Need a reliable thermometer

[Sailor Con Queso]

I have developed a spreadsheet to calculate equilibrium temperatures, time to target temp, etc based on my particular setup. Everything depends on volume of water, volume of milk, and the BTU values of your heat source. I have 35 gallons of heated water and 35 gallons of milk in a 70 gallon tank. Since the volumes are the same, quick math is easy for equilibrium temperatures. If the milk starts at 40F and the the water is 140F, then both will reach equilibrium at the average temperature of 90F. Put another way - 40+140=180 divided by 2 = 90F. Of course there is some heat loss, and the heat source keeps generating BTUs so that's why I start with 150F water in my vat. After the water hits 150F, I reset my digital controller to 90F or whatever my target temp is. It is MUCH better to overheat than underheat in the beginning. As the milk heats up, I can always drop the volume of water to slow down heat transfer and avoid overshooting the target temperature. And by the way, these calculations can be used even if you are making cheese in your kitchen sink. You just have to know volumes and temperatures.

Interesting physics and math. Not difficult, but it does take some head scratching. Here are some basic concepts:

It takes one BTU of energy to raise one pound of water, one degree F. Assuming water & milk are both 8.6 lb/gal means 70 gallons (combined water & milk in my vat) x 8.6 = 553 pounds. It will take 602 BTUs to increase that 70 gallons by one degree. Substitute your own volumes to calculate your needs.

By definition, a 1000w (1kw) electric water heating element running for one hour generates 3413 BTUs.

A 1500w element produces 5119 BTUs/hr. So 5119/602 = 8.5 degree rise per hour.

A 4500w would give 25.5 degrees per hour. A 6000w would give 34 degrees per hour.

So now you have everything you need to calculate temperature and times for your own setup.

Hey, If I gave you the spreadsheet, you'd miss out on all of the fun. Enjoy.

[KosherBaker]

Wow. This thread has turned into pure gold. LB, JD, Sailor and everyone else that have contributes so far. Thanks. May the cheese be with you.

[linuxboy]

Let me know if that's not clear of if you need help with figuring out how to calculate the bias curve for a thermometer. There's probably some guide you can find online for it, too. I just learned the lab methods long ago, so I still follow the same ones.

this would be absolutely great if you could post this.

Way better solution than spending $100 on a Thermapen.

Um, a little short on time. But seriously search on google for something like "thermometer calibration curve"

lots of info out there for how to do it.

[iratherfly]

Sailor, Linuxboy so sorry it took so long to respond to such comprehensive answers. Was away working and haven't had any time to cheese around...

Thanks for your offers to help. A friend of mine is a scientist-professor and said that in his university lab they have these special baths that mimic perfect atmosphere, elevation and pure water. They dump their alcohol thermometers into them to get the perfect universal temperature for calibration. Would love to see that. I am giving him all my thermometers to calibrate. Let's see how this works!

As for your help with my dream machine... Thanks!
The water circulation is not so important when making cheese unless it's a bit of water and a large vat. I just want to avoid hotspots and uneven temps. Milk is usually easy because it's so dense. The pump however is very important for sous vide so it will still be part of the machine. I will just turn it off and on as needed.
As for putting the thermocouple probe in the water Vs. the milk, I am a bit confused as to why. I get that you need the water to be hotter than the milk target but the PID doesn't know how hot is the heating elements and shouldn't care... it needs to start and stop the heating element to assure perfect temp of the milk. In other words, if it takes 137F water to heat the milk to 90F than shouldn't the PID be set to 90F and the thermocouple be in the milk? And what if another day the atmosphere and room temp make it 146F? How would the I (or the PID) know to adjust it? With the thermocouple in the milk the heating elements always switch on/off according to the milk temp; I shouldn't even know the water temp as long as the milk is correct. I have a feeling that I misunderstood a good point. Can you explain?

