Model HMP45C Temperature and Relative Humidity Probe
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).
V d = I
∗
R
= 4 mA
∗
13.9
Ω
/ 1000 ft
(1)
= 55.6 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 0.56
°
C and 0.56% per 100 feet of cable length,
respectively.
TABLE 6-1. Wiring for Differential Measurement
Examples
Colour Description CR1000
CR10(X)
Yellow Temperature
3H 3H
Jumper
to
4L 2L
3L
Blue Relative
Humidity 1H
4H
Purple Signal
Reference
1L
4L
Jumper from SW12V
Control
C1
Red Power 12V
12V
Black Power
Ground
G
G
Clear Shield
G
CR1000 Program using Differential Measurement Instructions Using 12V on Datalogger
'CR1000 program to measure HMP45C with differential measurements
Public AirTC
Public RH
DataTable(Temp_RH,True,-1)
DataInterval(0,60,Min,0)
Average(1,AirTC,IEEE4,0)
Sample(1,RH,IEEE4)
EndTable
BeginProg
Scan(1,Sec,1,0)
'HMP45C Temperature & Relative Humidity Sensor measurements AirTC and RH:
Portset
(1
,1
)
Delay(0,150,mSec)
VoltDiff
(AirTC,1,mV2500,2,True,0,_60Hz,0.1,-40)
VoltDiff
(RH,1,mV2500,1,True,0,_60Hz,0.1,0)
Portset
(1
,0
)
If RH>100 And RH<108 Then RH=100
CallTable(Temp_RH)
NextScan
EndProg
10
