionized H2O and stir until completely dissolved.
Pour the solution into a 100 mL graduated cylinder and add deionized H2O up to a total volume of 100.0mL.

Yep that's spot on, it's percentage by weight w/v. If you simply add 5g to 100ml you’d end up with more than 100ml by volume and it messes up the percentage of your solution.

[jasonaustin]

So, am I right? 10g sugar + 90g (or ml) water = a 10% solution?

[Andy]

So, am I right? 10g sugar + 90g (or ml) water = a 10% solution?

what was the final volume ? 100ml ?

you want a 100ml final volume of solution which has 10g of sugar in it.

[jasonaustin]

I didn't check it - I was too busy concentrating on the weight.
I've just created a 10% solution using the method you found (ie, adding 10g to some water then making up to 100ml). This came out slightly worse (as in further from what I expected) at 8.6 Brix.
A few solutions later...
12g of glucose dissolved and then made up to 100ml yields a 10% solution (as measured by my refractometer).
Presumably my glucose powder contains ~17% by weight of something other than glucose