[FarmerJd]

As for putting the thermocouple probe in the water Vs. the milk, I am a bit confused as to why. I get that you need the water to be hotter than the milk target but the PID doesn't know how hot is the heating elements and shouldn't care... it needs to start and stop the heating element to assure perfect temp of the milk. In other words, if it takes 137F water to heat the milk to 90F than shouldn't the PID be set to 90F and the thermocouple be in the milk? And what if another day the atmosphere and room temp make it 146F? How would the I (or the PID) know to adjust it? With the thermocouple in the milk the heating elements always switch on/off according to the milk temp; I shouldn't even know the water temp as long as the milk is correct. I have a feeling that I misunderstood a good point. Can you explain?

The problem is that if the thermocouple is in the milk; and the thermostat is set to 90F; and the water heats to 100F in order for the milk to get to 90F; then when the thermostat cuts off, the final temp will still rise to a point between 90 and 100 without the elements on. I think maybe the confusion was because I tried to tackle two issues at once: the initial heating ( when you are not concerned with the rate of increase) and the actual "cooking" phase when the rate is essential. Or it could be the incredibly long sentences.   Anyway, I hope that makes more sense. Let me know if I muddied the water even more or if I misunderstood the question.

[KosherBaker]

I am also planning on building a machine to do both sous vide and cheese. It will be made with industrial grade thermocouple connected to a PID unit that will be programmed to control a heating element based on the milk/water temp. Not sure how to calibrate that thermocouple either...  Has anyone here made such thing before? If so - advice will be helpful. If not, does anyone want to share the experiment? The parts should cost about $30-$60 only and it shouldn't take more than an afternoon to build.

Have you had any takers on your sous vide machine experiment? I would start a separate thread for it though. I may just jump in there ....

[iratherfly]

FarmerJD, I get it. I suppose the only question now is which is better? Putting the thermocouple in the water and Learning at what temp to shut it off in order to get a the correct milk tem? Or, putting it in the milk and learning how long before I reach the desired temp I need to shut it off and time it like that?  On the stove, I turn off the heat at 87F to get 90F milk and I feel comfortable and accurate about it...  One thing to remember is that water is less dense than milk. It doesn't hold temperature for long time and takes far more energy to heat up than milk, so if for example you have a double boiler configuration where it takes 100F heated water to get your 90F milk. Turning off the heater at 100F will not end up with a milk that's eventually 100F. The milk which is more stable and dense will cool the water down to 93F or so... it becomes the power source of the double boiler now. Does it make sense?

By the way, I always treat the speed of initial heating at the same importance as the cooking phase.  In traditional and farmstead cheese the milk comes from the animal and carries the animal's body heat, or sometimes through a pasteurizer with a quick cool down cycle. The keeping of temperature as well as the quick cooling and quick heating of milk are essential in preventing pathogen growth and development of unwanted acids. I always felt that the quicker I bring it back up to desired temp -the higher milk quality I preserve (and I grow less of the competing bacterium).

KosherBaker So far, some very quirky and sweet couple that happen to live a few blocks away found out about my sous vide experiments through Twitter. They have already built a couple of these and they are blogging about it here: http://qandabe.com/. We met last Friday and talked food and philosophy in a local coffee shop (got to love this town) and we are building one together in the new year. I am ordering the parts today! I will post photos on a separate thread when we do it. (If you start a thread, I will post it on yours, PM me in case I've missed the thread)

[FarmerJd]

FarmerJD, I get it. I suppose the only question now is which is better? Putting the thermocouple in the water and Learning at what temp to shut it off in order to get a the correct milk tem? Or, putting it in the milk and learning how long before I reach the desired temp I need to shut it off and time it like that?  On the stove, I turn off the heat at 87F to get 90F milk and I feel comfortable and accurate about it...  One thing to remember is that water is less dense than milk. It doesn't hold temperature for long time and takes far more energy to heat up than milk, so if for example you have a double boiler configuration where it takes 100F heated water to get your 90F milk. Turning off the heater at 100F will not end up with a milk that's eventually 100F. The milk which is more stable and dense will cool the water down to 93F or so... it becomes the power source of the double boiler now. Does it make sense?

