Campbell HMP45C User manual

INSTRUCTION MANUAL
Model HMP45C Temperature
and Relative Humidity Probe
Revision: 2/07
Copyright © 1990 - 2007
Campbell Scientific Australia Pty Ltd

Warranty and Assistance
The MODEL HMP45C TEMPERATURE AND RELATIVE HUMIDITY
PROBE is warranted by CAMPBELL SCIENTIFIC AUSTRALIA Pty Ltd. to be
free from defects in materials and workmanship under normal use and service for
twelve (12) months from date of shipment unless specified otherwise. Batteries
have no warranty. CAMPBELL SCIENTIFIC AUSTRALIA Pty Ltd’s obligation
under this warranty is limited to repairing or replacing (at CAMPBELL
SCIENTIFIC AUSTRALIA Pty Ltd.'s option) defective products. The customer
shall assume all costs of removing, reinstalling, and shipping defective products
to CAMPBELL SCIENTIFIC AUSTRALIA Pty Ltd. CAMPBELL SCIENTIFIC
AUSTRALIA Pty Ltd will return such products by surface carrier prepaid. This
warranty shall not apply to any CAMPBELL SCIENTIFIC AUSTRALIA Pty
Ltd. products which have been subjected to modification, misuse, neglect,
accidents of nature, or shipping damage. This warranty is in lieu of all other
warranties, expressed or implied, including warranties of merchantability or
fitness for a particular purpose. CAMPBELL SCIENTIFIC AUSTRALIA Pty
Ltd. is not liable for special, indirect, incidental, or consequential damages.
Products may not be returned without prior authorization. The following
contact information is for Australia and International customers residing in
countries served by CAMPBELL SCIENTIFIC AUSTRALIA Pty Ltd. directly.
Affiliate companies handle repairs for customers within their territories. Please
visit
www.campbellsci.com to determine which Campbell Scientific company
serves your country. To obtain a Returned Materials Authorization (RMA),
contact CAMPBELL SCIENTIFIC AUSTRALIA Pty Ltd., phone (07) 4772
0444. After an applications engineer determines the nature of the problem, an
RMA number will be issued. Please write this number clearly on the outside of
the shipping
container. CAMPBELL SCIENTIFIC's shipping address is:
CAMPBELL SCIENTIFIC AUSTRALIA Pty Ltd
RMA#_____
16 Somer St
Hyde Park, QLD 4812
Australia

HMP45C Table of Contents
PDF viewers note: These page numbers refer to the printed version of this document. Use
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1. General Description...............................1
2. Specifications…......................................2
2.1 Temperature Sensor ......................................................................2
2.2 Relative Humidity Sensor..............................................................2
3. Installation...............................................3
3.1 Siting..............................................................................................3
3.2 Assembly and Mounting................................................................3
4. Wiring......................................................5
5. Example Programs….............................6
6. Long Lead Lengths................................9
7. Absolute Humidity................................12
8. Sensor Maintenance.............................14
9. Troubleshooting ..................................15
10. References .........................................15
Figures 3-1. HMP45C and 41003-5 Radiation Shield on a Tripod Mast ................4
3-2. HMP45C and 41003-5 Radiation Shield on a CM200 Series Crossarm.4
Tables 1-1. Recommended Lead Lengths.............................................................. 1
4-1. Connections for Single-Ended Measurements.................................... 5
4-2. Connections for Differential Measurements....................................... 6
4-3. Power Connections using SW12V Peripherals................................... 6
5-1. Calibration for Temperature ............................................................... 7
5-2. Calibration for Relative Humidity ..................................................... 7
5-3. Wiring for Single-ended Measurement Examples.............................. 7
6-1. Wiring for Differential Measurement Examples ............................. 10
7-1. Wiring for Vapor Pressure Examples .............................................. 13
8-1. Chemical Tolerances of HMP45C.................................................... 16
i

