Campbell HMP155A User manual

Revision: 10/2021
Copyright © 1990 – 2021
Campbell Scientific, Inc.

Table of contents
1. Introduction 1
2. Precautions 1
3. Initial inspection 1
4. QuickStart 2
5. Overview 4
6. Specifications 5
6.1 Temperature sensor 7
6.2 Relative humidity sensor 8
7. Installation 8
7.1 Wiring to data logger 8
7.2 Data logger programming 10
7.2.1 VoltSE() instruction 10
7.2.2 VoltDiff() instruction 11
7.3 Mounting 11
7.3.1 Installation in a 41005-5 14-plate shield 13
7.3.2 Installation in a RAD14 14-plate shield 14
7.3.3 Mount the shield 14
8. Operation 15
8.1 Measurement 15
8.2 Long cable lengths 15
8.3 Absolute humidity 16
9. Troubleshooting and maintenance 17
9.1 Troubleshooting 17
9.2 Maintenance 17
9.2.1 Cleaning 18
9.2.2 Changing the probe filter 18
Table of Contents - i

10. References 19
Appendix A. Importing Short Cut code into CRBasic Editor 21
Appendix B. RS-485 output 22
B.1 SDM-SIO1A or SDM-SIO4A serial I/O module interface option 22
B.2 MD485 interface option 23
B.2.1 MD485 multidrop interface configuration 27
B.3 Numeric display 29
Table of Contents - ii

1. Introduction
The HMP155A probe monitors relative humidity (RH) for the range of 0 to 100% RH and
temperature for the range of –80 to 60°C. It provides reliable measurements for a wide range of
applications as part of a weather station system or as a single instrument. All Campbell Scientific
data loggers are compatible.
NOTE:
This manual provides information only for CRBasic data loggers. For retired Edlog data logger
support, see an older manual at www.campbellsci.com/old-manuals .
2. Precautions
lREAD AND UNDERSTAND the Safety section at the back of this manual.
lCare should be taken when opening the shipping package to not damage or cut the cable
jacket. If damage to the cable is suspected, contact Campbell Scientific.
lAlthough the HMP155A is rugged, it should be handled as a precision scientific instrument.
lDo not touch the sensor element.
lSantoprene® rubber, which composes the black outer jacket of the cable, will support
combustion in air. It is used because of its resistance to temperature extremes, moisture,
and UV degradation. It is rated as slow burning when tested according to U.L. 94 H.B. and
passes FMVSS302. However, local fire codes may preclude its use inside buildings.
3. Initial inspection
lUpon receipt of the HMP155A, inspect the packaging and contents for damage. File
damage claims with the shipping company.
lThe model number and cable length are printed on a label at the connection end of the
cable. Check this information against the shipping documents to ensure the expected
product and cable length were received.
HMP155A Temperature and Relative Humidity Probe 1

4. QuickStart
A video that describes data logger programming using Short Cut is available at:
www.campbellsci.com/videos/cr1000x-data logger-getting-started-program-part-3 .Short
Cut is an easy way to program your data logger to measure the sensor and assign data logger
wiring terminals. Short Cut is available as a download on www.campbellsci.com . It is included
in installations of LoggerNet,RTDAQ, and PC400.
The following procedure also shows using Short Cut to program the HMP155A.
1. Open Short Cut and click Create New Program.
2. Double-click the data logger model.
3. In the Available Sensors and Devices box, type HMP155A or locate the sensor in the
Sensors >Meteorological >Relative Humidity & Temperature >HMP155 Temperature and
Relative Humidity Sensor folder. Double-click either HMP155 (constant power) or HMP155
(panel switched power); the panel switched power option uses less current but requires a
scan rate that is greater than 5 seconds. Data defaults to degree Celsius. This can be
changed by clicking the Deg C box and selecting Deg F, for degrees Fahrenheit, or Kfor
Kelvin.
HMP155A Temperature and Relative Humidity Probe 2

4. Click the Wiring tab to see how the sensor is to be wired to the data logger. Click OK after
wiring the sensor.
5. Repeat steps three and four for other sensors.
6. In Output Setup, type the scan rate, meaningful table names, and Data Output Storage
Interval.
NOTE:
The HMP155A has a five second settling time after power up; therefore the scan rate
must be greater than five seconds when using the panel switched power option.
HMP155A Temperature and Relative Humidity Probe 3

