Loss of gravity is driving me mad!!!

[guypettigrew]

No, I'm not floating around, although I do feel as though I'm banging off the walls!

My wort post boil shows a much lower SG than would be expected from its pre boil value.

Following very helpful comments from Vacant and Jockey I've improved the run off efficiency from the boiler by using a syphon tube on the tap. Today's brew left approximately 1 litre in the boiler held in the 90g of hops in the boil. A great improvement!

I've also started measuring using the Brix scale on my refractometer and then converting to SG. Thanks again for the advice.

Today's brew gave the same gravity loss as always. Here's the figures.

The sparge gave me 7.5 gallons in the boiler. This had a Brix reading of 9.8 which, according to this site, equates to a gravity of 1.039.

After the boil there was 5.5 gallons in the fermenter, with less than two pints left in the boiler. This gave a Brix reading of 12.0, which equates to a gravity of 1.048.

This makes no sense!

A wort at 1.039 and a volume of 7.5 gallons should reduce down to a wort of 1.051 at 5.75 gallons (the 5.5 gallons in the FV plus the 0.25 gallons left in the boiler). The calculation is; (original SG x original volume)/ final volume = final SG. In this case the numbers are (39 x 7.5)/5.75 = 50.9. This should have been the SG at the final volume. Instead I got 48, a drop of 3 degrees or 6%.

Does anyone else have this problem, or is it just me? And, more importantly, does anyone have any thoughts on what's going wrong and how to put it right, please?

Thanks.

Guy

[TheSumOfAllBeers]

Check that you are measuring your post mash correctly. Refractometer readings need to be corrected for temperature.

Be sure you are measuring your post mash volume correctly too

[JamesF]

My first suspicion would be to check the temperature you're measuring at and that you're correcting for it in your calculations.

James

[a-slayer]

Don't bother to check, just enjoy the resulting beer.. that's what I do. Never bother to take the OG and just take what comes.

[Normski]

Have you tried taking a Hydrometer reading at 20c.
Just to check.
K.I.S.S.

[oz11]

Are you measuring pre and post boil at the same temperature? By which I mean the volumes i.e. allowing for thermal expansion.

[Jocky]

Have you tried taking a Hydrometer reading at 20c.
Just to check.
K.I.S.S.

I would really really suggest doing this - the default refractometer conversion factor used in calculators is a middle of the range value for all refractometers. In reality, the correction factor is specific to each refractometer, so you need to work out what that is using a hydrometer and several samples of 20C wort.

I only recently got a refractometer myself, and I've been putting aside samples I've measured to cool and cross check with the hydrometer. I believe my conversion would have been 5 points off otherwise, but now my converted refractometer figure and hydrometer agree... although I'm now paranoid that the hydrometer isn't right

[Jocky]

Are you measuring pre and post boil at the same temperature? By which I mean the volumes i.e. allowing for thermal expansion.

Also a good point - thermal expansion can account for a 4% change in volume.

Have you also calibrated whatever you are measuring your fermented volume? The difference between 1.051 as expected and the 1.048 you got is only about 1.5 litres, and the markings on fermenters are notoriously inaccurate.

[guypettigrew]

Thanks for all the advice and suggestions.

The pre boil gravity measurement is from wort taken by pipette from the boiler as the wort is coming up to temperature. A few drops are placed on the sight glass of the refractometer (previously calibrated using tap water) and the cover closed. The refractometer is then left for ten minutes or so for the temperature to stabilise before the reading is taken.

The post boil gravity measurement is taken from the FV once the wort is down to about 20C. Again, a few drops are placed on the refractometer and it's left for ten minutes or so before the reading is taken.

Any errors would be present in both readings and can't account for the amount of variance between the actual reading and the predicted one.

This suggests my measurement of the pre and post boil volumes may not be correct. Using the figures from my first post and calculating backwards shows the predicted and actual gravities would match if my pre boil volume was 7.1 gallons, not 7.5 gallons, or the final volume was 6.1 gallons and not 5.75 gallons. Or a combination of both!

For measurement of the pre and post boil volumes I use a Young's fermenting bucket on which I marked each half gallon with a felt tip pen some years ago. Sounds like a perfect source of inaccuracy!

Recalibration of the bucket is in order. How do you gents measure your wort volumes?

Guy

[vacant]

How do you gents measure your wort volumes?

I measure pre-boil volume roughly as I drain the mash tun to a bucket then transfer into the boiler with a 5 ltr jug. Then I measure the gravity and work out what I need to boil off. I can boil off 8 ltr/hr with two elements or 3.6 ltr/hr with one. That let's me put the hop schedule on track.

When done, I calculate the final volume from the gravity and weight. I know how much my FV and lid weigh empty.

The weight of alcohol after fermentation is the same as the weight of CO2 which is lost, the final volume will be even less I believe.

[guypettigrew]

Sorted--probably !!!!

First off, thanks for all the helpful thoughts. There are other (non-brewing) forums where the response to the query would have basically been 'you're an idiot, you can't even measure properly. Not so here, where people want to be positive.

It seems to be down to my inaccurate measurement of the pre and post boil volumes. Having recalibrated using the weight of water as the determinant I've discovered my 7.5 gallons of sparged wort is, in fact, 7 gallons and 1 pint. The final length was also under, but only by about a pint.

Plugging these back into the calculation means the actual and predicted gravity values are an almost exact match. Result!

Sadly, it also means my efficiency is less than expected. But that's another issue. Now the numbers are right I can work on the efficiency.

Thanks again.

Guy