Model HMP45C Temperature and
Relative Humidity Probe
(1) General Description
The HMP45C Temperature and Relative Humidity probe contains a Platinum
Resistance Temperature detector (PRT) and a Vaisala HUMICAP® 180
capacitive relative humidity sensor.
The -L option on the model HMP45C Temperature and Relative Humidity
probe (HMP45C-L) indicates that the cable length is user specified. This
manual refers to the sensor as the HMP45C.
The HMP45C can be powered continuously or the power may be switched to
conserve battery life. The HMP45C consumes less than 4 milliamperes current
at 12 volts. Approximately 0.15 seconds is required for the sensor to warm up
after power is switched on. At measurement rates slower than once per
second, the overall power consumption (datalogger and sensors) may be
reduced by switching power to the HMP45C. Most current Campbell
Scientific dataloggers have a built-in switched 12 volts that can be used to
control power.
The CR9000, CR510, CR500, CR7, CR10 and 21X dataloggers do not have a
built-in switched 12 volts. Users with these dataloggers can power the sensor
continuously or purchase the model SW12V to switch power.
Lead length for the HMP45C is specified when the sensor is ordered. Table 1-1
gives the recommended lead length for mounting the sensor at the top of
the tripod/tower with a 2 foot crossarm. Lead length can be 2 feet shorter
when the sensor is mounted to the tripod mast / tower leg without a crossarm.
The HMP45C ships with:
(1) Adjustment Screwdriver from mfg
(1) Calibration Sheet
(1) Instruction Manual
1
TABLE 1-1. Recommended Lead Lengths
CM6 CM10 CM110 CM115 CM120 UT10 UT20 UT30
11’ 14’ 14’ 19’ 24’ 14’ 24’ 37’

Model HMP45C Temperature and Relative Humidity Probe
2. Specifications
Operating Temperature: -40°C to +60°C
Storage Temperature: -40°C to +80°C
Probe Length: 25.4 cm (10 in.)
Probe Body Diameter: 2.5 cm (1 in.)
Filter: 0.2 μm Teflon membrane
Filter Diameter: 1.9 cm (0.75 in.)
Power Consumption: <4 mA @ 12 V
Supply Voltage: 7 to 35 VDC
Settling Time: 0.15 seconds
2.1 Temperature Sensor
Sensor: 1000 ΩPRT, IEC 751 1/3 Class B
Temperature Measurement Range: -40°C to +60°C
Temperature Output Signal range: 0.008 to 1.0 V
Temperature Accuracy:
2.2 Relative Humidity Sensor
Sensor: HUMICAP® 180
Relative Humidity Measurement Range: 0 to 100% non-condensing
RH Output Signal Range: 0.008 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: ±0.05% RH/°C
Typical Long Term Stability: Better than 1% RH per year
Response Time (at 20°C, 90% response): 15 seconds with membrane filter
NOTE The black outer jacket of the cable is Santoprene® rubber. This
compound was chosen for its resistance to temperature extremes,
moisture, and UV degradation. However, this jacket will
support combustion in air. It is rated as slow burning when
tested according to U.L. 94 H.B. and will pass FMVSS302.
Local fire codes may preclude its use inside buildings.
2

Model HMP45C Temperature and Relative Humidity Probe
3. Installation
3.1 Siting
Sensors should be located over an open level area at least 9 m (EPA) in
diameter. The surface should be covered by short grass, or where grass does
not grow, the natural earth surface. Sensors should be located at a distance of
at least four times the height of any nearby obstruction, and at least 30 m
(EPA) from large paved areas. Sensors should be protected from thermal
radiation, and adequately ventilated.
Standard measurement heights:
1.5 m +/- 1.0 m (AASC)
1.25 – 2.0 m (WMO)
2.0 m (EPA)
2.0 m and 10.0 m temperature difference (EPA)
See Section 10 for a list of references that discuss temperature and relative
humidity sensors.
3.2 Assembly and Mounting
Tools Required:
• 1/2” open end wrench
• small screw driver provided with datalogger
• UV resistant cable ties
• small pair of diagonal-cutting pliers
The HMP45C must be housed inside a radiation shield when used in the field.
The 41003-5 Radiation shield has a U-bolt for attaching the shield to tripod
mast / tower leg (Figure 3-1), or CM200 series crossarm (Figure 3-2). The
radiation shield ships with the U-bolt configured for attaching the shield to a
vertical pipe. Move the U-bolt to the other set of holes to attach the shield it to
a crossarm.
Loosen the split-nut on the bottom plate of the 41003-5. Remove the yellow
protective cap on the HMP45C, and insert the sensor into the shield. Tighten
the split-nut to secure the sensor in the shield. Route the sensor cable to the
instrument enclosure. Secure the cable to the tripod/tower using cable ties.
The HMP45C must be housed inside a radiation shield when used in the field.
The 41003-5 Radiation Shield (Figure 3-1 and 3-2) mounts to a tripod mast, tower leg,
or CM200 series crossarm.
3

Model HMP45C Temperature and Relative Humidity Probe
FIGURE 3-1. HMP45C and 41003-5 Radiation Shield on a Tripod Mast
FIGURE 3-2. HMP45C and 41003-5 Radiation Shield on a CM200 Series Crossarm
4
CM200 Series Crossarm
41003-5
Split Nut
U-
b
ol
t

