Campbell HC2S3, Product Manual, Page: 21 / 42

Model HC2S3 Temperature and Relative Humidity Probe
13
8.2 Low Power Operation
The HC2S3 draws approximately 2 mA powered from 12V. The HC2S3 can be
continuously powered from the 12V terminal, or power can be switched with
the SW12V terminal to conserve battery life. When power is switched, a three-
second warm-up period is required. This is programmed with the Delay()
instruction, using 0 for the delay option.
8.3 Long Cable Lengths
For cable lengths longer than 6.1 m (20 ft), Campbell Scientific recommends
measuring the voltage signals using differential inputs. Using single-ended
measurements with long cable lengths introduces the errors discussed below.
Connections for differential inputs are given in TABLE 7-2 .
The signal reference (yellow) and the power ground (gray) are connected inside
the HC2S3. When the HC2S3 temperature and relative humidity are measured
using a single-ended analog measurement, the signal reference and power
ground are both connected to ground at the datalogger, and both serve as the
return path for power. The voltage will drop along those leads because the wire
itself has resistance.
The HC2S3 draws approximately 2 mA when powered with 12 V. The wire
used in the HC2S3 (pn 27746) has resistance of 14.74 Ω /304.8 m (1000 ft).
Because the signal reference and the power ground are both connected to
ground at the datalogger, the effective resistance of those wires together is half
of 14.74 Ω /304.8 m (1000 ft), or 7.37 Ω /304.8 m (1000 ft). Using Ohm’s law,
the voltage drop (V
d
), along the signal reference/power ground, is given by Eq.
(1).
𝑉𝑉
𝑑𝑑
= 𝐼𝐼 × 𝑅𝑅
= 2 mA × 7.37 Ω 1000 ft ⁄
(1)
= 14.7 mV 1000 ft ⁄
This voltage drop will raise the apparent temperature and relative humidity
because the difference between the signal and signal reference lead at the
datalogger has increased by V
d
. The approximate error in temperature and
relative humidity is 0.15 °C and 0.15% per 30.5 m (100 ft) of cable length,
respectively (assuming a temperature range of –40 to 60 °C). When there are
not enough inputs available on the datalogger to allow for differential
measurements, single-ended measurements can be made and the errors
associated with cable length subtracted as offsets.
9. Toubleshooting and Maintenance
All factory repairs and recalibrations require a returned material
authorization (RMA) and completion of the “Declaration of
Hazardous Material and Decontamination” form. Refer to the
Assistance page at the beginning of this manual for more
information.
NOTE