7. Select the measurement and its associated output option.
8. Click Finish and save the program. Send the program to the data logger if the data logger
is connected to the computer.
9. If the sensor is connected to the data logger, check the output of the sensor in the data
display in LoggerNet,RTDAQ, or PC400 to make sure it is making reasonable
measurements.
5. Overview
The HMP155A Temperature and Relative Humidity probe contains a platinum resistance
temperature detector (PRT) and a Vaisala HUMICAP®180 capacitive relative humidity sensor.
The HMP155A outputs a 0 to 1VDC signal for temperature and relative humidity that can be
measured by all models of Campbell Scientific data loggers with model HMP155ACBL1 cable. The
HMP155A also has RS-485 outputs for temperature and relative humidity that can be directly
interfaced to the CR6 and CR1000X data logger using the HMP155ACBL2 cable. The RS-485
output can also be used with our CR800, CR850, CR3000, and CR1000 data loggers, but they
require both the HMP155ACBL2 cable and the SDM-SIO1A or SDM-SIO4A Serial I/O Module
(RS-485 output (p. 22)). The RS-485 output has a higher current drain than that listed in the
specifications.
The HMP155A can be powered continuously, or the power may be switched to conserve battery
life. The HMP155A consumes less than 3 milliamps current at 12VDC. Approximately 2 seconds is
required for the sensor to warm up after power is switched on. At measurement rates slower than
once per 5 seconds, the overall power consumption (data logger and sensors) may be reduced
HMP155A Temperature and Relative Humidity Probe 4

by switching power to the HMP155A. Current Campbell Scientific data loggers have a built-in
switched 12VDC that can be used to control power.
NOTE:
HMP155A sensors purchased directly from Vaisala with serial numbers < E4430001 require
approximately 5 seconds warm up time.
Features:
lWell-suited for long-term, unattended applications
lAccurate and rugged
lCompatible with the following data loggers: CR6, CR1000X, CR800-series, CR300-series,
CR3000, CR1000
6. Specifications
Dimensions in mm [inches]
FIGURE 6-1. Probe dimensions
HMP155A Temperature and Relative Humidity Probe 5

8-Pin Connector
FIGURE 6-2. Wiring of HMP155A 8-pin connector
*HMP155ACBL1 Cable provided by Campbell Scientific
1 = VOUT1 (yellow, temp)
2 = no connection
3 = AGND (white)
4 = VOUT2 (blue, RH)
5 = no connection
6 = no connection
7 = VCC (red)
8 = GND (black)
– = SHIELD (clear)
*Note: HMP155ACBL2 for RS-485 is described in
RS-485 output (p. 22).
Operating temperature range for
humidity measurement: –80 to 60 °C (–112 to 140 °F)
Storage temperature range: –80° to 60 °C (–112 to 140 °F)
Electromagnetic compatibility: Complies with EMC standard EN61326-1, Electrical
equipment for measurement control and laboratory use
- EMC requirements for use in industrial locations
Filter: Sintered PTFE
Housing material: PC
Housing classification: IP66
HMP155A Temperature and Relative Humidity Probe 6

Weight: 86 g (3 oz)
Voltage outputs: 0 to 1 VDC
Average current consumption: <3 mA (analog output mode)
Operating voltage: 7 to 28 VDC
Settling time at power-up: 2 s
6.1 Temperature sensor
Measurement range: –80 to 60 °C (–112 to 140 °F)
Accuracy with voltage output
at –80 to 20 °C: ±(0.226 – 0.0028 x temperature) °C
at 20 to 60 °C: ±(0.055 + 0.0057 x temperature) °C
See the following graph:
Temperature sensor: Pt 100 RTD 1/3 Class B IEC 751
Response time (63 %) for additional
temperature probe in 3m/s air flow: 63% <20 s
90% <35 s
HMP155A Temperature and Relative Humidity Probe 7

