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fabbie said: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?
fabbie said:"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 ---fabbie said:"You would have to calibrate it against a known accurate device. "
Do u happen to know any accurate device i can use? Any examples?
williB said:thats easy ---fabbie said:"You would have to calibrate it against a known accurate device. "
Do u happen to know any accurate device i can use? Any examples?
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..
What in my post was inaccurate..Oznog said:williB said:thats easy ---fabbie said:"You would have to calibrate it against a known accurate device. "
Do u happen to know any accurate device i can use? Any examples?
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 said:What in my post was inaccurate..
i believe i stated that the thermister wasnt linear
Oznog said:williB said: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.
not true , i have done itBoiling water and ice are two classic reference points, but they don't work for thermistors
i believe i said that also..it's a huge load of work and still quite inaccurate when you could just use the mfg's data for that part.
thanks .. looks like a nice application..csaba911 said:Here is a small app from ATEECC to calculate resistance of THR
**broken link removed**
what adc are you going to be using?fabbie said: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