[time-nuts] What do people use for measuring temperature?

[Bob kb8tq]

Hi

Going back a bit to get closer to the original request …..

Indeed a thermistor is the “high resolution” king of the hill when it comes to temperature
measurement. Resistance change of 3% (30,000 ppm) per degree is not uncommon, you can do better …
They come in all sorts of packages for not a lot of money. eBay will happily supply you with 
a ton of parts. Go with high resistance parts to reduce the self heating issues mentioned below ….

Since the original request was not concerned about accuracy ( only resolution) the cheap
glass body eBay parts may do just fine. You would need to do a bit of characterization / 
curve fitting to work out what’s what. 

If you are after 0.001C resolution you would need to measure resistance to 30 ppm. That
can be done. Indeed a bridge setup and looking at voltage is a more normal way to do this.
For narrow ranges an op-amp on the bridge will give you lots of voltage. The main issue is
indeed living with the range limitation. 

Yes this is a bit of “cobble it together” nonsense. You can indeed get a pretty good setup that
will give you mili C resolution and tens of mili C accuracy. Best guess is that a single sensor
system you can trust will be over $5K. The sensors will be giant RTD probes that are a bit 
useless for looking at small devices …. sorry about that ….

======

Since this is time nuts ……

One can build an R/C oscillator with a thermistor. You can measure the period of the output 
and get temperature that way. Resolution (obviously) is not going to be hard to get with a 
Time Nut grade counter. Don’t bother … the jitter is going to get in the way long before you get
to 0.01, let alone 0.001C ….

Bob

> On Sep 26, 2020, at 7:18 AM, Manfred Bartz <vk3aes at gmail.com> wrote:
> 
> A thermistor should do the job.  You can buy them in SMD packages and
> down to 0.1% accuracy.
> How much resolution you get depends on the measurement range and the
> ADC you are using.
> 
> A platinum RTD would be another candidate but requires more signal
> conditioning.
> In 3-wire or 4-wire probe configuration you can compensate for long probe wires.
> 
> Any sensor you choose should have a thermal mass less that the item
> you want to measure.
> Generally, a smaller sensor means smaller thermal mass.
> 
> If you really need to resolve 0.01ºC or 0.001ºC then you also need to
> pay attention to:
> * sensor self-heating
> * consider turning off excitation between measurements
> * with a thermistor, go with a high R25,  i.e. 100kΩ which will help
> with keeping self-heating low.
> * Temperature coefficient and environment of the thermistor's series resistor.
> * Stability of the supply/reference voltage.
> 
> BTW, 0.1% resistors are sensitive to static discharge. A zap can
> easily produce a 0.5% change!
> 
> Having really good and stable thermal contact is essential.
> The item you are measuring and the sensor should be in an isolated
> micro environment.
> Airflow or proximity to anything of a different temperature will
> potentially cause a temperature gradient between sensor and the
> measured item.
> 
> All passive temperature sensors require some sort of linearization,
> but that could be done away from the sensor or during post-processing.
> For thermistors search for the "Steinhart Hart equation".
> 
> I am not aware of active (smart) sensors with have better than 0.1ºC
> resolution, but I also have not done any significant search lately.
> 
> Cheers
> --
> Manfred VK3AES
> 
> On Sat, Sep 26, 2020 at 12:46 PM Hal Murray <hmurray at megapathdsl.net> wrote:
>> 
>> 
>> I've got a collection of 1-wire gizmos and USB thumb drives.  They are great
>> for many applications but I'm looking for something better/different.
>> 
>> I'd like something that reads to 0.01 degree or 0.001 degree.  I don't need
>> accuracy.  What I want is reasonable linearity so I can make pretty graphs.
>> 
>> I'd like the actual probe to be small enough so I can poke it inside gear like
>> a PC and attach it to a crystal.
>> 
>> I'm looking for a USB or serial connection so I can log the data.
>> 
>> Is there an obvious brand/whatever I should be looking at? thermistor?
>> thermocouple?  ...
>> 
>> I don't care about a display on the device.  I don't want a logger, they fill
>> up.  I want to grab the data on the fly and do my own logging.  (But I'm happy
>> to use a logger if it will do what I want.)
>> 
>> 
>> --
>> These are my opinions.  I hate spam.
>> 
>> 
>> 
>> 
>> _______________________________________________
>> time-nuts mailing list -- time-nuts at lists.febo.com
>> To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com
>> and follow the instructions there.
> 
> 
> 
> -- 
> Manfred  VK3AES
> 
> _______________________________________________
> time-nuts mailing list -- time-nuts at lists.febo.com
> To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com
> and follow the instructions there.