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Old 06-05-2012, 02:48 AM   #21
kladue
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Thermistors are not seen in much beyond the cheapo commercial controllers, the temperature range limitation and non linearity work against their wide spread use.
A better choice is a 0-5V output amplified temperature sensor that is precalibrated and delivers linear response to temperature change like the AD22100K from Linear, no curve correction formulas are needed, and no calibration when switching sensors.
With the low cost single chip solutions out there now for thermocouples, it would seem to be better to devote time and effort in that direction as thermocouples do not require external drive power, are readily available, and usually do not additional calibration when switching sensors.

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Old 06-05-2012, 10:37 PM   #22
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Hi

Well, for some applications (like high precision single set point stuff) thermistors are very much the sensor of choice and have been for at least 50 years. For wide range work, indeed their rapid rate of change works against them.

Bob

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Old 06-07-2012, 06:08 AM   #23
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Thermistors are much more accurate than thermocouples. Keep in mind that thermocouples need a second temp sensor for the cold junction reference. Therefore the thermocouple is no more accurate than its cold junction reference. I have seen thermistors used for cold junction compensation on almost all of the thermocouple circuits I have encountered. National instruments uses them on their $500 plus precision data acquisition instruments.

The non linearity comment is bogus. You can easily compensate for this by using a look up table.

Also thermistors are extremely easy to implement. All you need is a single resistor and a voltage source to bias them.

Thermocouples produce extremely small voltages that need to be amplified with a gain of at least 30 to be able to read them with an ADC. Any inaccuracies in your amplifier circuit will be multiplied by your gain value. You will have to use resistors with very high tolerances. In the end you will still have to calibrate your amplifier circuit to account for these errors.

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Old 06-07-2012, 04:24 PM   #24
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Hi

With a thermistor both the nominal resistance and the beta (rate of change) vary from part to part. For enough money you get pretty tight tolerances on both numbers. You can also calibrate each one and eliminate the issue.

The gotcha comes when you want to do wide range (say -55 to +125C) measuremnet. Thermistors simply change to much over a range like that for a simple circuit to work well. Thermocouples or RTD's are a better choice there. Thermocouples are the bang for the buck kings when it comes to wide range measurement.

Bob

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Old 04-20-2014, 02:05 AM   #25
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You weld these yourself or did you buy these somewhere?
Thanks,
-Chris

Quote:
Originally Posted by TheFlyingBeer View Post
I am using the DS18B20's throughout, made some tri-clamp thermowells for easy removal/cleaning:



The cost of monitoring additional temperatures throughout the brewing room becomes very low when using these digital sensors. Even at 750ms polling for the highest resolution you can use basic filtering to provide more timely inputs to control algorithms if needed.
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