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HMP50
Instruction Manual
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Campbell HMP50 Instruction Manual

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HMP50 Temperature and
Relative Humidity Probe
Revision: 10/09
Copyright © 1995-2009
Campbell Scientific, Inc.
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HMP50 Table of Contents
PDF viewers note: These page numbers refer to the printed version of this document. Use the Adobe Acrobat® bookmarks tab for links to specific sections.
1. General .........................................................................1
2. Specifications ..............................................................1
2.1 Temperature Sensor..................................................................................2
2.2 Relative Humidity Sensor .........................................................................2
3. Installation....................................................................3
3.1 Siting.........................................................................................................3
3.2 Mounting and Assembly...........................................................................3
4. Wiring............................................................................6
5. Example Programs......................................................6
5.1 Example for CR1000................................................................................7
5.2 Example for CR10X.................................................................................8
6. Long Lead Lengths......................................................9
7. Absolute Humidity.......................................................9
7.1 CR1000 Vapor Pressure Example..........................................................10
7.2 Sample CR10(X) Program that Computes Vapor Pressure and Saturation
Vapor Pressure...................................................................................11
8. Maintenance...............................................................12
8.1 Procedure for Removing RH Chip .........................................................13
9. References .................................................................13
Figures
1. HMP50 as Shipped..................................................................................... 4
2. HMP50 and 41303-5A Radiation Shield on a Tripod Mast .......................4
3. HMP50 and 41303-5A Radiation Shield on a CM202 Crossarm...............5
4. HMP50 and 41303-5A Radiation Shield....................................................5
5. HMP50 Wiring...........................................................................................6
6. Exploded View of HMP50 (as shipped)...................................................13
i
HMP50 Table of Contents
Tables
1. Recommended Lead Lengths..................................................................... 1
2. Datalogger Connections.............................................................................6
3. Calibration for Temperature....................................................................... 7
4. Calibration for Relative Humidity ............................................................. 7
5. Wiring for CR1000 and CR10X Examples................................................ 7
6. Wiring for Vapor Pressure Examples ...................................................... 10
ii
HMP50 Temperature and Relative Humidity Probe
1. General
The HMP50 Temperature and Relative Humidity probe contains a Platinum Resistance Temperature detector (PRT) and a Vaisala INTERCAP® capacitive relative humidity sensor.
The -L option on the model HMP50 Temperature and Relative Humidity probe (HMP50-L) indicates that the cable length is user specified. Lead length is specified when the sensor is ordered. Table 1 gives the recommended lead length. This manual refers to the sensor as the HMP50.
TABLE 1. Recommended Lead Lengths
2 m Height Atop a tripod or tower via a 2 ft crossarm such as the CM202
Mast/Leg CM202 CM6 CM10 CM110 CM115 CM120 UT10 UT20 UT30
9' 11' 11' 14' 14' 19' 24' 14' 24' 37'
Note: Add two feet to the cable length if you are mounting the enclosure on the leg base of a light-weight tripod.
2. Specifications
Operating Temperature: -40°C to +60°C
Storage Temperature: -40°C to +80°C
Probe Length: 7.1 cm (2.8 in.)
Probe Body Diameter: 1.2 cm (0.47 in.)
Filter: 0.2 μm Teflon membrane
Filter Diameter: 1.2 cm (0.47 in.)
