ADC for temperature

[fabbie]

Hi guys, i was just wondering if i used a thermistor to measure the temperature of a motor, how would i know '1 celcius = how many mV?'.

Is there any kind of datasheet to refer to?

Thx

 

[Nigel Goodwin]

Hi guys, i was just wondering if i used a thermistor to measure the temperature of a motor, how would i know '1 celcius = how many mV?'.

Is there any kind of datasheet to refer to?

You would have to calibrate it against a known accurate device.

A further complication is that thermistors are not usually linear, so you would need to plot a graph of temperature against resistance - you can then use the graph to read off the temperature for a specific resistance. With a microcontroller solution you would do this with a lookup table.

 

[fabbie]

"You would have to calibrate it against a known accurate device. "

Do u happen to know any accurate device i can use? Any examples?

 

[Nigel Goodwin]

"You would have to calibrate it against a known accurate device. "

Do u happen to know any accurate device i can use? Any examples?

An accurate thermometer!.

 

[williB]

"You would have to calibrate it against a known accurate device. "

Do u happen to know any accurate device i can use? Any examples?

thats easy ---
you take ice measure its voltage output (0 centegrade)
boil water ( 100 centegrade)
then like Nigel said you have to ' interpolate ' the results ..usually the graph of the in between results come with the part..

 

[sarang1_in]

Better use LM35 temp. sensor. It is linear and need not be caliberated. It gives a 10mV rise/degree celcius. Its simple and does have good accuracy. I used a lot of them.
Check this link for datasheet. It is 3 pin device. Pin 1- +5V, Pin 2- o/p, Pin 3- Gnd.
https://www.google.co.in/search?hl=en&ie=ISO-8859-1&q=LM35&meta=

 

[sarang1_in]

Better use LM35 temp. sensor. It is linear and need not be caliberated. It gives a 10mV rise/degree celcius. Its simple and does have good accuracy. I used a lot of them.
Check this link for datasheet. It is 3 pin device. Pin 1- +5V, Pin 2- o/p, Pin 3- Gnd.
https://www.google.co.in/search?hl=en&ie=ISO-8859-1&q=LM35&meta=

 

[Oznog]

"You would have to calibrate it against a known accurate device. "

Do u happen to know any accurate device i can use? Any examples?

thats easy ---
you take ice measure its voltage output (0 centegrade)
boil water ( 100 centegrade)
then like Nigel said you have to ' interpolate ' the results ..usually the graph of the in between results come with the part..

I'm afraid you are being fed inaccurate advice!!

A thermistor is nowhere near linear and interpolation is useless over 100 deg span. It is accurate for most purposes with around a 5 deg span between data points.

Nor would you ever need to measure a bunch of data points for a thermistor against a known temp. Their response is a fixed formula (I do not have it handy, look it up). Now the data sheet will only guarantee its constants to 2%, 5%, 10%, whatever. If you want to be more accurate than the tolerance, you can measure a single temp, figure out the exact resistance coefficient for that device, and you know what you need to know to calculate temp quite accurately via interpolation with a lookup chart or a nonlinear calc.

The LM35 and others ARE linear, thus much easier to use. They do have significant inaccuracies (calibration compensated for it). I also noted none of the ones I saw handled cold temps below freezing.

There's the DS18S20 temp sensor, has a wide temp range, very accurate, already digital. If you've got a microcontroller, it's a tempting option.

 

[williB]

"You would have to calibrate it against a known accurate device. "

Do u happen to know any accurate device i can use? Any examples?

thats easy ---
you take ice measure its voltage output (0 centegrade)
boil water ( 100 centegrade)
then like Nigel said you have to ' interpolate ' the results ..usually the graph of the in between results come with the part..

I'm afraid you are being fed inaccurate advice!!

A thermistor is nowhere near linear and interpolation is useless over 100 deg span. It is accurate for most purposes with around a 5 deg span between data points.

