What's going wrong with my gravity calculation?

[guypettigrew]

Something is going awry with my OG. It's frequently less than anticipated from my post mash measurement.

Here's an example from a beer brewed today.

The sparge gave 7.5 gallons (34 litres) at a gravity of 1.039 according to my refractometer. To work out what this would give at 5 gallons (23 litres) I used a calculation given by Dave Line in his book from 40 years ago. The calculation is; 7.5 gallons at 1.039 will give a gravity of (7.5 x 39)/5 at 5 gallons. Which is 58.5, or 1.059, rounded up.

What actually happened was the 5 gallons came in at 1.049. Somewhat disappointing.

Anyone got any thoughts, please?

Guy

[Jocky]

Well your math seems correct. Are you using your refractometer for both measurements?

While I mention refractometer, are you taking the reading in brix and converting it, or are you reading in SG? The reason I ask is that all the refractometers I've seen that have the SG scale put it as a linear scale against the brix scale. The problem is that brix to SG is not a linear conversion, and the SG scale on such refractometers should be ignored, you should instead read in brix and use a tool to convert to SG.

[vacant]

23 ltr to the FV plus 4 litres of dead space/trub would account for it

(23+4) x 49 = 1323 which is close enough to 34 x 39

[guypettigrew]

Well your math seems correct. Are you using your refractometer for both measurements?

While I mention refractometer, are you taking the reading in brix and converting it, or are you reading in SG? The reason I ask is that all the refractometers I've seen that have the SG scale put it as a linear scale against the brix scale. The problem is that brix to SG is not a linear conversion, and the SG scale on such refractometers should be ignored, you should instead read in brix and use a tool to convert to SG.

Interesting. Using the Brix to SG conversion gives 1.040 at the post sparge 7.5 gallons, and an amazing 1.052 at the final volume of 5 gallons. This is 6% higher than using the SG readings directly from the refractometer. This is news to me. Should I buy a new refractometer which just measures Brix very accurately?

23 ltr to the FV plus 4 litres of dead space/trub would account for it

(23+4) x 49 = 1323 which is close enough to 34 x 39

Even using the info from Jockey and recalculating, it shows over 3 litres were left behind.

It seems I'll need to improve my technique if the boiler is holding this much after the run off. It's a Buffalo with a 26cm DX hop filter from Mr Lard, which I bought a short while ago. This is only the third time I've used it.

Thanks for the advice. All other thoughts welcome.

Guy

[vacant]

If you're leaving 3 ltr behind and the boiler internal diameter is 38cm(?) that's ending the flow when 3000/19/19/pi = 2.6 cm or an inch deep remains.

Perhaps you could stuff a bit of silicone tube up the filter such that the bottom of the tube almost touches the base of the boiler. Providing the end of the tap also extends below the base of the boiler (airtight - more silicone tube?) the syphon should get most of those 3 ltr out.

[Jocky]

I have the same hop filter - the problem you have is that the wort needs to go up to get out, leaving quite a bit of dead space below it. If you put some silicon tube on the tap that it is connected to outside of the boiler, with the end of the tube below the bottom of the boiler it will syphon out everything, and leave you with nearly zero dead space.

[guypettigrew]

Thanks vacant and Jocky.

Just been experimenting with water in the boiler and discovered how important the syphon is.

If the silicone tubing from the boiler tap isn't full of water (ie the water is running out very slowly) and the tubing is acting simply as a guide for the wort into the receiving vessel, then there is about 1.8 litres of water left in the boiler when the flow stops.

If the silicone tube from the boiler tap is full (ie, a fast flow), then the syphon effect takes over and there is a little under 300ml left in the boiler.

So that's the syphon theory proven to be correct. How do I put it into practice? I've previously had a slow run off from the boiler. The hops covering the filter determine the flow rate.

How do you work it so the flow is sufficient to fill the tubing and create the syphon? Short tubing, narrow bore tubing?

Thanks again for your advice on this.

Guy

[vacant]

When you get to the last few litres, clamping the lower end of the tube almost shut to slow the flow down even more should keep the air out and the syphon going. The tube only needs to be long enough to be just below the level of bottom of the liquid so not too much air to displace.

[guypettigrew]

Might have found a solution. I've shoved a smaller diameter bit of tubing up the larger diameter silicone tubing.

It's reduced the effective internal diameter from 12mm to 7.5mm. It's tightly secured with a couple of cable ties at the exit end to stop air running in and creating problems with the flow.

Using this it's possible to get the full syphon effect with a very slow flow. Amazingly it also reduces still further the volume of liquid left in the boiler at the end of the run off. With this tube only 175ml was left behind.

Not entirely sure how be certain it's sanitised, though, because of the inevitable small gap between the outer wall of the smaller tube and the inner wall of the larger tube. Perhaps I'll just leave it permanently in the bucket of starsan and hope it penetrates into the tube. Or perhaps I'll try to come up with a better design!

Guy

[Jocky]

Regarding your refractometer - you don't need to throw it away, just use the brix scale and convert to SG. The tool I use to convert in beersmith needs calibration using a hydrometer but after that I've found it accurate - usually within a point of the hydrometer reading.