# What happens when reaching the max temperature on a temperature sensor?

I need to monitor the temperature of a hot plate so it stays at 210C

I am just using a Arduino and lcd to display the temp using a DS18S20 (I have it ready but not actually using it to measure the plate with this) but that's only rated to 100C so I am buying a LM34CZ DKF503N5 / DKF103N5 which goes to 250C.

My question is what will happen if it goes above 250? will it de calibrate it? will it explode? or something else? It is possible that it could go higher than that but for short periods of time until I can get the correct temperature on the plate.

I did read a few datasheets and it does not say anything about exceeding the rated temperatures.. that is why I am asking you experienced guys..

EDIT

Sorry I was on a US site and never realised it was F i switched over to UK and got the correct ones.. completely different models though

• Correct me, if I'm wrong, but this datasheet: datasheet.octopart.com/… says that the LM34CZ has a max. temp of 230°_F_, which is 110°C. Maybe you have to take a step back and think of another sensor (maybe you have to go for a thermocouple). – 0x6d64 Dec 9 '11 at 9:28
• My Bad.. found another one actually that it 250C. Sitll. waht happens if i cross the thresh hold? – Piotr Kula Dec 9 '11 at 10:17
• I am not a masterful writer, but it really helps if the titles gives at least a quick idea of what you want to know. It was a bit broad before, please tweak it to read better if the idea catches your fancy. – Kortuk Dec 9 '11 at 10:29
• @Kortuk What don't you understand from the title. It asks what happens when you reach the max temperature of a sensor.. That is pretty specific to me, unless you want me to include part numbers in the title? – Piotr Kula Dec 9 '11 at 10:35
• @ppumkin, I wrote "What happens when reaching the max temperature on a temperature sensor?". It originally said, "What happens at high temperatures?" I did miss part of my thought in my comment. I meant to say "it was a bit broad before, I have edited it, please tweak it to read better if the idea catches your fancy." However, I was also trying to point out it can be valuable to have a more specific title then the original. I am sorry for the confusion. – Kortuk Dec 9 '11 at 12:01

Thanks to 0x6d64 for pointing out the C/F discrepancy.

The data sheet says:

Rated for full −50° to +300°F range

In C that is -45°C to 148°C - not enough for your needs.

Your main problem at that kind of temperature is going to be the melting of the solder you're using to attach the LM34CZ.

Personally I'd use a thermocouple. Rated an hundreds of degrees (usually 400+), they allow you to keep all sensitive circuitry away from the hot area.

You'll need a converter chip to allow you to reliably use a thermocouple of course. Most people use the MAX6675, but that is being phased out to be replaced by the MAX31855.

Both are surface mount, so you'll need to etch a PCB to mount it on, or get a breakout board (8 pin SOIC).

Both the chips are pin compatible with each other, and provide an SPI interface the Arduino can talk to easily.

I have had a MAX6675 working on my Arduino, and am waiting for some MAX31855's to arrive so I can check to see if my Thermocouple Library for the Arduino works with them.

• that kind of temperature is going to be the melting of the solder - Yup - that is what I want :) – Piotr Kula Dec 9 '11 at 10:15
• I have found an IR gun on ebay for like 9quid. I might just buy that and save all the headaches making the thing.. but thermoupler is the correct answer in this case - and if i want ot build a controlled environment.. but still- what will hapen if i go voer the rated temperatures? – Piotr Kula Dec 9 '11 at 10:21
• With a thermocouple? The junction will break down / melt. It'll be hard for you to get hot enough - the element in the hot plate will burn out before then. – Majenko Dec 9 '11 at 10:34
• And with a thermistor? – Piotr Kula Dec 9 '11 at 10:36
• You will basically cook it. It will never work again. Exactly what happens depends on the technology, but it won't be pretty. It should be able to "cope" with heats in it's reflow zone for small amounts of time (30 seconds or so) but not much more – Majenko Dec 9 '11 at 11:05

You ask specifically about what happens if a temperature exceeds a certain temperature. The answer is (like in a lot of other cases): Look in the datasheet. Usually you will find a value called "absolute maximum ratings": You can't really know what happens if you exceed these values. Your chip might take an internal damage (this can also mean: reduced performance), or the case might melt. Do not go there, so you don't have to care how exactly the chip will take damage or fail.

Other than that, there is the operating temperature range, where the manufacturer guarantees the accuracy of a part. Always chose your sensor so that all expected temperatures where you want reliable measurements are in that range. Notice: Sensors usually are more accurate in the middle of the measurement range, manufacturers will sometimes tell you in their datasheet:

So what sensor should you then use? In every case you have to answer these questions (maybe another more):

1. Will this sensor handle the temperature? Will it be accurate enough?
2. How do I read the sensor value? The DS18S20 can be read via 1-Wire interface, a "naked" NTC will need some circuit around (just like a thermocouple).
3. In your case: How will I mount the sensor, can I solder it

The NTC you choose will not be very accurate at this temperature, as the resistance changes only a little per degree at high temperatures. As this document says (p. 27), thermistors are somewhat sensitive to overtemperatures. You have also to look for a method to read out the sensor and calculate the temperature (NTCs are non-linear). You will also have to think about how to connect the sensor to your Arduino (soldering is not an option, you might have to crimp or sth. similar).

Considering this, I would go with the answer from Majenko: Thermocouples are good at measuring high temperatures (the accuracy under 100°C is not important to you, I guess) and they come with their leads already properly attached. If you combine this with the IC Majenko named, you don't have to mess around with calculating the actual temperature or how to do an A/D conversion. You might only be careful about the different types of thermocouples and chose the right thermocouple/IC combination.