Temperature sensor accuracy

Question

Hi all, 
 I have two temperature sensors near each other, outside. In the winter I was using one to keep track of the nectar temperature in a hummingbird feeder, but I just left it hanging there for the summer. 
 Here is the typical result: 
 
 The top line is a dht11 or could be 12. It includes pressure and humidity. The bottom line is a ds18b20 in a metal tube. 
 Both are digital, but one is nearly always about 1 or 2 degrees higher than the other. Generally it&rsquo;s fine to give me a close enough idea of what&rsquo;s going on, but I think I should be more accurate when looking at the freezing point. 
 Do I need to calibrate them? How would I go about doing that? Ie, what can I trust as a proper baseline? I don&rsquo;t have access to any really guaranteed kind of thermometer, I think. 
 Thanks in advance for any wisdom you can send my way! 
 -Nico

dougw · Accepted Answer

If they are waterproof you can stick them in a slush of ice and water to get a zero degrees reference. If they aren't waterproof you can put them in a plastic bag in the ice/water mix. 
 If they can take 100 degrees, you can put them in a plastic bag in boiling water for a second reference point. 
 This will tell you which sensor is more accurate and allows 2-point (linear curve fit) calibration.

beacon_dave · Answer

Comparing data sheets it looks like the accuracy of the DHT11 is &plusmn;2&deg;C 
 
 whereas the DS18B20 is &plusmn;0.5&deg;C 
 
 It will be interesting to see what your testing shows.

wolfgangfriedrich · Answer

During the ice-water test, you have to be careful not to touch the ice-cubes. They are still colder than zero degC. I learnt this during a thermocouple roadtest.

shabaz · Answer

Hi Nico, 
 If required, these types of boards are quite neat to act as a sort of semi-reference. The accuracy is claimed to be +- 0.1 degrees C within a certain range. I have an older model that claims +- 0.3 degrees C I think. 
 They can be accessed via Bluetooth on mobile, and can show historical data as well. Link: https://www.newark.com/sensirion/sht4x-smartgadget/ref-design-board-temp-humidity/dp/74AJ2003

Temperature sensor accuracy

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