Model HMP45C Temperature and Relative Humidity Probe
4. Wiring Connections to Campbell Scientific dataloggers are given in Tables 4-1
through 4-3. The probe can be measured by two single-ended or differential
analog input channels.
When measuring the HMP45C with single-ended
measurements, the purple or white and black leads must
both be connected to AG on the CR10(X) and
CR500/CR510 or to on the CR1000, CR5000, and
CR23X. Doing otherwise will connect the datalogger’s
analog and power ground planes to each other, which in
some cases can cause offsets on low-level analog
measurements. To avoid 4 mA flowing into analog ground,
switch the sensor on/off for its own measurement.
5
CAUTION
TABLE 4-1. Connections for Single-Ended Measurements
CR1000,
CR3000,
CR800, CR10,
CR5000, CR510,
Color Description CR10X CR23X CR500 21X, CR7
Yellow Temperature Single-Ended Single-Ended Single-Ended Single-Ended
Signal Input Input Input Input
Blue Relative Humidity Single-Ended Single-Ended Single-Ended Single-Ended
Signal Input Input Input Input
Purple Signal Reference AG AG
Black Power Ground AG AG
Shield Shield G G
Red Power 12V 12V 12V 12V
Oran
g
e Power Control Control Port Control Control Port Control

Model HMP45C Temperature and Relative Humidity Probe
5. Example Programs
This section is for users who write their own datalogger programs. A
datalogger program to measure this sensor can be created using Campbell
Scientific’s Short Cut Program Builder software. You do not need to read this
section to use Short Cut.
The temperature and relative humidity signals from the HMP45C can be
measured using a single-ended analog measurement or a differential analog
measurement.
Use a single-ended analog measurement when the HMP45C signal lead length
is less than 6.1 m (20 ft.) or if the probe will be turned on and off under
datalogger control between measurements. For lead lengths greater than 6.1 m
(20 ft.) or when the probe will be continuously powered, use a differential
analog measurement. For a discussion on errors caused by long lead lengths
see Section 6.
6
TABLE 4-2. Connections for Differential Measurements
CR1000,
CR3000,
CR800, CR10,
CR5000, CR510,
Color Description CR10X CR23X CR500 21X, CR7
Yellow Temperature Differential Differential Differential Differential
Signal Input -H Input -H Input -H Input -H
Blue Relative Humidity Differential Differential Differential Differential
Signal Input -H Input -H Input -H Input -H
Purple Signal Reference Differential
Input - L
Differential
Input - L
Differential
Input - L
Differential
Input - L
Black Power Ground G
G AG
Shield Shield G G
Red Power 12V 12V 12V 12V
Orange Power Control Control Port Control
Port
Control Port Control
Port

TABLE 5-2
Calibration for Relative Humidity
Units Multiplier
(% mV-1)Offset
(%)
Celsius 0.1 0
Fahrenheit 0.001 0
Model HMP45C Temperature and Relative Humidity Probe
The HMP45C output scale is 0 to 1000 millivolts for the temperature range of
-40°C to +60°C and for the relative humidity range of 0 to 100%. Tables 5-1
and 5-2 provide calibration information for temperature and relative humidity
7
TABLE 5-1. Calibration for Temperature
Units Multiplier
(degrees mV-1)Offset
(degrees)
Celsius 0.1 -40
Fahrenheit 0.18 -40
TABLE 5-3.
Wiring for Single-ended Measurement Examples
Color Description CR1000 CR10(X)
Yellow Temperature SE 2 (1L) SE 3 (2H)
Blue Relative Humidity SE 1 (1H) SE 4 (2L)
Purple Signal Reference AG
Orange Power Control C1 C1
Red Power 12V 12V
Black Power Ground AG
Clear Shield
G

Model HMP45C Temperature and Relative Humidity Probe
CR1000 Program using Single-Ended Measurement Instructions Using 12V on Datalogger
'CR1000 program to measure HMP45C with single-ended 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)
VoltSE(AirTC,1,mV2500,2,0,0,_60Hz,0.1,-40.0)
VoltSE(RH,1,mV2500,1,0,0,_60Hz,0.1,0)
Portset (1 ,0 )
If RH>100 And RH<108 Then RH=100
CallTable(Temp_RH)
NextScan
EndProg
CR10(X) Program using Single-Ended Measurement Instructions Using 12V on Datalogger
;Turn the HMP45C on.
;
01: Do (P86)
1: 41 Set Port 1 High ;Orange wire (C1)
;control port
;Pause 150 mSec before making measurements so the
;probe can stabilize on true readings.
;
02: Excitation with Delay (P22)
1: 1 Ex Channel
2: 0 Delay W/Ex (units = 0.01 sec)
3: 15 Delay After Ex (units = 0.01 sec)
4: 0 mV Excitation
8