Housing Material: chrome-coated aluminum and chrome-coated ABS plastic
Power Consumption: <2 mA
Supply Voltage: 7 to 28 VDC
Settling Time after power is switched on: 1 second
1
HMP50 Temperature and Relative Humidity Probe
2.1 Temperature Sensor
Sensor: 1000 Ω PRT, DIN 43760B
Temperature Measurement Range: -40°C to +60°C
Temperature Output Signal range: 0 to 1.0 VDC
Temperature Accuracy:
1.6
1.2
0.8
0.4
o
( C)
0
-0.4
Error
-0.8
-1.2
-1.6
-20-40 0
2.2 Relative Humidity Sensor
20 40 60
Temperature( C)
o
Sensor: INTERCAP®
Relative Humidity Measurement Range: 0 to 98% non-condensing
RH Output Signal Range: 0 to 1.0 VDC
Accuracy at 20°C
±3% RH (0 to 90% Relative Humidity) ±5% RH (90 to 98% Relative Humidity)
Temperature Dependence of Relative Humidity Measurement:
1.5
1.0
o
0.5
C
0
-0.5
( %RH/ )
-1.0
-1.5
Temperature Dependence
10 20 30 40 50 60 70 80 90 100
o
60 C
-40 C
RH (%)
o
Typical Long Term Stability: Better than 1% RH per year
Response Time (at 20°C, 90% response to a steep change in humidity):
15 seconds with membrane filter
2
3. Installation
3.1 Siting
HMP50 Temperature and Relative Humidity Probe
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 9 for a list of references that discuss temperature and relative humidity sensors.
3.2 Mounting and Assembly
Pull off the yellow shipping cap (see Figure 1).
The HMP50 must be housed inside a solar radiation shield when used in the field. The 41303-5A 6-Plate Radiation Shield (Figures 2 and 3) mounts to a tripod mast, UT10 tower leg, or CM202, CM204, or CM206 crossarm. The HMP50 is held within the 41303-5A by a mounting clamp (Figure 3).
The UT6P 6-plate Radiation Shield mounts to a UT10, UT20, or UT30 tower with the UT018 horizontal mounting arm.
NOTE
The black outer jacket of the cable is Santoprene 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.
®
rubber. This
3
HMP50 Temperature and Relative Humidity Probe
)
FIGURE 1. HMP50 as Shipped
Yellow Shipping Cap (remove before installation
4
FIGURE 2. HMP50 and 41303-5A Radiation Shield
on a Tripod Mast
HMP50 Temperature and Relative Humidity Probe
FIGURE 3. HMP50 and 41303-5A Radiation Shield
on a CM202 Crossarm
Mounting Clamp
FIGURE 4. HMP50 and 41303-5A Radiation Shield
5
HMP50 Temperature and Relative Humidity Probe
FIGURE 5. HMP50 Wiring
TABLE 2. Datalogger Connections
Black Temperature Signal
White Relative Humidity Signal
Blue Signal & Power Reference
Brown Power
Clear Shield
Wire Label
Temp Signal Black Single-Ended Input Single-Ended Input Single-Ended Input
RH Signal White Single-Ended Input Single-Ended Input Single-Ended Input
Power & Signal
Ground
Power 12V Brown 12 V 12 V 12 V
Shield Clear
Color
Blue G G
CR800, CR3000,
CR200, CR23X,
CR1000
CR10(X), CR510
G
21X, CR7
4. Wiring
Connections to Campbell Scientific dataloggers are given in Table 2. The probe is measured by two single-ended analog input channels, one for temperature and one for relative humidity.
CAUTION
Always connect the Blue lead to the datalogger first, followed by the Black, White, and Clear leads. Connect the Brown (Power) lead last.
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.
6
HMP50 Temperature and Relative Humidity Probe
The temperature and relative humidity signals from the HMP50 are measured using two single-ended analog measurements (Instruction 1).
The probe 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 3
and 4 provide calibration information for temperature and relative humidity.
