# Designing a sensor.....

Discussion in 'Sensors and Actuators' started by Trippin, Nov 1, 2012.

1. ### Trippin

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Nov 1, 2012
I am in the process of designing a temp sensor that will be submerged in an oil bath via a probe/housing that I have had custom made for my appplication. The problem is I'm trying my dammedest to obtain the correct sensor to install in my probe. The manufacturers are asking for what accuracy I need and my B value....This is where I got lost. I know my temp range, I believe I need a 50K ohm thermistor (NTC) but this B value has got me stumped, I've found all sorts of complex math to aid my decision as to which B value I need but that's where I cant keep up. Equasions that combine letters and numbers just do me in.

So can someone here aid in my selection of the proper thermistor to work with the gauge package I have selected? I am far wiser today than I was yesterday on this subject, but not wise enough.

Thanks...

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2. ### Harald KappModeratorModerator

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Nov 17, 2011
You won't get off without some math. An NTC has a non-linear characteristic. The B-value essentially defines the resistance as a function of temperature. Reversely you can compute the temperature from the resistance, too.
It's all wrapped up here.

3. ### Trippin

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Nov 1, 2012
I have already been down that road and as I mentioned earlier, its the math that stumps me. Thanks for the link although I've already seen it a dozen times. My goal here is to find someone that might be able to put the process into laymans terms for me. If you'll notice the pic on WIki is the same pic as mine.

4. ### duke37

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Jan 9, 2011
You have not said what your temperature range is or the circuit which needs to be driven. I have not looked into the specification of thermistors but I thought they had quite a wide tolerance.
As to the B factor, I would chose a device with a common value rather than an exotic one.

5. ### Trippin

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Nov 1, 2012
Thanks Duke,

The temp range is 0C-300C, the circuit that is being driven falls into the reference voltage category of 5vts or less. It will be an oil temperature sensor. Every manufacturer is asking for the B value, is that the slope? I see there is like 10 different slopes or more. I really didnt think was going to be that difficult.

Last edited: Nov 2, 2012
6. ### duke37

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Jan 9, 2011
I have calculated the resistance for B values 3500,4000,4500.

I could not upload an XLS file so printed the results and scanned them.

The resistances vary very rapidly at low temperatures but little at high temperatures.
You may be better off using a thermocouple which will be almost linear.

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7. ### Trippin

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Nov 1, 2012
Thanks again Duke. Im still a little lost, but im gaining on it.