High Measured Mash Efficiency

[Goose]

This is something that has been bugging me for some time.  When I brew a beer I assume between 80% and 85% mash efficiency depending on the beer and write my recipes as such.  When I have the kettle full, I measure the gravity with my refractometer and plug that number converted to degrees Plato (I work in Plato rather than SG) into the measured pre-boil volume area.  BeerSmith always tells me my mash efficiency is well over 100%.  For example I made a Tripel the other day and it told me that the mash efficiency was 128%, which is obviously impossible.
I have been doing the calculations manually to determine what the kettle full gravity is to achieve my post-boil gravity and ignore what BeerSmith is telling me.  I have my equipment profile set for a 15 gallon keggle and use no temperature adjustment for the the thermal mass of the mash tun since I heat the mash liquor in the MT.
I have looked for a way to adjust the estimated mash efficiency so that it more closely tracks what I am actually seeing but have not found a way to do it.
Any suggestions?  I am using BeerSmith 3.
A a sidebar, I have my brewhouse efficiency set to 80% and am always right in the ballpark on this.

............Goose Steingass
AHA Governing Committee Member

[Oginme]

Hi Goose!

From what you describe, the first thing I would look at is your volume measurements.  Somewhere, your measurements are not accurate in order to account for mash efficiency above 100%.  How are you measuring your volumes?

If you take this last recipe where you achieved a whopping 128% mash efficiency and post your actual volumes and gravity readings maybe we can make some sense of it.  Alternatively, if you export that recipe as a .bsmx file and post it here, we can have a better idea of your total process to see if anything else pops up that may give us a clue.

[Goose]

I have a sight glass on my kettle that I calibrated when I built my system.  It is pretty accurate.  I have attached the .bsmx file here.

Like I said, I get consistent results by doing the calculations by hand, it is just a bit perplexing that I get numbers that are so far off from the software.

[Oginme]

I have a sight glass on my kettle that I calibrated when I built my system.  It is pretty accurate.  I have attached the .bsmx file here.

Like I said, I get consistent results by doing the calculations by hand, it is just a bit perplexing that I get numbers that are so far off from the software.

First glance, you have entered the preboil gravity in for your post mash gravity.  I am guessing that your post mash gravity contains the contribution of the sugar addition.  If you drop out the sugar addition, your post mash gravity would drop 17 points.  This 17 points brings the post mash gravity reading to 1.051, which still gives you a mash efficiency of 96%. 

The reason that the post mash gravity measurement is in the program is specifically to remove the impact of the sugar/extract additions made before the boil in order to correctly calculate the mash efficiency.  Otherwise, this number would be influenced by the sugar contribution and give you an inflated value, which you are seeing in your recipe.

You will please forgive me for using metric units below, but I work in metric as it is so much easier for conversions:

The second issue is that your sugar point balance does not work out properly.  You have 28.39 liters of wort at a gravity of 1.068 at the beginning of the boil.  This gives you 68 points x 28.39 liters = 1930 sugar points.  Following your boil, with the zero trub loss you have indicated in your equipment profile.  You end up with 19.61 liters at a 1.087 gravity reading.  19.61 liters with the thermal expansion is 20.43 liters.  Your ending sugar point total is 20.43 liters x 87 points = 1777 sugar points.  With no other losses in the system, these numbers should be the same (or nearly the same as some depends upon how accurately you can measure the volumes).  This makes me believe that there is one measurement which is not correct.

Another good reason for taking a post mash gravity reading, which with a refractometer is really easy, before the addition of the sugars is to help identify if there is a number which is inaccurate.  If you have gotten a post mash gravity reading of 1.046 and then measured your pre-boil gravity at 1.068, I would think that you either over added the sugar or that one of these readings was in error.  In fact, if you take your target gravity reading of 1.063 and volume of 28.39 liters, you end up with 1788 sugar points, which very nearly balances out and is probably within your standard error for volume measurement.

Edit:  As an additional check, I looked at your boil off rate.  You have 7.57 liters/hr in your equipment profile (2.0 gal/hr) and taking your volume measurements as actual you are getting 7.96 liters or 2.1 gallons for a 60 minute boil.  This is pretty good confirmation that your equipment profile is pretty good for boil off rate.  This points even more strongly to the pre-boil gravity measurement being off.

[Goose]

I see your point.  However, I have the same issue here when I have no additions to the kettle.  I also measure the gravity of the first runnings to see if I am in going to be in the ballpark since the first runnings obviously will be quite a bit higher than the pre-boil gravity and gives some data on what the end of mash gravity is before I start sparging.  I always enter the first runnings gravity in the notes section.  Plus, I always do a calculation of the desired OG times the EOB volume and then divide it by my pre-boil volume to determine if I have to boil longer or add DME if the kettle gravity is low on my bigger beers which sometimes happens.  We did it this way during my pro-brewer days and I have been doing it ever since since it is a good double check.

I will re-check everything again so see if I do in fact have a measurement error somewhere.  I have learned from experience that if my refractometer reading is about 3-4 BX or so higher than the predicted pre-boil gravity, I normally hit my desired OG within about plus or minus 2% (close enough for me).  In addition, I always calibrate the refractometer with distilled water before using it to make sure that if is properly zeroed to eliminate any possible error on that end and I do use the conversion in BeerSmith to correct the reading from BX to degrees Plato.

Finally I note that the thermal expansion of the hot wort is about 4% from you calculations.  I let the wort settle a bit (after removing the hop bag) which allows it to cool a bit and look at my EOB volume.  I then multiply that by 0.96, for the 4% contraction that results for the chilling process

Thanks for the help!  I'll let you know what I find out.

[Oginme]

OK, my next question is have you checked the calibration of your refractometer?  I don't mean zeroing the unit, but checking against a known sugar solution for accuracy.

I've had to do this in a dispute against NB about their herculometer hydrometer sets which are no where near accurate from the zero point to higher gravity readings.  While I had the sugar solutions made, I checked my other hydrometers and my refractometers and found one of my refractometers woefully off at the higher gravity measurement (25 Plato/1.106 sg).  To do this you will need access to a high precision scale or make the solution in a quantity such as to guarantee the accuracy of your solution. 

[Goose]

I can do that the next time I make a starter.  I have a scale that reads to the nearest 0.01 grams (or 0.0001 oz if I set it to ounces) and know that 100 grams of DME in a liter of water is 10 degree Plato/1.040 or 10.4 BX. I can check it vs a calibrated hydrometer that I got from Weber Scientific that has been certified by them.
I can also calculate and make a table sugar solution as well.

I'll let you know.