TABLE 3. Calibration for Temperature
Units Multiplier
(degrees mV-1)
Celsius 0.1 -40
Fahrenheit 0.18 -40
TABLE 4. Calibration for Relative Humidity
Units Multiplier
(% mV-1)
Percent 0.1 0
Fraction 0.001 0
TABLE 5. Wiring for CR1000 and CR10X Examples
Description Color CR1000 CR10(X)
Temperature Black SE 1 SE 3 (2H)
Relative Humidity White SE 2 SE 4 (2L)
Signal & Power Reference Blue G G
Power Brown 12 V 12 V Shield Clear
Offset
(degrees)
Offset
(%)
G
5.1 Example for CR1000
'CR1000 'Created by SCWIN (2.1)
Public AirTC Public RH
DataTable(Table1,True,-1) DataInterval(0,60,Min,0) Average(1,AirTC,FP2,0) Sample(1,RH,FP2) EndTable
7
HMP50 Temperature and Relative Humidity Probe
BeginProg Scan(5,Sec,1,0) 'HMP50 Temperature & Relative Humidity Sensor measurements AirTC and RH: VoltSE(AirTC,1,mV2500,1,0,0,_60Hz,0.1,-40.0) VoltSE(RH,1,mV2500,2,0,0,_60Hz,0.1,0) If (RH>100) And (RH<108) Then RH=100 CallTable(Table1) NextScan EndProg
5.2 Example for CR10X
;Measure the HMP50 temperature. ;
01: Volt (SE) (P1) 1: 1 Reps 2: 5 2500 mV Slow Range ;CR510 (2500 mV); CR23X (1000 mV); 21X, CR7 (5000 mV) 3: 3 SE Channel ;Black wire (SE 3), Blue wire (G) 4: 1 Loc [ T_C ] 5: .1 Mult ;See Table 3 for alternate multipliers 6: -40 Offset ;See Table 3 for alternate offsets
;Measure the HMP50 relative humidity. ;
02: Volt (SE) (P1) 1: 1 Reps 2: 5 2500 mV Slow Range ;CR510 (2500 mV); CR23X (1000 mV); 21X, CR7 (5000 mV) 3: 4 SE Channel ;White wire (SE 4), Blue wire (G) 4: 3 Loc [ RH_pct ] 5: .1 Mult ;See Table 4 for alternate multipliers 6: 0 Offset
;Limit the maximum relative humidity to 100%. ;
03: If (X<=>F) (P89) 1: 3 X Loc [ RH_pct ] 2: 3 >= 3: 100 F 4: 30 Then Do
04: Z=F (P30) 1: 100 F 2: 0 Exponent of 10 3: 3 Z Loc [ RH_pct ]
05: End (P95)
8
6. Long Lead Lengths
Long lead lengths cause errors in the measured temperature and relative humidity. The approximate error in temperature and relative humidity is
0.52°C and 0.52% per 100 feet of cable length, respectively.
When long lead lengths are required and the above errors in temperature and relative humidity are unacceptable, use the HMP45C temperature and humidity probe.
Understanding the following details are not required for the general operation of the HMP50 with Campbell Scientific’s dataloggers. The signal reference and the power ground (black) are the same lead in the HMP50. When the HMP50 temperature and relative humidity are measured, both the signal reference and power ground are connected to ground at the datalogger. The signal reference/power ground lead serves as the return path for 12 V. There will be a voltage drop along this lead because the wire itself has resistance. The HMP50 draws approximately 2 mA when it is powered. The wire used in the HMP50 (P/N 18159) has resistance of 26.2 Ω/1000 feet. Using Ohm’s law, the voltage drop (V Eq. (1).
HMP50 Temperature and Relative Humidity Probe
), along the signal reference/power ground lead, is given by
d
This voltage drop will raise the apparent temperature and relative humidity because the difference between the signal and signal reference, at the datalogger, has increased by V relative humidity is 0.35°C and 0.35% per 100 feet of cable length, respectively.
7. Absolute Humidity
The HMP50 measures the relative humidity. Relative humidity is defined by the equation below:
where RH is the relative humidity, e is the vapor pressure in kPa , and e 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).
V
d
∗= RI
(1)
Ω∗=
=
. The approximate error in temperature and
d
e
RH
=∗
e
s
ft 1000 mV 2.45
100
ft 1000 6.22 mA 2
(2)
is the
s
9
HMP50 Temperature and Relative Humidity Probe
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 in 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 from 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, there is more energy available to vaporize the water. 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. CRBasic dataloggers use the VaporPressure instruction to calculate vapor pressure from temperature and relative humidity measurements (see Section 7.1). Edlog dataloggers must first calculate the saturation vapor pressure and then calculate vapor pressure (see Section 7.2).
