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PICAXE Humidity Measuring Using The HopeRF HH10D

Recent SILICON CHIP articles on HopeRF 433MHz data transceivers have drawn our attention to other monitoring modules made by this Chinese firm. Amongst their offerings is a well-priced and calibrated humidity sensor, the HH10D. This has shown itself capable of extremely simple interfacing to even the humblest PICAXE, although factory calibration values first need reading via an I2C-level PICAXE such as the 18X.

By Wayne Geary and Stan Swan

Although atmospheric humidity levels may not be considered as interesting as temperature, numerous environmental situations arise when humidity insights can be crucial.

Relative humidity (RH) levels range from a bone dry 0% to 100% (when the air is so saturated that dew forms). Although temperature dependant, human comfort levels typically range between 40-60% RH.

Drier air may cause skin conditions, static discharges and thirst due to excessive sweating. Higher RH levels are associated with food spoiling, disease, moulds and human discomfort, due to an inability for perspiration to evaporate.

In contrast plants may wilt in dry air (and hence usually favour higher humidity levels) but food preservation, equipment storage and crop drying best suits low RH levels.

Naturally the likely onset of rain may be associated with a rise in the atmospheric RH levels too – a technique used by many home weather stations.

Relative humidity is classically measured by techniques ranging from paper and hair stretching to more scientific dual “wet and dry bulb” thermometers, with simple instruments based around this latter approach still capable of very good results.

Although inconvenient, a couple of similar thermometers and a moist cotton shoelace can readily give do-it-yourself RH insights.

The moist bulb cools as the evaporating water takes thermal energy from it, with the degree of “wet” temperature drop being inversely related to the RH.

(Refer to www.picaxe.orconhosting.net.nz/humtable.jpg for the resulting wet/dry tables).

Electronic humidity measuring has predictably developed, being normally now done with specialised capacitors, since an exposed and porous dielectric slowly changes its moisture content in response to humidity levels in surrounding air. If used in an RC oscillator circuit, small frequency changes can be linked to the altered capacitance value.

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