can i attach a needle probe to a temperature sensor?

[BraveWart]

Maybe somebody with some experience with temperature sensors can help me out with this:

I am trying to measure the temperature of an egg (refrigerated to room temperature).

I did various tests doing this via an IR thermal sensor, but it is giving me headaches, as it only measures the temperature of the egg shell and not the inside of the egg. The shell warms up quickly after taking out of the fridge, so the measurements are not really representative of the egg temperature.

My thought now is, to take the temperature measurement via a needle probe. As I couldn't find any such thing online, I was wondering if I can just take a temperature sensor like this LM35 and somehow attach a needle probe to it.

Would that even work?

 

[danadak]

Yes, preferably a Cu needle would be best. But even a SS one would work.

To minimize errors needle and sensor should be close, eg. not a long needle where
a significant portion of needle not immersed in egg fluid. Diameter of course issue,
thicker better to minimize thermal Resistance error.

Note the heat must transfer up the needle into the sensor thermal mass to raise
its T. So maybe a needle with a short small diameter shell penetrator, balance of
needle external to egg much thicker to min thermal R. Or thermal insulation
around needle external to egg. Or both.....

Alternatively you could consider characterizing eggshell thermal R so that you can
calculate the internal T from the thermal path parameters. Hence use the IR sensor. this
Complicated as shell thickness affects this approach. Maybe fluid is relatively fixed and
stable parameters, egg to egg, so you could do an acoustic sensor approach.....

https://www.analog.com/media/en/technical-documentation/application-notes/an131f.pdf

Regards, Dana.

 

[BraveWart]

Hey Dana

Thank you so much for the super helpful and detailed answer! The acoustic approach sounds very interesting. I had never heard of such a thing previously. my design wouldn't immediately allow for such a design, but I will definitely have a closer look at it, should the needle probe not work out after all.
Your thoughts on the IR sensor also support something a friend told me. I do have an accurate room temperature measurement already. If the Tshell IR reading is not a spot reading, but a continuous reading instead, then Tegg could be derived from some function of Delta Tshell over time.
But ultimately: great to hear that attaching a needle to a Temperature sensor would indeed work. I'll rig up some test with needle glued to the mass body of the sensor this weekend and see how it behaves.

Many thanks!

Markus

 

[rjenkinsgb]

If this is a one-off experiment to create a temperature plot?

An alternate approach is to use a batch of eggs, all cooled for the same time and all removed from the refrigerator at the same time and separated afterwards so their proximity does not affect the temperature rise.

At regular intervals, take an egg and wrap in in some good, thick insulating material to [almost] halt the temperature rise, with a temperature probe inside the insulation.

The shell and internal temperature should equalise fairly quickly, giving the actual temperature at that point in time, before the temperature falls slowly again.

You can only use each egg once during the experiment, but they remain undamaged and can be used again.


(Any bare metal needle is going to loose some heat to the shell, so cannot be absolutely accurate. You would need to drill a small hole and insert a tiny bead thermistor or similar, to get a direct reading of the interior).

 

[BraveWart]

I am experimenting at the moment, yes, but I am looking for a solution that will consistently in a repeatable task. That being said: The egg in question will be boiled after being tested. The intention was to pierce the shell with the temperature probe. That didn't bother me, because I am piercing the shell before cooking anyways to equalize pressure and prevent the shell from cracking during boiling.

 

[Pommie]

I am experimenting at the moment, yes, but I am looking for a solution that will consistently in a repeatable task. That being said: The egg in question will be boiled after being tested. The intention was to pierce the shell with the temperature probe. That didn't bother me, because I am piercing the shell before cooking anyways to equalize pressure and prevent the shell from cracking during boiling.

So what is the point of the "experiment". RJ came up with a very good idea to map the temperature change and you appear to have completely ignored it.

Mike.

 

[atferrari]

The "batch" per se would not ensure they come from the same chicken, so to speak, thus with different shell thickness if that comes to be of concern. Just coming from checking so in a 6-units pack in my fridge.