By the way, I always treat the speed of initial heating at the same importance as the cooking phase.  In traditional and farmstead cheese the milk comes from the animal and carries the animal's body heat, or sometimes through a pasteurizer with a quick cool down cycle. The keeping of temperature as well as the quick cooling and quick heating of milk are essential in preventing pathogen growth and development of unwanted acids. I always felt that the quicker I bring it back up to desired temp -the higher milk quality I preserve (and I grow less of the competing bacterium).

IMHO, putting it in the water is better because you are adding heat not taking it away. If you keep the water at 100F, the milk will eventually get there although at an ever decreasing rate and the good news is you don't have to worry about going over. As far as density is concerned the difference is very small especially whole milk like I use with a 4-6% fat value. (1 to 1.03)


I agree about the importance of quick initial heating. My point was that you did not have to worry about keeping it at a slow rate like the cooking phase; you could just heat it as fast as you could. Good discussion, good luck with the sous vide experiment.

[iratherfly]

Thanks FarmerJD!
My experience using water for sous vide in the same vat usually used for milk -cheesemaking has shown me that the difference is very significant. 2 Gal. milk seem to hold temperature about 3 times longer than ...get this; 4 Gal. water.  But it makes me wonder - maybe I should use something thicker than water in that external water bath. Just thicken it. That could be interesting.

Anyway, my only fear remaining is ...what if it takes too long to heat up to the desired temp? Other than that I think you have a good point. I will keep the thermocouple on the outside.  I just ordered the parts and it includes a high power 1000W bucket heater instead of travel immersion heater. I assume if that can bring the water bath to temp fast enough -I have nothing to worry about regarding the milk.

[FarmerJd]

1000w will not work for a significant amount of milk/water. I use a 2500w and barely have enough power. Wayne has five 1500w I think and sailor uses two 3000w. I'm not sure how big your setup is but there is a thread on here where we talked about how to do the thermal calculations. You might want to check it out.

[iratherfly]

thanks ...but how big of vats are we talking here? This is a 5 gallon stockpot. In fact, I got the 1000W heater in the mail and it's bigger than the pot!!! I am sending it back and replacing with 2-3 immersion heaters (the ones you stick in your mug to boil water to make yourself tea when you travel).
Also, remember that in sous vide the food never touches the water so I can fill it in hot water from my faucet and begin the process at 115F so I only need to raise it by 20F-25F for most dishes

[FarmerJd]

oops! I was under the impression this was a much larger setup. Just ignore my last post. hard to keep all the threads from merging in my scattered brain.

[KosherBaker]

JD perhaps if you have a recommendation of number of watts per unit of measure? Then everyone can calculate the power needed based on their requirements.

[FarmerJd]

Without putting a pencil to it, I would say about 30W per gallon would give you the ability to raise the temp 1 degree F every five minutes and 75W would increase that rate to 1 degree F every 2 minutes. This would be fine for the cooking phase most of the time. The initial heating rate with this wattage would be pretty slow so if you could get 3 elements with appropriate wattages you could conceivably operate all three during the initial heating phase (say a combined 150W per gallon with a 30W/gal, 40w/gal and a 80w/gal element), then cut back to only one (30w/gal) for the initial cooking phase, then add one more element (40w/gal) for the last third of the cooking phase or as needed. My recommendation is at least 100W per gallon but if you can't swing this, shoot for at least 50-75W per gallon. BTW, when I say per gallon, I am referring to the total amount of liquid including the water in the outside jacket. I have not allowed for heat loss, mass of the pots, kids pouring their ice tea in the jacket or other variables and I may have made a major mistake in the calculations since I am doing this in my head. My system is 40 gallons and I have a 2500W element (62.5W/gal) that doesn't meet the "1 degree F in 2 minutes" requirement and that is consistent with my practical results. I am going to rest my brain now. 

[KosherBaker]

Awesome stuff JD as always.

BTW, when I say per gallon, I am referring to the total amount of liquid including the water in the outside jacket.

That's water and milk, right?