HMP45C Temperature and RH Probe
10
6. Long Lead Lengths
This section describes the error associated with measuring the HMP45C with a
single-ended measurement if the probe has a long cable. To avoid these
problems, CSI recommends measuring the HMP45C using a differential
analogue measurement (Instruction 2) when long lead lengths are required.
Generic datalogger connections for measuring the HMP45C using a differential
measurement are given in Table A-2.
Understanding the details in this section are not required for the general
operation of the HMP45C with Campbell Scientific’s dataloggers.
The signal reference (blue) and the power ground (black) are in common inside
the HMP45C. When the HMP45C temperature and relative humidity are
measured using a single-ended analogue 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 12 V. There will
be a voltage drop along those leads because the wire itself has resistance. The
HMP45C draws approximately 4 mA when it is powered. The wire used in the
HMP45C (P/N 9721) has resistance of 27.7 Ω/1000 feet. 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 27.7 Ω/1000 feet, or
13.9 Ω/1000 feet. Using Ohm’s law, the voltage drop (V
d
), along the signal
reference/power ground, is given by Eq. (1).
ft 1000 mV 6.55
ft 1000 9.13 mA 4
=
Ω∗=
∗= RI
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 0.56°C and 0.56% per 100 feet of cable length, respectively.
Table 7. Wiring for Differential Measurement Examples
Description Colou
CR10(X) CR1000
Temperature White 2H 2H
Jumper to 1L 2L 2L
Relative Humidity Green 1H 1H
Signal Reference Blue 1L 1L
Jumper from SW12V Control C1 N/A
Powe
Red SW12 V SW12 V
Power Ground Black G G
Shield Clear G
NOTE: These wire colours differ from HMP45C probes supplied in the USA.
(50 pagine)
Manymanuals.com
Manymanuals.de
Manymanuals.fr
Manymanuals.it
Manymanuals.pl
Manymanuals.cz
Manymanuals.es
Manymanuals-pt.com
Commenti su questo manuale