Campbell Scientific HMP35C Temperature and RH Sensor - Manual (Page 13)

HMP35C TEMPERATURE AND RH PROBE
11
;Compute the saturation vapor pressure in kPa. The temperature must be in degrees Celsius.
;
06:
Saturation Vapor Pressure (P56)
1: 1
Temperature Loc [ T_C ]
2: 3 Loc [ e_sat ]
;Compute the vapor pressure in kPa. Relative humidity must be a fraction.
;
07:
Z=X*Y (P36)
1: 3
X Loc [ e_sat ]
2: 2 Y Loc [ RH_frac ]
3: 4 Z Loc [ e_kPa ]
9. INSTRUCTION 11 DETAILS
Understanding the details in this section are not
necessary for using the HMP35C Probe with
Campbell Scientific's dataloggers.
Instruction 11 outputs a precise 2 VAC
excitation (4 V with the 21X) and measures the
voltage drop due to the sensor resistance
(Figure 5). The thermistor resistance changes
with temperature. Instruction 11 calculates the
ratio of voltage measured to excitation voltage
(Vs/Vx) which relates to resistance as shown
below:
V
V R
s
x s
=
+ +
1000
249000 1000
(2)
where Rs is the resistance of the thermistor.
See the measurement section of the datalogger
manual for more information on bridge
measurements.
Instruction 11 then calculates temperature using
a fifth order polynomial equation correlating
Vs/Vx with temperature. The polynomial
coefficients are given in Table 10. The
polynomial input is (Vs/Vx)
∗
800. Resistance
and datalogger output at several temperatures
are shown in Table 11.
FIGURE 5. Thermistor Probe Schematic
TABLE 10. Polynomial Coefficients
Coefficient Value
C0 -53.4601
C1 90.807
C2 -83.257
C3 52.283
C4 -16.723
C5 2.211
10. MAINTENANCE
The HMP35C Probe requires minimal
maintenance. Check monthly to make sure the
radiation shield is free from debris. The black
screen on the sensor's end should also be
checked.
When installed in close proximity to the ocean
or other bodies of salt water (e.g., Great Salt
Lake), a coating of salt (mostly NaCl) may build
up on the radiation shield, sensor, filter and
even the chip. NaCl has an affinity for water.
The humidity over a saturated NaCl solution is
75%. A buildup of salt on the filter or chip will
delay or destroy the response to atmospheric
humidity.
The filter can be rinsed gently in distilled water.
If necessary, the chip can be removed and
rinsed as well. Do not scratch the chip while
cleaning.
Recalibrate the HMP35C annually. Obtain an
RMA number before returning the HMP35C to
Campbell Scientific for recalibration.