Campbell Scientific HMP35C Temperature and RH Sensor - Manual (Page 3)

1
MODEL HMP35C
TEMPERATURE AND RELATIVE HUMIDITY PROBE
1. GENERAL DESCRIPTION
The HMP35C Temperature and Relative
Humidity probe contains a thermistor for
measuring temperature and a Vaisala
capacitive polymer H chip for measuring relative
humidity.
The -L option on the model HMP35C
Temperature and Relative Humidity probe
(HMP35C-L) indicates that the cable length is
user specified. This manual refers to the
sensor as the HMP35C.
2. SPECIFICATIONS
Operating Temperature: -35
°
to +60
°
C
Storage Temperature: -40
°
to +80
°
C
Probe Length: 25.4 cm (10 in.)
Probe Body Diameter: 2.5 cm (1 in.)
Filter: 0.2
µ
m Teflon membrane
Filter Diameter: 1.9 cm (0.75 in.)
Power Consumption: < 4 mA
Supply Voltage (via CSI switching circuit):
7 to 35 VDC
Settling Time after power is switched on:
0.15 seconds
2.1 TEMPERATURE SENSOR
Sensor: thermistor (100 k
Ω
@ 25
°
C)
Temperature Measurement Range:
-35
°
to +55
°
C
Thermistor Interchangeability Error:
Typically <
±
0.2
°
C over 0
°
to +60
°
C,
±
0.4
°
C
@ -35
°
C
Polynomial Linearization Error:
<
±
0.5
°
C over -35
°
to +50
°
C
2.1.1 Temperature Sensor Accuracy
The overall probe accuracy is a combination of
the thermistor's interchangeability specification,
the precision of the bridge resistors, and the
polynomial error. In a "worst case" all errors
add to an accuracy of
± 0.4
°
C over the range of
-24
°
C to 48
°
C and
±
0.9
°
C over the range of
-38
°
to 53
°
C. The major error component is the
interchangeability specification of the
thermistor, tabulated in Table 1. For the range
of 0
°
to 50
°
C the interchangeability error is
predominantly offset and can be determined
with a single point calibration. Compensation
can then be done with an offset entered in the
measurement instruction. The bridge resistors
are 0.1% tolerance with a 10 ppm temperature
coefficient. Polynomial errors are tabulated in
Table 2 and plotted in Figure 1.
TABLE 1. Thermistor Interchangeability
Temperature
(
°
C)
Temperature
Tolerance (
±°
C)
−
40 0.40
−
30 0.40
−
20 0.32
−
10 0.25
0 to +50 0.20
TABLE 2. Polynomial Error
Temperature
Range(
°
C)
Error (
°
C)
−
40 to +56 <
±
1.0
°
C
−
38 to +53 <
±
0.5
°
C
−
24 to +48 <
±
0.1
°
C
2.2 RELATIVE HUMIDITY SENSOR
Sensor: Vaisala capacitive polymer H chip
Relative Humidity Measurement Range:
0 to 100% non-condensing
RH Output Signal Range:
0.002 to 1 VDC
Accuracy at 20
°
C
±
2% RH (0 to 90% Relative Humidity)
±
3% RH (90 to 100% Relative Humidity)
Temperature Dependence of Relative Humidity
Measurement:
± 0.04% RH/
°
C
Typical Long Term Stability:
Better than 1% RH per year
Response Time (at 20
°
C, 90% response to
steep change in humidity):
15 seconds with membrane filter