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bknox81 · 11-05-2009, 03:12 AM

I'm working on developing a transient thermal model of my mash tun/heat input system to help refine my mash temperature control. I have a single heat input (resistive heating element) and a single thermistor in my closed-loop control circuit. There is a definate temperature gradient across the grain bed I am trying to model to help reduce overheating the mash. Does anyone have info or an idea on where to find bulk thermal properties of a mash, or any grain/water mixture? Specifically I'm looking for thermal conductivity and how it relates to grain concentration.

Thanks!

The Pol · 11-05-2009, 03:17 AM

Do you have the heating element IN the mash?

bknox81 · 11-05-2009, 03:27 AM

Yes. The mash tun is a Rubbermaid Cooler, so having an external heat source gets tricky. I've made 12-15 beers this way. It works good (ie. the beers taste great!), but I'm trying to improve my process repeatability and have more discrete temperature steps.

The Pol · 11-05-2009, 11:03 AM

I use the coolers too, I use an electric HLT and an external heat exchanger to heat the mash.

What sort of temp controller do you have on the element currently?

The issue is that the mash has some serious insulative properties, making it necessary for a mash mixing device, or at the VERY least (you would still get hot and cold spots) recirculate the wort that is in the mash to keep it moving.

Even with a PID, I think youd have issues with the mash temps not being uniform, because of the sheer power output of the element, insulated in the mash. Even heating a tank of pure water takes some agitation to avoid temp. stratification.

**rocketman768** · 11-05-2009, 05:24 PM

The specific heat of grain is about 0.4 cal / (g*K) (little 'c' calories). Water is 1 cal / (g*K), so you can easily find the specific heat of the mixture with a weighted average of the two. Also, The Pol is right... that heat isn't gonna go anywhere unless you are really stirring your mash.

bknox81 · 11-06-2009, 02:11 PM

I have a home built temp controller on the element - a programmed PIC reads the temperature, runs a PID loop, then outputs a PWM signal (on/off) to the element. The large thermal capacitance of the mash smooths the temperature response from a PWM input. I have a matlab model of the system right now to find the gains for the PID loop, and it works spot on for just water.

I'd like to add some sort of delay element based on the thermal transients of the mash (ie thermal conductivity) to where the thermistor is located. Even if you don't stir the mash, the temperature across the mash will be uniform after some time period based on the mash thermal conductivity and geometry (assuming a perfectly insulated container). I agree that the best method is to stir/keep the mash moving, but right now a software upgrade is more in my budget than a hardware one.

Just a side note, specific heat is a different measure than thermal conductivity.

**rocketman768** · 11-06-2009, 02:36 PM

Quote:

Originally Posted by bknox81

Just a side note, specific heat is a different measure than thermal conductivity.

You are right.

Looks like it's somewhere between 169e-3 and 288e-3 W/(m*K) for barley.

http://faculty.ksu.edu.sa/dr_hobani/Documents/Bulk%20Thermal%20Conductivity%20and%20Diffusivity% 20of%20Barley.pdf

http://linkinghub.elsevier.com/retrieve/pii/S073352100300047X

As far as water/grain mixtures, I dunno. Probably pretty close to a linear approximation between that of grain at 0% water and that of water at 100% water.