SG calculation and hydrometer reading differing

As the title, the short of it is the hydrometer gives an SG for 1.090 for my must, the Mead Making calculator gives at least 1.102 (at least, as I've not topped up to my final volume yet, so it should be higher). I guess the nub of it is, I'm not sure which I should be trusting.

Not including small stuff like nutrients & enzymes, ingredients so far are:

8l Merlot grape juice (15.2% sugar)
2lb crystalised honey (76.4% sugar)
2-300ml water from rinsing out and the yeast and nurtient additions

Final vol is 2 IMP GAL / 9L / 2.4 US GAL, but as I've not topped up yet the current volume will be a little shy of this.

Honey was warmed a little for pouring, the juice heated a little as it had been chilled. I didn't measure the must temp, but it wasn't far off 25C when pitching. The temp seems only to make a point or two difference anyhow. As I say the must measured 1.090 - I checked a couple of times - but I was expecting something in the region of 1.100 -1.115.

Any ideas? Which to trust?

I probably wouldn't bother too much, as most of my batches to date - on which there has been a fairly good correspondance between the measured SG and calculated SG - have been aimed between 12 and 15%. I've been fairly relaxed about the final ABV. Here I'm aiming for a fairly heavy red pyment, and further honey additions, to take it too maybe 16-18% and an error like this will end up in it finishing too sweet.

Just thought I still have some of the juice maybe I should test it's got the % sugar it sez on the packet.

fatbloke said:

The mead calculator presumes 80% sugar (I understand), so there's bound to be some difference.

12 points ? Don't know if it would be as much as that.

You could of course, check the hydrometer for errors i.e. distilled water at whatever temperature is listed as tbe calibration temp for the hydrometer (either on the packaging, a separate piece of paper or even printed on the scale). It should read 1.000 or give you the inaccuracy to allow for.

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It's a fair bit. You're right I'll check the hydrometer in water at room temperature, along with the pure juice - it's one thing to rule out easily.

bernardsmith said:

Do you have more of the Merlot? I would take a reading of the Merlot and use a Pearson's Square to calculate approximately the SG of the honey and Merlot combo. You know the volume of the honey and water and you know the volume of the Merlot so using a PQ you can calculate what the SG should be if the calculator is more or less reasonable (my own experience is to assume that 1 lb of honey in 1 US gallon (a very small British gallon) gives me a SG of about 1.036)

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Yep I have a still have a litre of the Merlot I was gonna top up with. I'll take a reading tonight before my OH guzzles it with her breakfast and try the calculation.

psychopomp23 said:

Yeah making sure your hydrometer gives you a reading of 1.000 for water. These things can be pretty fragile sometimes but at least they're not expensive!

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I realise! It was at least working for a good while, tho it could have got cracked. I noticed it's easy to let it drop to the bottle of the test tube too fast after washing.

Cheers all for the help.

Meant to post earlier but I've had visitors all weekend. Okay I think the nub of this is me not using the calculator correctly with regards to juice...

I checked the gravity of tap water and Merlot at approx 20C: 1.002 and 1.065 respectively. I'm happy with 1.002 for tap water. For the Merlot, the gravity gives a sugar content of 176 g/l. If you work out the SG from the total sugar in the must including the honey it agrees well with the SG I measured (i.e. 1.092 vs the 1.090 I got with hydrometer).

I think this issue never raised it's head before as I never had such a high % of the total sugars coming from juice in past recipes. I'm not sure what I'm doing wrong exactly, I put in the % sugar and quantity of juice in volume (in this case litres) and let the calculator do the rest.

Lesson then - and I should know better - don't be spoonfed and double check with basic maths.

fatbloke said:

S'pose its down to having faith in the tools you have at your disposal.......

Or how sciencey you want to get...... there's plenty hereabouts who try to plan their batches down to the enth degree, but IMO, a close approximation is good enough.

Good luck with your batch.....

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Cheers! I'm fine with being within a couple of degrees - experimental error etc - just when it gets into double figures and things don't add up.

The water is pretty hard around here, so that might explain the SG of 1.002 of tap water? Although, it's good enough for me if the hydrometer is close enough. Like you said earlier, perhaps I could calibrate it with distilled if I was bothered enough.