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Calculating Thermal Mass

Jimatthelake's picture

Calculating Thermal Mass

How do you go about calculating "thermal mass" in an oven?  Are the "mass" items in the oven weighed and heat transfer rates attached to each type of material?  How do you account for the mass of the oven "box" itself?

I'm dealing with a new (to me) deck oven, and trying to understand and predict baking out comes.  Thanks for your help.  Jim

tpassin's picture

I don't think you will be able to do it very well.  In principle, you would weigh or calculate the mass of each part that will heat up, and look up the thermal mass of each in a handbook or on line.  Heat transfer rates are a different thing.  In practice, you may not know what the materials are, and also, as you speculated, it's hard to work out the effective mass of the oven itself.  An oven designer might need to have a good idea of both the thermal mass and the heat transfer characteristics so that the heat source can be matched to the oven structure.

In general, metals transfer heat by conduction more readily than stone or ceramic. 

I think you would be better off measuring different places in the oven to learn how long it all takes to come up to a fairly steady state.  Your IR thermometer will be helpful, as well as the oven temperature sensor and any other probes you might want to put inside.

In addition, each oven design will probably drop a different amount of temperature when a large mass of dough is inserted and when steam is produced.  A big commercial oven will no doubt lose less than a consumer grade home oven would.


semolina_man's picture

Yes, you are thinking along the proper lines.  Each material has its own mass and coefficient of specific heat, Cp, aka heat capacity.  Cp units are kJ/kg*K. 

For each degree Kelvin (or Celsius), all of the oven materials increase in heat energy as measured in kJ by the amount proportional to their specific heat. 

For each increment of energy (Watts, or current * voltage * time in seconds), of the oven materials will increase in deg K (or Celsius) proportional to specific heat. 

Calculating anything useful in the real world based on this information would be challenging.   At best you could arrive at the blended Cp for the oven as a complete unit inside of the insulation, if any, and understand how long it takes to change the oven temperature by 1 degree.   The calculation will be an estimate. 

For purposes of simplification, I would exclude any materials outside of an insulating blanket, if insulation is present.  If the oven has no insulation or air gaps, for examples is 100% masonry or masonry+steel, you could use the entire mass of the oven. 

Along a different path, the concept that more mass is always better, in terms of heat retention, is a good thought to be aware of.  Conversely, less mass means faster heating and cooling times, but less heat retention or stability.  

Heat stability is why I seek out induction cookware with its heavy steel bottoms, and I do not own or use an induction cooking surface.

Please tell us more about your goal. 

Jimatthelake's picture

My goal is to better determine baking temps. and times for my oven.  I've fitted it with a 55 lb. granite deck, and a 6 lb. steel steam tray, and was wondering what impact they would have on it's performance.  I guess I just need to practice with different settings and see how the bakes come out.  Jim

semolina_man's picture

More mass takes longer to heat up, and longer to cool down, and temperature will change more slowly (will be more stable) if/when heat input changes. 

alcophile's picture

"Theory guides, experiment decides" was the mantra of my my analytical chemistry professor in college. The maxim is attributed to the father of modern analytical chemistry, I. M. Kolthoff, and his mentor, Nicholas Schoorl.

Your idea of testing different conditions is the best way to know what works with your equipment. The calculated thermal mass approximations were only going to be just that—approximations. Empirical data are always necessary to confirm (or disprove) a theory.