Nor would you ever need to measure a bunch of data points for a thermistor against a known temp. Their response is a fixed formula (I do not have it handy, look it up). Now the data sheet will only guarantee its constants to 2%, 5%, 10%, whatever. If you want to be more accurate than the tolerance, you can measure a single temp, figure out the exact resistance coefficient for that device, and you know what you need to know to calculate temp quite accurately via interpolation with a lookup chart or a nonlinear calc.

The LM35 and others ARE linear, thus much easier to use. They do have significant inaccuracies (calibration compensated for it). I also noted none of the ones I saw handled cold temps below freezing.

There's the DS18S20 temp sensor, has a wide temp range, very accurate, already digital. If you've got a microcontroller, it's a tempting option.

What in my post was inaccurate..
i believe i stated that the thermister wasnt linear

 

[Oznog]

What in my post was inaccurate..
i believe i stated that the thermister wasnt linear

Boiling water and ice are two classic reference points, but they don't work for thermistors. A thermistor is far too nonlinear for this type of interpolation. Even if you had a reference thermometer, it's a huge load of work and still quite inaccurate when you could just use the mfg's data for that part.

 

[williB]

What in my post was inaccurate..
i believe i stated that the thermister wasnt linear

Boiling water and ice are two classic reference points, but they don't work for thermistors. A thermistor is far too nonlinear for this type of interpolation. Even if you had a reference thermometer, it's a huge load of work and still quite inaccurate when you could just use the mfg's data for that part.

Boiling water and ice are two classic reference points, but they don't work for thermistors

not true , i have done it

it's a huge load of work and still quite inaccurate when you could just use the mfg's data for that part.

i believe i said that also..

 

[csaba911]

Here is a small app from ATEECC to calculate resistance of THR
**broken link removed**

 

[williB]

Here is a small app from ATEECC to calculate resistance of THR
**broken link removed**

thanks .. looks like a nice application..

 

[lavenatti]

While boiling water is an easy reference point for the amatuer, ice is not.

Waters temperature won't rise above 100 degrees C unless it's under pressure, it'll just boil harder. (Water will boil at lower temperature at higher elevations, the 100 degree C boiling point is at sea level) Ice will be the temperature of your freezer when you take it out, this is usually considerably colder than 0 degrees C.

A good thermometer is your best bet. Even a lousy one will suffice if you don't need the greatest accuracy.

 

[Beefer3]

actually, if you want a 0 C reference you could use ice. all you need is a good insulated thermos, crushed ice and a little bit of water.

crush the ice into little particles and put into the thermos. add a little water to make a slurry. use a ratio of about 75% ice and 25% water. insert your probes into the slurry and stir. cap the top and let the probes stabilize for about 10 minutes then take your readings.

if properly sealed, this will give you about a 1 to 2 degree C accuracy for about 30 minutes.

 

[fabbie]

Im not really sure how to calibrate the results, but should i perform this:

1) Get the voltage across resistance at 0 C
2) Use the voltage at 0 C as my reference point
3) Continously Add 10mV to the reference point from 0 C to 100 C

Since the LM35 is linear, should i do it this way? OR should i use the tedious way of observing the thermometer reach from 0 to 100 C at the same time reading the voltage drop across the resistors

 

[williB]

Im not really sure how to calibrate the results, but should i perform this:

1) Get the voltage across resistance at 0 C
2) Use the voltage at 0 C as my reference point
3) Continously Add 10mV to the reference point from 0 C to 100 C

Since the LM35 is linear, should i do it this way? OR should i use the tedious way of observing the thermometer reach from 0 to 100 C at the same time reading the voltage drop across the resistors

what adc are you going to be using?
This is from the LM35 data sheet

https://www.electro-tech-online.com/custompdfs/2004/11/LM35.pdf

 

[fabbie]

oh sorry for not mentioning it.
Im planning to use the PIC ADC module. Is my method correct to determine the correct temperature?