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For a cooking experiment, I'd like to be able to measure the temperature inside a piece of meat while it's being cooked. Due to the mechanics of the cooking process, it's going to be difficult to use a wired temperature probe, so I was wondering if there's some way to do it wirelessly.

Is it possible to set up a thermistor, capacitor and tiny coil in some way that its resonant frequency could be measured, and thus the resistance of the thermistor could be measured wirelessly?

I'm especially interested in very small solutions which could fit into a slim spike which could be inserted into the meat. For this reason, I would also like it to be a batteryless solution.


Added:

Sample rate very low. I'd be happy with one sample per minute for Sous Vide, although one sample per second would be useful for frying.

The maximum temperature this device will have to survive will probably be about 80º just to be safe.

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  • \$\begingroup\$ @RussellMcMahon - I'm sure you could do this with IR. \$\endgroup\$ – Rocketmagnet Sep 6 '12 at 22:58
  • \$\begingroup\$ What kind of cooking process is it? \$\endgroup\$ – Chris Laplante Sep 6 '12 at 23:29
  • \$\begingroup\$ Remote temperature sensors with resonant coil and thermistor were implanted into small lab animals. So, this is possible. How are you going to prevent your cooking sensor from becoming a gastric temperature probe? If it's tiny, it will be difficult to find in the meat after it's cooked. \$\endgroup\$ – Nick Alexeev Sep 7 '12 at 2:42
  • \$\begingroup\$ @SimpleCoder - Actually several kinds of cooking, including frying and Sous Vide. \$\endgroup\$ – Rocketmagnet Sep 7 '12 at 8:47
  • \$\begingroup\$ @NickAlexeev - Partly I'm just going to have to remember not to swallow it. But also it's going to be inserted into a spike, so it will actually be about an inch or two long. \$\endgroup\$ – Rocketmagnet Sep 7 '12 at 8:49
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Edit: there are a lot of products already on the market exactly like this if you don't want to make it yourself. Look at numbers 2 through 5: http://homecooking.about.com/od/kitchenequipmentreviews/tp/electricthermom.htm

If you can get a resonant coil and thermistor system with enough SNR to reliably detect temperature remotely, then that seems to be the best solution. If not, here's some information about wireless sensor nodes.

In terms of wireless data transmission, infrared radiation of an object increases in proportion with temperature. Unfortunately, this information could only tell us the temperature of the surface of the meat, and not the center. At first I thought maybe you could use a spike with very high thermal conductivity so that you could see the temperature of the inside by looking at the spike with a thermal camera. However, the spike would just effectively cook whatever its touching so it would probably do more harm than good. For this reason, whatever design you choose to go with, the spike you insert should have very low thermal conductivity so that it doesn't cook the meat from the inside. Unless you want that.

The IR radiation coming from the oven/pan also introduces another problem: noise. You will have to select an IR transmitter with a frequency that doesn't overlap with the IR noise from the cooking implements. Maybe all IR transmitters are designed this way anyway, I don't know. Just something to be aware of. You could use an RF transmitter instead, at the cost of higher power. RF has the advantage that it will penetrate through more obstacles than IR.

A thermoelectric generator would be the most effective way to harvest energy. Because there is expected to be a large temperature difference between the outside of the meat, and the insulated inside, you could place a TEG along this gradient. But then you probably need to have a voltage regulator circuit to power your digital control circuit, temperature sensor and transmitter. I don't know if the TEG can produce the power you would need. It may be easier to just use an energy storage element like a super capacitor or chemical battery. Here is a tiny battery that operates up to 85°C: http://www.infinitepowersolutions.com/images/stories/downloads/ips_thinergy_mec225_product_data_sheet_ds1014_v1-1_final_20110913.pdf

They say, "Standard electrochemical degradation is proportional to temperature increase. Contact IPS for performance information regarding higher temperature applications up to 150°C."

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  • \$\begingroup\$ Why did you remove your earlier content - it would have made this answer more comprehensive. \$\endgroup\$ – boardbite Sep 26 '12 at 10:32
  • \$\begingroup\$ I thought it was excessively lengthy, but I rolled it back. Thanks. \$\endgroup\$ – travisbartley Sep 26 '12 at 10:49
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Unless you're into eating charcoal, the temperature inside the meat should never exceed 170°F (75°C) which is within the Industrial/Automotive range of -40°C to +85°C. So you can get electronics that will survive; the problem is how do you power that. I know you said you prefer a batteryless solution, but there are NiCd batteries that are rated to 55°C. See high temperature nicd cells

I've heard of chemistries that work at higher temperatures but don't know exactly what they are.

A batteryless solution to this would be interesting. I'm guessing from your description that you can't get closer than about 6-10" (150mm to 250mm) from the food, which makes inductive powering difficult without wasting lots of energy. Wonder if there's a way to get energy out of eddy currents.

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  • \$\begingroup\$ Yes, you're right about the temperatures. I'd love a batteryless solution, but I think I'm resigned to the fact that it'll need a battery. I hadn't thought about the temperature limit of the battery though. It will need to be rated for at least 70ºC. \$\endgroup\$ – Rocketmagnet Sep 11 '12 at 22:20
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From what I read, Sous-Vide starts by vacuum sealing food in bags and then submerge the bag into a temperature controlled bath of water well below boiling (130-150°F, or 55°C-65°C). The idea is to cook the food evenly at the same temperature inside and out. So after a specific amount of time, the food will be uniformly cooked and at the same temperature of the water. So you only need to monitor the water and control a heating element with a PID loop based thermocouple controller (plenty out there for under a $100).

BUT I gather you want to be able to cook a fat piece of meat and not have to guess or calculate the time it takes for the entire piece of meat to heat and cook evenly, essentially an electronic pop-up timer (like those used on turkeys.). Your going to need to spec all your parts to withstand the cooking temps which isn't hard at all since the water temps are low and you can get parts that withstand 70°C, military parts over 100°C. But the size will be tough.

I wouldn't want batteries in my food, especially since they could burst poisoning the food. I am thinking a dead simple voltage controlled RF transmitter that uses an RF signal OR induction coupled power supply (think wireless power). AM modulate a low frequency carrier wave with a thermistor controlled oscillator. Or you could get real clever and use the thermal gain sensitivity of a transistor, crystal or other component to vary the oscillator. Then a simple receiver that feeds a frequency to voltage converter to get your temperature level. Anytime you want to sample the food temperature you simply "ping" the probe with RF or induction power and look at the frequency. This is probably the easiest route to go as its low-tech yet fun to experiment with. You could even use an AM radio as a receiver and if you use audio frequencies for the temperature level tone, hear the temperature. AM transmitters can be built with only a few components so you could solder them together into a ball and stuff inside a largish sealed pill shaped capsule. You could even use a tone decoder like the LM567 to trigger when the right frequency is received.

For a more "digital" option look up "RFID temperature sensor" and check this article out: low power long range ism wireless measuring node

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Isn't it possible to measure the dimensions of the meat, then measure the heat, and do the calculations on how hot the inside of the meat should be?

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  • \$\begingroup\$ It is certainly possible to estimate the internal temperature from the meat thickness, water temperature and time by referring to a table. However, the dangers of sous vide when cooking things like fish is that the internal temperature stays within the danger zone temperature for too long (where the bacteria are being incubated) or never quite reaches pasteurisation temperature. These are real dangers, especially if you have a pregnant woman or a child in the house. In this case, I would really like to be sure the meat was safe. \$\endgroup\$ – Rocketmagnet Sep 21 '12 at 9:27

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