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HTU21D / Si72021 Temperature and Humidity Sensor

I have been been evaluating the HTU21D temperature and humidity sensor made by Measurement Specialties as a possible DHT22 replacement for the emonTH. This is quite a new sensor, released in 2013. The Si72021 is also a posiblity with an identical pin-out and specs.

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The metrics speak for themselves:

Metric HTU21D Si72021 DHT22 DHT22 vs HTU21D Difference
Cost in 1k off £1.42 (July16) £2.15 £4.57 (July16) 3.2 times cheaper (£3.15 less!)
Vcc 2.1v - 3.6V 1.9V - 3.6V 3.3-6V  
Humidity accuracy ±2% RH ±2% RH ±2%RH n/a
Humidity Range 0-100% RH 0-100% RH 0-100% RH n/a
Temperature accuracy ±0.3°C ±0.4°C ±0.5°C 40% more accurate
Temperature Range -40°C +125°C -40°C +125°C -40°C +80°C 56% more accurate
Sleep Current 0.02uA 0.06uA 15uA 750 times less power
Measurement Current 0.045mA 0.09 mA 0.5mA 11 times less power
Measurement time 0.01s - 0.0026s 0.01s - 0.0026s 2s 200 times faster
Energy consumed per sample 1.5uJ 2.97uJ 3300uJ 2000 times less power
Time sampling per day* 14.4s   2800s  
Time sleeping per day* 86386s   83600s  
Energy consumed per day* 2.36mW [1]   2836mW [2] 1201 times less energy per day! 
energy consumed (joule, J) = potential difference (volt, V) × charge (coulomb, C)  
charge (coulomb, C) = current (ampere, A) × time (second, s) 
energy consumed (joule, J)  = potential difference (volt, V) × current (ampere, A) × time (second, s)

*Assuming 1 sample per min and sleeping in between samples, 1440 min per day = 86400s per day. See HTU21D testing repo a sub-repository of the emonTH for full calculations.

An upgraded emonTH with a HTU21D sensor instead of DHT22 should benefit from substantially increased battery life, increased accuracy and lower cost.

I’ve obtained HTU21D Sparkfun breakout to test and easily got it working with an Arduino using the Adafruit library.

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There is some confusion with regard to the variants of this sensor I believe from my research that the HTU21D replaces the SHT21 made by Sensirion. Both sensors have identical pin-outs.

I hope to get a prototype emonTH with HTU21D up and running soon. I would be interested to hear if anyone has experience working with this sensor.

For disucssion see HTU21D forum thread.

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