HUMIDITY AND TEMPERATURE

This little sensor gives its data to a library program #HUMID.TDS that does all the hard mathematics of deriving the useful readings you need.

SHT75 humidity and temperature sensor
There are also surface-mount versions without leads.

The transducer connects to TDS2020F card computer using two wires (plus power). The data can be displayed or logged, for example to a Compact Flash card. For information on the SHT1x and SHT7x series of transducers, download the datasheet. There are four versions of the product but the library file will work with any. At Triangle Digital Support Ltd we stock the most sensitive part with leads, the SHT75.

The interface is similar to I²C but is not identical and I²C software will not work. However, it will co-exist with I²C chips so can share the same pair of wires as other serial devices. The sensor works from +5V and consumes only half a milliamp. It automatically sleeps between measurements, taking less than one microamp.

The device measures not only humidity, but also temperature. This is important not only in its own right, but because a third measurement, dewpoint, can be derived mathematically if you know both humidity and temperature. The library routine has this calculation.

q       Relative Humidity is expressed as a percentage, 80% means the air is 80% as saturated with water vapour as it can hold. The best sensors have an accuracy of 2% R.H. and can measure from 0% to 100% R.H. Conversion of the raw reading to R.H. and linearisation is done in software in the TDS2020F’s library routine.

q       Temperature is measured to an accuracy of 0.5°C over 5 to 40°C and 2°C over -4 to +123°C. Conversion of the raw reading to both degrees C and F is done in software in the TDS2020F’s library routine.

q       Dewpoint is a temperature mathematically derived from these two. The dewpoint is the temperature at which, if you cooled the air, water would begin to condense from the atmosphere and is available from the library software in both degrees C and F.

The Datalogger Wizard can automatically produce the source code for programs that log any (or all) of Humidity Temperature & Dewpoint.