6.2 Relative humidity sensor
Measurement range: 0 to 100% RH
Accuracy (including non-linearity,
hysteresis and repeatability)
15 to 25 °C (59 to 77 °F): ±1% RH (0 to 90% RH)
±1.7% RH (90 to 100% RH)
-60 to -40 °C (-76 to -40 °F): ± (1.4 + 0.032 × reading) % RH
-40 to -20 °C (-40 to -4 °F): ± (1.2 + 0.012 × reading) % RH
-20 to 40 °C (-4 to 104 °F): ± (1.0 + 0.008 × reading) % RH
40 to 60 °C (104 to 140 °F): ± (1.2 + 0.012 × reading) % RH
Factory calibration uncertainty (20 °C): ±0.6% RH (0 to 40% RH)
±1.0% RH (40 to 97% RH)
(Defined as ±2 standard deviation limits. Small
variations possible, see also calibration certificate.)
Humidity sensor: HUMICAP®180R
Response time for HUMICAP®180R(C)
at 20 °C in still air with sintered PTFE
filter and a 0% to 75% RH step change: 20 s (63% step change);
60 s (90% step change)
7. Installation
If you are programming your data logger with Short Cut, skip Wiring to data logger (p. 8) and
Data logger programming (p. 10). Short Cut does this work for you. See QuickStart (p. 2) for a
Short Cut tutorial.
7.1 Wiring to data logger
The HMP155A probe can be measured with two single-ended or differential analog input
channels. Differential measurements are recommended for cable lengths greater than 6.1m
(20ft). See Long cable lengths (p. 15).
HMP155A Temperature and Relative Humidity Probe 8

Connections to CRBasic data loggers are given in Table 7-1 (p. 9) and Table 7-2 (p. 9). To wire an
Edlog data logger, see an older manual at www.campbellsci.com/old-manuals , or contact
Campbell Scientific for assistance.
Table 7-1: Wire color, function, and data logger connection for single-ended measurements
Wire color Wire function Data logger connection terminal
Yellow Temperature signal Uconfigured for single-ended analog input1,
SE (single-ended, analog-voltage input)
Blue Relative humidity signal Uconfigured for single-ended analog input, SE
White Signal reference ⏚(analog ground)
Black Power ground ⏚
Clear Shield ⏚
Red Power SW12V
1Uterminals are automatically configured by the measurement instruction.
CAUTION:
When measuring the HMP155A with single-ended measurements, the white and black wires
must both be connected to ⏚. Doing otherwise will connect the data logger analog and
power ground planes to each other, which in some cases can cause offsets on low-level
analog measurements. To avoid 3mA flowing into analog ground, switch the sensor on/off
for its own measurement.
Table 7-2: Wire color, function, and data logger connection for differential measurements
Wire color Wire function Data logger connection terminal
Yellow Temperature signal Uconfigured for differential analog input H1,
Diff H
Jumper to
White
Temperature signal
reference
Uconfigured for differential analog input L,
Diff L
Blue Relative humidity signal Uconfigured for differential analog input H,
Diff H
White Signal reference Uconfigured for differential analog input L,
Diff L
HMP155A Temperature and Relative Humidity Probe 9

Table 7-2: Wire color, function, and data logger connection for differential measurements
Wire color Wire function Data logger connection terminal
Black Power ground G
Clear Shield ⏚
Red Power 12V or SW12V
1Uterminals are automatically configured by the measurement instruction.
7.2 Data logger programming
Short Cut is the best source for up-to-date programming code for Campbell Scientific data
loggers. If your data acquisition requirements are simple, you can probably create and maintain a
data logger program exclusively with Short Cut. If your data acquisition needs are more complex,
the files that Short Cut creates are a great source for programming code to start a new program
or add to an existing custom program.
NOTE:
Short Cut cannot edit programs after they are imported and edited in CRBasic Editor.
AShort Cut tutorial is available in QuickStart (p. 2). If you wish to import Short Cut code into
CRBasic Editor to create or add to a customized program, follow the procedure in Importing
Short Cut code into CRBasic Editor (p. 21). Programming basics for CRBasic data loggers are
provided in the following sections. Downloadable example programs are available at
www.campbellsci.com/downloads/hmp155a-example-programs .
Measure the HMP155A with either the VoltSE() or VoltDiff() measurement instruction as
described in the following sections.
For a discussion on errors caused by long cable lengths, see Long cable lengths (p. 15).
NOTE:
HMP155A sensors purchased directly from Vaisala with serial numbers < E4430001 require
approximately 5 seconds warm up time.
7.2.1 VoltSE() instruction
When cable lengths are shorter than 6.1 meters or when power is switched, use the VoltSE()
measurement instruction to measure the temperature and relative humidity. The HMP155A
output scale is 0 to 1000 millivolts for the temperature range of –80 to 60°C and for the relative
humidity range of 0 to 100%.
HMP155A Temperature and Relative Humidity Probe 10

