# Temperature & Humidity Sensor Evaluation and Testing



| Metric              | [HTU21D](https://octopart.com/htu21d-measurement+specialties-30374934)             |[Si72021](https://octopart.com/si7021-a20-im-silicon+labs-52675246)| DHT22          | DHT22 vs HTU21D Difference| [BME280](https://octopart.com/bme280-bosch-52931981) |
| ------------- | -------------            |------------- | ------------- | ------------- | ------------- |
| Cost in 1k off      | £1.42 (July16)     |£2.15|  £4.57 (July16)    | 3.2 times cheaper (£3.15 less!) | £3.07 |
| Vcc                 | 2.1v - 3.6V        | 1.9V - 3.6V | 3.3-6V | | 1.71 V - 3.6V |
| Humidity accuracy   | ±2% RH             | ±2% RH |  ±2%RH         | n/a | ±3%RH |
| Humidity Range      | 0-100% RH          | 0-100% RH |  0-100% RH     | n/a|  0-100% RH  |
| Temperature accuracy | ±0.3°C            | ±0.4°C|  ±0.5°C        | 40% more accurate | ±0.5 - ±1.0 |
| Temperature Range   | -40°C +125°C       |  -40°C +125°C  |  -40°C +80°C   | 56% more accurate | -40°C +80°C  |
| Sleep Current       | 0.02uA             | 0.06uA |  15uA          | 750 times less power | 0.1uA - 0.3uA |
| Measurement Current | 0.045mA            | 0.09 mA |  0.5mA         | 11 times less power | 0.714mA - 0.350mA |
| Measurement time    | 0.01s - 0.0026s    | 0.01s - 0.0026s |  2s            | 200 times faster | 0.013s |
| Energy consumed per sample | 0.00045mW   ||  1mW           | 2222 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 per day*

- **HTU21D:** (14.4s * 0.045mA) + (86386 * 0.00002mA) = 0.63mW + 1.73mW = **2.36mW** [1]
- **DHT22:**  (2800s * 0.55mA) + (86386 * 0.015mA) = 1540mW + 1295.8mW = **2836mW**  [2]

\*Assuming 1 sample per min and sleeping in between samples, 1440 min per day = 86400s per day

[Adafruit HTU21D Library](https://github.com/adafruit/Adafruit_HTU21DF_Library)