I'm looking to get a very high precision temperature measurement over a somewhat small range. Ideally, I want at least a 0.001 Celsius increment, over a -5C to 5C range. Searching Digikey doesn't seem to leave me with anything that could accomplish this. Is there a sensor that I could use to accomplish this?
High Precision Temperature reading, over a small range
sensortemperature
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Your original question mentioned a sensor, which now seems to be gone. How precise do you want to keep that luminosity constant? If it's for visual it's pointless: your eye's sensitivity follows a logarithmic curve, so that it can cope with both 1 lux moonlight and 100 000 lux sunlight. Therefore it is not very sensitive to small changes in luminosity.
Most LED manufacturers will give a linear relationship between current and luminosity, so to get the latter constant you only have to keep current constant.
Note however that you have to derate current at higher temperatures:
So for this particular LED you can't go higher than 5mA if you want the same luminosity for 25°C and 85°C.
The Supertex CL25 is a temperature compensated constant current source for LEDs.
This has a 0.01%/°C typical temperature coefficient. It is 25mA however, so you'll have to use a LED which doesn't need to be derated below that.
Rocketmagnet makes an important remark: "It may be the phototransistor which is sensitive to temperature." (RM, I hope you don't mind that I copy it here.)
Indeed, it's no use to have a temperature controlled LED driver if the sensor's reading varies highly with temperature. You'll have to look into that too.
Output voltage pec is 750mV @ 25'C nom and your reading is what from cold start to after a few minutes? Does it change?
Spec is 1000 pF load max you are using twisted cable which is typically 5 pF/cm depending on tpi, diam. etc., so 50cm x 5 = 250 pF, so cable seems ok to prevent instability and oscillation. If you don't have a microamp analog meter get one or use series drop R to estimate.
Self heating from 5V should be less < 0.05'C unless it is damaged so measure current against spec to ensure self-heating is not the issue.
open air thermal time constant is < 1 minute.. immersed in ice water is 3 seconds . what are the calibration results you get?
- Do you have 0.1 µF cap across IC?
- Did you add an op amp to compensate for offset only inherent to TP36 and not TMP35 or TMP37? VOUT @ 1mV/°F – 58°F It is designed for 'F
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Best Answer
I would use a platinum RTD for this purpose. Some precision thermistors might also be suitable.
If you calibrate the sensors and signal chain individually at the triple point of water (single point calibration) then the accuracy of the sensor should be fairly good over such a narrow range.
The optimal current will depend on whether the sensor is in contact with the water and on sensor construction (self heating variation) but at say 0.2mA a 100 ohm RTD will give you a fairly healthy 77 uV per millikelvin, so a good performance signal chain should not be limiting.