Model HC2S3 Temperature and Relative Humidity Probe
The signal reference (yellow) and the power ground (gray) are in common
inside the HC2S3. When the HC2S3 temperature and relative humidity are
measured using a single-ended analog measurement, both the signal reference
and power ground are connected to ground at the datalogger. The signal
reference and power ground both serve as the return path for power. There will
be a voltage 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 (P/N 27746) has resistance of 14.74
Ω
/304.8 m (1000 ft).
Since 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).
)
'
(1000
m
304.8
mV
4.7
1
)
'
(1000
m
304.8
.37
7
mA
2
=
Ω
∗
=
∗
=
R
I
V
d
(1)
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.
8. Sensor Maintenance
Corroded, discolored or clogged filters should be replaced. To replace the
filter, unscrew the filter from the probe and pull it straight away, being careful
not to bend or damage the sensors. Before putting on the replacement filter
check the alignment of the sensors with the probe, and if necessary, carefully
correct the alignment before installing the filter.
The Teflon filter tip is recommended when the sensor is installed in close
proximity to the ocean or other bodies of salt water. A coating of salt (mostly
NaCl) may build up on the radiation shield, sensor, filter and even the sensors.
A buildup of salt on the filter or sensors will delay or destroy the response to
atmospheric humidity.
Long term exposure of the relative humidity sensor to certain chemicals and
gases may affect the characteristics of the sensor and shorten its life. The
resistance of the sensor depends strongly on the temperature and humidity
conditions and the length of the pollutant influence.
In general, the HC2S3 requires minimal maintenance. The radiation shield
should be kept clean and free of debris, and the sensor should be calibrated
annually. Please obtain an RMA number before returning the HC2S3 to
Campbell Scientific for calibration. Please refer to Warranty and Assistance
sections at the beginning of the manual.
20