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 4 for alternative multipliers
6: -40 Offset ;See Table 4 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 for alternative multipliers
6: 0 Offset
;Turn the HMP45C off.
;
05: Do (P86)
1: 51 Set Port 1 Low ;Orange wire (C1)
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.
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
9

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 (Vd),
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 Vd. 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

Model HMP45C Temperature and Relative Humidity Probe
CR10(X) Program using Differential Measurement Instructions Using 12V on Datalogger
;Turn the HMP45C on.
;
01: Do (P86)
1: 41 Set Port 1 High ;Orange wire (C1)
;Pause 150 mSec before making measurements so the
;probe can stabilize on true readings.
;
02: Excitation with Delay (P22)
1: 1 Ex Channel
2: 0 Delay W/Ex (units = 0.01 sec)
3: 15 Delay After Ex (units = 0.01 sec)
4: 0 mV Excitation
;Measure the HMP45C temperature.
;
03: Volt (Diff) (P2)
1: 1 Reps
2: 5 2500 mV Slow Range ;CR510, CR500 (2500mv); CR23X (1000 mV);
21X, CR7 (5000 mV)
3: 3 DIFF Channel ;Yellow wire (3H), jumper (3L to 4L)
4: 1 Loc [ T_C ]
5: .1 Mult ;See Table 4 for alternative multipliers
6: -40 Offset ;See Table 4 for alternative offsets
;Measure the HMP45C relative humidity.
;
04: Volt (Diff) (P2)
1: 1 Reps
2: 5 2500 mV Slow Range ;CR510, CR500 (2500mv); CR23X (1000 mV);
21X, CR7 (5000 mV)
3: 4 DIFF Channel ;Blue wire (4H), white or purple wire (4L)
4: 2 Loc [ RH_pct ]
5: .1 Mult ;See Table 5 for alternative multipliers
6: 0 Offset
;Turn the HMP45C off.
;
05: Do (P86)
1: 51 Set Port 1 Low ;Orange wire (C1)
11

Model HMP45C Temperature and Relative Humidity Probe
7. Absolute Humidity
The HMP45C measures the relative humidity. Relative humidity is defined by
the equation below:
100
e
e
RH
s
∗= (2)
where RH is the relative humidity, e is the vapor pressure in kPa , and es is the
saturation vapor pressure in kPa. The vapor pressure, e, is an absolute measure
of the amount of water vapor in the air and is related to the dew point
temperature. The saturation vapor pressure is the maximum amount of water
vapor that air can hold at a given air temperature. The relationship between
dew point and vapor pressure, and air temperature and saturation vapor
pressure are given by Goff and Gratch (1946), Lowe (1977), and Weiss
(1977).
When the air temperature increases, so does the saturation vapor pressure.
Conversely, a decrease in air temperature causes a corresponding decrease in
saturation vapor pressure. It follows then from Eq. (2) that a change in air
temperature will change the relative humidity, without causing a change
absolute humidity.
For example, for an air temperature of 20°C and a vapor pressure of 1.17 kPa,
the saturation vapor pressure is 2.34 kPa and the relative humidity is 50%. If
the air temperature is increased by 5°C and no moisture is added or removed
rom the air, the saturation vapor pressure increases to 3.17 kPa and the
relative humidity decreases to 36.9%. After the increase in air temperature, the
air can hold more water vapor. However, the actual amount of water vapor in
the air has not changed. Thus, the amount of water vapor in the air, relative to
saturation, has decreased.
Because of the inverse relationship between relative humidity and air
temperature, finding the mean relative humidity is meaningless. A more useful
quantity is the mean vapor pressure. The mean vapor pressure can be
computed on-line by the datalogger as shown in the following examples.
12
TABLE 7-1. Wiring for Vapor Pressure Examples
Color Description CR1000 CR10(X)
Yellow Temperature SE 2 (1L) SE 3 (2H)
Blue Relative Humidity SE 1 (1H) SE 4 (2L)
Purple Signal Reference AG
Jumper from
SW12V Control
Red Power SW12V SW12 V
Black Power Ground
AG
Clear Shield
G

