SHT11 :: Humidity and Temperature sensor by Sensrion FromParallax

Original Report by Keunyoung Oh Fall 2005

I. Measurering Humidity

0. Hygrometer

1.Resistive Humidity Sensors

Resistive humidity sensors measure the change in electrical impedance of a hygroscopic medium such as a conductive polymer, salt, or treated substrate. Resistive sensors usually consist of noble metal electrodes either deposited on a substrate by photoresist techniques or wire-wound electrodes on a plastic or glass cylinder. The substrate is coated with a salt or conductive polymer. When it is dissolved or suspended in a liquid binder it functions as a vehicle to evenly coat the sensor. Alternatively, the substrate may be treated with activating chemicals such as acid. The sensor absorbs the water vapor and ionic functional groups are dissociated, resulting in an increase in electrical conductivity.

Humidity Frequency Converter and Humidity Voltage Converter (Galltec)

C7-M3 (Shinyei)

2, Capacitive Humidity Sensors

Capacitive Humidity Sensors consist of a substrate on which a thin film of polymer or metal oxide is deposited between two conductive electrodes. The sensing surface is coated with a porous metal electrode to protect it from contamination and exposure to condensation. Thin film capacitive sensors may include monolithic signal conditioning circuitry integrated onto the substrate. The most widely used signal conditioner incorporates a CMOS timer to pulse the sensor and to produce a near-linear voltage output.

3, Thermal Conductivity Humidity Sensors

These sensors measure the absolute humidity by quantifying the difference between the thermal conductivity of dry air and that of air containing water vapor. In general, absolute humidity sensors provide greater resolution at temperatures >200°F than do capacitive and resistive sensors, and may be used in applications where these sensors would not survive.(such as clothes dryers and both microwave and steam-injected ovens, kilns for drying wood; machinery for drying textiles, paper, and chemical solids; pharmaceutical production; cooking; and food dehydration.)

II. SHT11 (Digital Capacitive Humidity Sensor)

The SHTxx is a single chip relative humidity and temperature multi sensor module comprising a calibrated digital output. Application of industrial CMOS processes with patented micro-machining (CMOSensĘ technology) ensures highest reliability and excellent long term stability. The device includes a capacitive polymer sensing element for relative humidity and a bandgap temperature sensor. Both are seamlessly coupled to a 14bit analog to digital converter and a serial interface circuit on the same chip.


* serial interface

1. sending a command

Serial Data(DATA)
Serial Clock Input(SCK)
 - code

2.Reading thd data

MSB : Most Significant Bit

LSB : Least Significant Bit

Measurement Sequence

0000 1001 0011 0001 = 2353 = 75.79%RH @ 25C

3,Converting output to physical valure

' linearize humidity

rhLin = (soRH ** $67AE) - (soRH ** $83 * soRH ** $5B) - 40

' temperature compensated humidity

rhTrue = (tC - 250) * (soRH ** $34) + rhLin

III. Applications

_ HVAC (Heating, Ventilation and Air Conditioning)

_ Automotive

_ Consumer Goods

_ Weather Stations

_ (De-) Humidifiers

_ Test & Measurement

_ Automation

_ White Goods

_ Medical (incubator)

_ Copy machine

_ Air purifier Sharp FPR45CX(for specialmode for high temp & high humid)

IV. Code Samples

V. (non-)Typical Behavior

Warning : not recommend use more than 5V. Try below 4.5V unless you need super fast respond.

SCK changes its frequency upto VDD.( VDD>4.5V : 10MHz(max), VDD<4.5V : 1MHz(max))

As a result, my SHT11 was burned out :-(

--- correction.

just use right resistor :4.7 K then, it should be fine... I guess.