User manual

1-Wire Application Guide v1.04 Page 17 of 41
There are lots of choices for temperature sensors available and all have slightly
different characteristics. Your choice might depend on accuracy or precision, or a
combination of both (see Figure 9). Be careful not to confuse the two. Precision
is the resolution of the measurement, but accuracy is the variance between the
actual temperature and the returned result. Many users have confused this and
have been puzzled why two devices placed side by side would produce a
variance in temperature. Look at the worst case: two devices, each with a
precision of ±0.5°C and an accuracy of ±2.0°C, could lead to a worse case
difference of 5°C (9°F) between the two sensors.
3.3 Rain Gauge
All rain gauges developed for the 1-Wire measurement scheme are based on the
original Dan Awtrey design shown in Figure 10.
Figure 10 Original 1-Wire Rain Gauge
The rain gauge counts rainfall via a tipping bucket, which is magnetically coupled
to the reed switch SW1. The original design called for a tipping bucket design,
which caused a tip to occur for every 0.01 inch of rainfall. The DS2423 inputs A
and B are connected to an internal counter, which will then increment for every
0.01 inch of rain.
Rain gauges are still available from some vendors in this original configuration.
Also, some vendors have developed a retrofit kit for non-1-Wire rain gauges to
allow them to be read from a 1-Wire interface. There are many wireless or wired
rain gauges that can be retrofitted in this manner.
When used with TEMP08, set the DS2423 type to “R” (rain).
3.4 Wind Speed
Some Wind Speed sensors available are based on the same 1-Wire circuit as the
rain gauge. The reed switch is coupled to an anemometer vane, which rotates
with a speed proportional to the speed of the wind flowing through the cups. The
original wind speed sensor from Dallas Semiconductor used only 1 reed switch,