DHTxx family Humidity Sensors
(Models DHT11/DHT21/DHT21 and also known as AM2303)
Figure 1. Typical DHT humidity sensor. [1]
Introduction
The DHT family of humidity sensors consist of a capacitive humidity sensor and a thermistor. It is available in a few package types, most commonly in a 15mm x 25mm plastic box with four leads. The different models work with the same principles, but they have slightly different specifications and accuracy.
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Low cost
3 to 5V power and I/O
2.5mA max current use during conversion (while requesting data)
Good for 20-80% humidity readings with 5% accuracy
Good for 0-50°C temperature readings ±2°C accuracy
No more than 1 Hz sampling rate (once every second)
Body size 15.5mm x 12mm x 5.5mm
4 pins with 0.1" spacing
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Applications: Arduino + Humidity Sensor + Temperature Sensor
Sensor details: https://www.youtube.com/watch?v=C7uS1OJccKI
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Arduino code: http://playground.arduino.cc/Main/DHTLib
DHT11 power consumption
Figure 2. Power consumption sheet.
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Arduino requires about 46.5 mA current in order to operate properly and sleeping power consumption 34.4 mA.
DHT11 material (used in component)
‘This sensor includes a resistive-type humidity measurement component and an NTC temperature measurement component, and connects to a highperformance 8-bit microcontroller, offering excellent quality, fast response, anti-interference ability and cost-effectiveness.’ [8]
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So basicly humidity sensor is made out of regular material such as silicon microchip, ceramic body and plastic casing?
Relative Humidity
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‘Relative humidity is the ratio of the current absolute humidity to the highest possible absolute humidity (which depends on the current air temperature). A reading of 100 percent relative humidity means that the air is totally saturated with water vapor and cannot hold any more, creating the possibility of rain. This doesn't mean that the relative humidity must be 100 percent in order for it to rain -- it must be 100 percent where the clouds are forming, but the relative humidity near the ground could be much less.’ [2]
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The higher the percentage, the more humid. The higher the relative humidity, the less drying takes place. This affects the human bodies' perception of heat, as explained next.’ [3]
Figure 3. [7]
Figure 4. [7]
Capacitive Humidity Sensor and Physical principles models
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A capacitive humidity sensor is used to measure relative humidity with a usual range of 5 to 95 % relHum. The sensor is a capacitor, made from two metal electrodes with a porous dielectric substance between them. Water vapour is able to penetrate this layer, changing the total capacitance. Typical capacitance at 50% relHum is 100 to 500pF, with 0.2 to 0.5pF change per 1% relHum.
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AM2302 / DHT22 | DHT11 | |
Range | 0-100% | 20-90% |
Absolute accuracy | ±2% | ±5% |
Repeatability | ±1% | ±1% |
Long term stability | ±0.5% per year | ±1% per year |
Typical street price | US$ 4-10 | US$ 1-5 |
Raw usage
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Most humidity sensors available are sold as units that include the capacitive humidity sensor, a temperature sensor and a microcontroller that reads, calculates and transmits this data.
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To actually get any usable humidity values, a relatively accurate temperature measurement is also needed, as relative humidity is very dependant on temperature.
http://www.arduino.cc/en/Tutorial/CapacitanceMeter
Communication
DHT sensor family uses single wire serial communication.
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Sensor proceeds to send the data, each bit starts with 50µs low, then the bit data itself; 26 to 28µs for ‘0’, 70µs for ‘1’
Figure 5. Timing Diagram. [5]
Summary
‘Humidity sensors are sensors that convert the moisture content in air, gases, bulk materials or soil into an electric output signal.
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Major amounts of humidity sensors are required by the various process industries such as the food industry, the mining sector, the cement industry, the chemical industry, the pharmaceutical industry and many more. They are used for product quality control, in-process quality testing and for research and development. In the food industry, humidity sensors help to guarantee the quality level of flour and potato chips. Moisture sensors are also of importance in the pharmaceutical industry; here they help to avoid excessive humidity, which could alter the quality features of the final products.’ [6]
Reference
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[1] | Figure 1. Picture - |
[2] | http://science.howstuffworks.com/dictionary/meteorological-terms/question651.htm |
[3] | |
[4] | Chart 1. - http://www.kandrsmith.org/RJS/Misc/calib_dht22_dht11_sht71.html |
[5] | Timing Diagram - http://mcuoneclipse.com/2015/03/27/using-the-dht11-temperaturehumidity-sensor-with-a-frdm-board/ |
[6] | |
[7] | Datasheet - http://www.ist-usadivision.com/objects/media/data-sheets/product/humidity/MK33-W.pdf |
[8] | DHT11 Datasheet - http://www.micropik.com/PDF/dht11.pdf |
[9] | DHT22 Datasheet - http://www.adafruit.com/datasheets/DHT22.pdf |
[10] | Capacitive Sensor - http://machinedesign.com/news/sensor-sense-humidity-sensors |
[11] | Hygrometer - http://en.wikipedia.org/wiki/Hygrometer#Capacitive_humidity_sensors |
