Campbell Scientific HMP45C Temperature and RH Probe - Manual (Page 13)

Model HMP45C Temperature and Relative Humidity Probe
;Measure the HMP45C temperature.
;
03: Volt (SE) (P1)
1: 1
Reps
2: 5 2500 mV Slow Range ;CR510, CR500 (2500mv); CR23X (1000 mV);
21X, CR7 (5000 mV)
3: 3 SE Channel ;Yellow wire (SE 3), white or purple wire (AG)
4: 1 Loc [ T_C ]
5: .1 Mult ;See Table 5-1 for alternative multipliers
6: -40 Offset ;See Table 5-1 for alternative offsets
;Measure the HMP45C relative humidity.
;
04: Volt (SE) (P1)
1: 1
Reps
2: 5 2500 mV Slow Range ;CR510, CR500 (2500 mV); CR23X (1000 mV);
21X, CR7 (5000 mV)
3: 4 SE Channel ;Blue wire (SE 4), white or purple wire (AG)
4: 2 Loc [ RH_pct ]
5: .1 Mult ;See Table 5-2 for alternative multipliers
6: 0 Offset
;Turn the HMP45C off.
;
05: Do (P86)
1: 51
Set Port 1 Low ;Jumper wire from SW12V control to C1
;Orange wire (C1) if older wiring
;Green wire (C1) if using SW12V device
;For CR23X or CR5000 use 59 for SW12V internal
;control port
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
analog 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 (white or purple) and the power ground (black) are in
common inside the HMP45C. When the HMP45C 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 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
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