TABLE 6. Wiring for Vapor Pressure Examples
Description Color CR10(X) CR1000
Temperature Black SE 3 (2H) SE 1 (1H)
Relative Humidity White SE 4 (2L) SE 2 (2H)
Signal & Power Reference Blue G G
Power Brown 12 V 12 V Shield Clear G
7.1 CR1000 Vapor Pressure Example
The VaporPressure instruction has the following syntax:
VaporPressure ( Dest, Temp, RH )
Where:
Dest—the variable in which the results of the instruction will be sto red.
Temp—the program variable that contains the value for the temperature sensor. The temperature measurement must be in degrees C.
RH—the program variable that contains the value for the relative humidity sensor. The RH measurement must be in percent of RH.
10
HMP50 Temperature and Relative Humidity Probe
'CR1000
Public AirTC Public RH Public VP
DataTable(Table1,True,-1) DataInterval(0,60,Min,0) Average(1,AirTC,FP2,0) Sample(1,RH,FP2) Average(1,VP, FP2,0) EndTable
BeginProg Scan(5,Sec,1,0)
'HMP50 Temperature & Relative Humidity Sensor measurements AirTC and RH:
VoltSE(AirTC,1,mV2500,1,0,0,_60Hz,0.1,-40.0) VoltSE(RH,1,mV2500,2,0,0,_60Hz,0.1,0) If (RH>100) And (RH<108) Then RH=100 VaporPressure(VP,AirTC,RH) CallTable(Table1) NextScan EndProg
7.2 Sample CR10(X) Program that Computes Vapor Pressure and Saturation Vapor Pressure
;Measure the HMP50 temperature. ;
01: Volt (SE) (P1) 1: 1 Reps 2: 5 2500 mV Slow Range ;CR510 (2500 mV); CR23X (1000 mV); 21X,
CR7 (5000 mV)
3: 3 SE Channel ;Black wire (SE 3), Blue wire (G) 4: 1 Loc [ T_C ] 5: .1 Mult ;See Table 3 for alternate multipliers 6: -40 Offset ;See Table 3 for alternate offsets
;Measure the HMP50 relative humidity. ;
02: Volt (SE) (P1) 1: 1 Reps 2: 5 2500 mV Slow Range ;CR510 (2500 mV); CR23X (1000 mV); 21X,
CR7 (5000 mV)
3: 4 SE Channel ;White wire (SE 4), Blue wire (G) 4: 2 Loc [ RH_frac ] 5: .001 Mult ;See Table 4 for alternate multipliers 6: 0 Offset
11
HMP50 Temperature and Relative Humidity Probe
;Limit the maximum value of relative humidity ;to 1 (expressed as a fraction). ;
03: If (X<=>F) (P89) 1: 2 X Loc [ RH_frac ] 2: 3 >= 3: 1 F 4: 30 Then Do
04: Z=F (P30) 1: 1 F 2: 0 Exponent of 10 3: 2 Z Loc [ RH_frac ]
05: End (P95)
;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 ]
8. Maintenance
12
The HMP50 Probe requires minimal maintenance. Check monthly to make sure the radiation shield is free from debris. The white screen at the tip of the probe should also be checked for contaminants.
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 (see Figure 6 and Section 8.1). Do not scratch the silver chip while cleaning. It might be necessary to repeat rinsing.
HMP50 Temperature and Relative Humidity Probe
Protective Cap and Filter
9598 RH Chip
FIGURE 6. Exploded View of HMP50 (as shipped)
Shipping Cap (remove prior to installation)
8.1 Procedure for Removing RH Chip
CAUTION
9. References
The offset and gain on the HMP50 electronics can not be adjusted as part of a recalibration. Replace the RH chip as needed.
1. Unscrew the protective cap.
2. Hold the plastic sides of the RH chip and unplug it.
To prevent scratching, avoid touching the silver RH chip, and handle the RH chip with care.
3. Rinse the RH chip or dispose of the old RH chip.
4. Hold the sides of the rinsed or new chip and plug it in.
5. Screw on the protective cap.
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.
13
HMP50 Temperature and Relative Humidity Probe
14
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