Author Topic: Specific Heat Calculations Conflicting  (Read 2363 times)

Offline FunkedOut

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Specific Heat Calculations Conflicting
« on: April 07, 2018, 09:49:22 AM »
Conflict is between a recipe and Mash Adjust tool...


New mash tun (insulated stainless) and wanted to get the specific heat dialed in.

Let it acclimate to ambient temps (78*F) overnight and pumped 5.5gallons of (172.5*F) water in. 
Water was pumped through a preheated chugger pump and 10' of 1/2" silicone hoses.  Took 90 seconds.
Preheated pump and hoses by recirculating strike water while heating.

10 minutes later, temperature stabilized at 166*F and then began to lose temp (3*F over 2 hours).

I created a new recipe with 0.0000000001 lbs of grain and nothing else.
  • mash tun weight set to 28.6 lbs
  • mash tun & grain temp set to 78*F
  • step temp set to 166*F
  • strike water set to 5.5 gallons (22qts)
Then proceeded to adjust specific heat value until the calculated strike water temp was 172.5*F.
That value ends up being 0.1811 cal/gram-deg C.

Now I try the Mash Adjust tool.
Set the same values as above along with 0.0000000001 gallons of starting water volume.
Specific Heat value ends up being 0.1186 cal/gram-deg C.

Adding 10 lbs of grain to the equation still has conflicting results.
Adding 1 gallon of water to an initial step still has conflicting results.

I'm inclined to trust the Mash Adjust tool because it aligns with the suggested value for stainless (0.12 cal/gram-deg C).
But I need the recipe calculator to perform, as that is what I use, not the Mash Adjust Tool.

Am I missing something?  Or is there another value factoring into one calculation and not the other?
Mash Tun Additions and Tun Deadspace are set to zero.

Thanks in advance.
« Last Edit: April 07, 2018, 03:44:20 PM by FunkedOut »

Offline GigaFemto

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Re: Specific Heat Calculations Conflicting
« Reply #1 on: April 07, 2018, 10:33:18 AM »
You have enough information to calculate the mash tun specific heat yourself. You know how much energy was lost by the water to heat the mash tun:
Q=(Kw)(T2-T1)(mw) = (1 cal/gm-C)(3.6 C)(20800 gm)=74880 calories

Now use that to calculate your kettle specific heat because you know the energy and mass and the temperature change
Kk=Q/(mk)(T4-T3) = (74880)/(12970 gm)(52.5 C) = 0.10997 or 0.11 considering the accuracy of the readings

This is close to what the mash adjust tool gave and the default value for stainless, so I would go with 0.11 or 0.12.

--GF

Offline FunkedOut

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Re: Specific Heat Calculations Conflicting
« Reply #2 on: April 07, 2018, 10:36:59 AM »
Appreciate the info and formulas.
I guess my biggest concern is that it seems I cannot trust the recipe calculator for the strike temp.

I've always used the recipe calculator with the box for adjusting for equipment off.
I've always brewed single vessel BIAB and heated the water in the same vessel I mashed.
Growing pains.

Offline GigaFemto

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Re: Specific Heat Calculations Conflicting
« Reply #3 on: April 07, 2018, 01:28:41 PM »
I forgot to congratulate you on making a complete set of careful measurements with extraneous items like pump and hoses accounted for. I have always done BIAB, so I don't need to worry about heating my equipment, and I can't vouch for the strike calculator with the box checked. Try it a couple of times and you should be able to dial in a profile that works for you.

I once went a bit crazy trying to figure out how to calculate in detail the thermal load of a mash tun. BeerSmith asks for the volume of the tun and the mass (or weight), but if you have a large tun and only fill it halfway then the upper half will not get nearly as hot as the bottom part. Stainless conducts heat poorly for a metal. I had formulas that took into account the shape of the tun and assumed that the mass of the bottom was always heated and after that the sides came into play, but only the fraction of the sides that was in contact with the mash. This got complicated quickly, and I finally decided it is just not worth it. The differences in strike temperature are pretty minor and those who really want accuracy can dial in their own numbers. And, if you are really particular, you can preheat your mash tun and then it doesn't matter!

--GF