Question for all you HERM/RIMS recirculators :) !

I've done about 4 AG brews on my HERMS rig and I'm loving it. My question has to do with the grain bed temp versus temperature of the recirculating liquid. My RTD sensor measures the temperature of the wort entering back into the MLT keggle after passing through my CFC/heat exchanger. If my temp sensor is measuring 159 on the wort, the middle of my grainbed is usually at 155. I'm about to do another batch soon and will have my digital thermometer to get a better and more accurate reading. So assuming that I want a mash temp of 152, do I want the temp to be the middle of the grain bed or the liquid entering the MLT? I'm assuming it's the middle of the grain bed.

I'd love to hear what your experiences are.

Flananuts

What you want is the average grain bed temperature and my best guess for where to measure it would be half way between the center and the outside wall of the MT and halfway down in the grain bed. You might think that half way down and in the center would be best, but IMO it probably won't indicate the average very well. The grain bed won't be of a completely uniform temperature even with continuous circulation. You must also allow sufficient time for any temperature changes to be distributed. How long that might be for certain I don't know. I measure the return wort temperature as you are doing and also monitor the grain bed. There will always be a loss of heat through the circuit so you will probably have to keep the returning wort a few degrees higher than your target temp. A lot will depend on the ambient temps and if you are outside, how breezy it is. I'm basing this on my experience and my RIMS system arrangement. Others may have something completely different to report or suggest.

+1 on Cat's reply. The temperature will never be constant throughout the mash. I agree with Cat's suggested thermometer position.

That's very helpful. So it safe to assume that the return temperature of the circulating wort is less important as the average grain bed temp? My thought is I just set my PID to hold temp at 159 for an average grain bed temp of 156 for example.

flananuts said:

That's very helpful. So it safe to assume that the return temperature of the circulating wort is less important as the average grain bed temp? My thought is I just set my PID to hold temp at 159 for an average grain bed temp of 156 for example.

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Yes, IMO that is the best way to do it. Let the grain bed temperature govern. The optimum difference between the two will depend on the system design, what you are trying to do and the ambient conditions. A number of variables must usually be considered. The heat source, flame intensity, circulation rate, insulated or not, ramping up or simply maintaining temps etc, etc, must all be considered and only you can figure out what works best for your system.

Thanks for the input. I'm fortunate with the setup that I don't have too huge of a temp swing during dough in and don't have to strike more than 2-3 degrees above my mash temp. My HLT is circulating approximately 11 gallons or so through my cfc/heat exhchanger and temp recovers quickly. I'm brewing in a week so I'll try out the suggestions.