The HS07 humidity sensor

PRODUCT DESCRIPTION

The Smartec HS07 humidity sensor is a two terminal capacitor, which increases in value as water molecules are absorbed into its active polymer dielectric. The humidity sensor is based on silicon technology on glass wafer.

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The sensor will be delivered with gold-plated lead wires attached. Besides the active dielectric, polymers give protection to the base plate, prevent polarisation and pinholing and generally ruggedise the sensor so that it can be used in a wide range of hostile environments.

FUNCTIONAL DESCRIPTION

The measuring of humidity is difficult. Generally the humidity in air is measured as the fraction of the maximum amount of water that can be absorbed at a certain temperature. In air, at a given temperature, this fraction can vary between 0 (absolute dry) and 100 (the point where condensation will begin to form). Many classical sensing techniques, such as mechanical devices and resistive type sensors, are temperature dependent and the wet bulb thermometer is pressure dependent. With the SMARTEC Humidity sensor RH can be measured without the complicating factor of temperature and pressure.

MEASURING THE CAPACITY VALUE

The small capacitor value of the sensor gives you the relative humidity. The capacity is about 330pF @ 25°C and 55% RH. The variation of the capacity equals 0.6 pF / %HR. (Meaning 60pF over 0-100% HR). The capacity can be measured in several different ways.

Analogue solution
A very simple analogue circuit, using two TLC555, has a DC output. This application has a very limited accuracy. It is described in application note APPHS1001.PDF.

Digital solution
In the digital corner a much more sophisticated solution is available: the UTI.

If you want to know more about how this works, please read APPUTI06.PDF "Startup guide for measurement of capacitors with three signal technique, January 2003"

For our national readers a French version is available: "Mesurer précisément des petits signaux issus de capteurs distants avec un microcontrôleur, janvier 2003" (APPUTI12FR.PDF).

CALIBRATING THE SENSORS

In order to obtain the maximum possible accuracy, it is necessary to calibrate the sensor and the measuring circuitry. How humidity sensors can be calibrated, is explained in application note APPHS1002.PDF.

SUPPORT

All relevant documentation and files (data sheet, application notes, software etc.) for our products can be downloaded from the SUPPORTSHOP page.

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