Model HMP45C Temperature and Relative Humidity Probe
CR1000 Program that Computes Vapor Pressure and Saturation Vapor Pressure
'CR1000 program that calculates Vapor Pressure
Public AirTC
Public RH
Public RH_Frac, e_Sat, e_kPa
DataTable(Temp_RH,True,-1)
DataInterval(0,60,Min,0)
Average(1,AirTC,IEEE4,0)
Sample(1,RH,IEEE4)
Sample(1,e_kPa,IEEE4)
EndTable
BeginProg
Scan(1,Sec,1,0)
'HMP45C Temperature & Relative Humidity Sensor measurements AirTC and RH:
Portset (1,1)
Delay(0,150,mSec)
VoltSE(AirTC,1,mV2500,2,0,0,_60Hz,0.1,-40.0)
VoltSE(RH,1,mV2500,1,0,0,_60Hz,0.1,0)
Portset (1,0)
If RH>100 And RH<108 Then RH=100
'Calculate Vapor Pressure
'Convert RH percent to RH Fraction
RH_Frac = RH * 0.01
'Calculate Saturation Vapor Pressure
SatVP(e_Sat, AirTC)
'Compute Vapor Pressure, RH must be a fraction
e_kPa = e_Sat * RH_Frac
CallTable(Temp_RH)
NextScan
EndProg
CR10(X) Program that Computes Vapor Pressure and Saturation Vapor Pressure
;Turn the HMP45C on.
;
01: Do (P86)
1: 41 Set Port 1 High ;Orange wire (C1)
;Pause 150 mSec before making measurements so the
;probe can stabilize on true readings.
;
02: Excitation with Delay (P22)
1: 1 Ex Channel
2: 0 Delay W/Ex (units = 0.01 sec)
3: 15 Delay After Ex (units = 0.01 sec)
4: 0 mV Excitation
;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
6: -40 Offset
13

Model HMP45C Temperature and Relative Humidity Probe
;Measure the HMP45C relative humidity.
;
04: Volt (SE) (P1)
1: 1 Reps
2: 5 2500 mV Slow Range ;CR510, CR500 (2500mv); CR23X (1000 mV);
21X, CR7 (5000 mV)
3: 4 SE Channel ;Blue wire (SE 4), white or purple wire (AG)
4: 2 Loc [ RH_frac ]
5: .001 Mult
6: 0 Offset
;Turn the HMP45C off.
;
05: Do (P86)
1: 51 Set Port 1 Low ;Orange wire (C1)
;Compute the saturation vapor pressure.
;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.
;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 ]
8. Sensor Maintenance
The HMP45C Probe requires minimal maintenance. Check monthly to make
sure the radiation shield is free from debris. The black screen at the end of the
sensor should also be checked for contaminates.
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.
Long term exposure of the HUMICAP® relative humidity sensor to certain
chemicals and gases may affect the characteristics of the sensor and shorten its
life. Table 8-1 lists the maximum ambient concentrations, of some chemicals,
that the HUMICAP® can be exposed to.
14

Model HMP45C Temperature and Relative Humidity Probe
Recalibrate the HMP45C annually. Obtain an RMA number before returning
the HMP45C to Campbell Scientific for recalibration.
9. Troubleshooting
Symptom: -9999, NAN, -40 deg C, or 0 % relative humidity
1. Check that the sensor is wired to the correct excitation and analog input
channels as specified by the measurement instructions.
2. Verify the Range code is correct for the datalogger type.
3. Verify the red power wire is correctly wired to the 12V, Switched 12V, or
SW12V module. The terminal the wire is connected to will depend on the
datalogger program.
Connect the red wire to a 12V terminal to constantly power the sensor for
troubleshooting purposes. With the red wire connected to12V, a
voltmeter can be used to check the output voltage for temperature and
relative humidity on the yellow and blue wires respectively (temperature
°C = mV * 0.1 – 40.0; relative humidity % = mV * 0.1).
Symptom: Incorrect temperature or relative humidity
1. Verify the multiplier and offset parameters are correct for the desired
units (Table 5-1).
10. References
Goff, J. A. and S. Gratch, 1946: Low-pressure properties of water from -160°
to 212°F, Trans. Amer. Soc. Heat. Vent. Eng., 51, 125-164.
Lowe, P. R., 1977: An approximating polynomial for the computation of
saturation vapor pressure, J. Appl. Meteor., 16, 100-103.
Weiss, A., 1977: Algorithms for the calculation of moist air properties on a
hand calculator, Amer. Soc. Ag. Eng., 20, 1133-1136.
15
TABLE 8-1. Chemical Tolerances of
HMP45C
Chemical Concentration
(PPM)
Organic Solvents 1000 to 10,000
Aggressive chemicals
(e.g. SO2, H2SO4,
H2S, HCl, Cl2, etc.) 1 to 10
Weak Acids 100 to 1000
Bases 10,000 to 100,000