VoltSE(Dest, Reps, Range, SEChan, MeasOff, SettlingTime, Integ/FNotch, Mult, Offset)
Variations:
lTemperature reported as °C – set Mult to 0.14 and Offset to –80
lTemperature reported as °F – set Mult to .252 and Offset to –112
lHumidity reported as a percent – set Mult to 0.1 and Offset to 0
lHumidity reported as a fraction – set Mult to 0.001 and Offset to 0
7.2.2 VoltDiff() instruction
When cable lengths are longer than 6.1 meters or when the sensor is constantly powered, the
VoltDiff() measurement instruction is used to measure the HMP155A sensor. The HMP155A
output scale is 0 to 1000 millivolts for the temperature range of -80 to 60°C and for the relative
humidity range of 0 to 100%.
VoltDiff(Dest, Reps, Range, DiffChan, RevDiff, SettlingTime, Integ/FNotch, Mult,
Offset)
Variations:
lTemperature reported as °C – set Mult to 0.14 and Offset to –80
lTemperature reported as °F – set Mult to .252 and Offset to –112
lHumidity reported as a percent – set Mult to 0.1 and Offset to 0
lHumidity reported as a fraction – set Mult to 0.001 and Offset to 0
7.3 Mounting
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 the natural earth surface where grass does not grow.
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 (AASC)
1.25 to 2.0 m (WMO)
2.0 m (EPA)
See References (p. 19) for a list of references that discuss temperature and relative humidity
sensors.
HMP155A Temperature and Relative Humidity Probe 11

Tools required:
l1/2 inch open-end wrench
lsmall screwdriver provided with data logger
lUV-resistant cable ties
lsmall pair of diagonal-cutting pliers
lAdjustable wrench with a minimum 1-7/8 inch jaw size
To install the HMP155A, you will need the 41005-5 or RAD14 14-Plate Radiation Shield. The
HMP155A is packaged with a white connector cover and a yellow sensor head protective cap. See
FIGURE 7-1 (p. 12).
FIGURE 7-1. HMP155A in shipping box
1. Slide the white connector cover off the sensor head before removing the sensor from the
box.
HMP155A Temperature and Relative Humidity Probe 12

2. Squeeze the sides of the white connector cover and insert the cable’s connector through it
(see FIGURE 7-2 (p. 13)).
FIGURE 7-2. Cable routed through the connector cover
3. Attach the cable to the mating connector on the bottom of the HMP155A (see FIGURE 7-3
(p. 13)).
FIGURE 7-3. Cable connected to the sensor
4. Slide the white connector cover over the connector and gently push it up as far as it will go.
The white connector cover has built-in molded stops that will only allow it to slide up so far
(see FIGURE 7-4 (p. 13)).
FIGURE 7-4. HMP155A with connector cover in place and without protective cap
7.3.1 Installation in a 41005-5 14-plate shield
1. Loosely thread the collar adapter into the base of the 14-plate shield.
2. Remove the yellow protective cap on the HMP155A, and insert the sensor into the shield.
See FIGURE 7-5 (p. 14) (left) and FIGURE 7-6 (p. 15).
3. Hold the collar and sensor, and finish threading the collar into the shield by hand. Tighten
the hex plug so it compresses against the body of the HMP155A to hold it inside the
41005-5 radiation shield. Use an adjustable wrench if necessary, but do not overtighten the
collar.
HMP155A Temperature and Relative Humidity Probe 13

7.3.2 Installation in a RAD14 14-plate shield
1. Loosen the nut on the entry gland at the bottom of the shield.
2. Remove the yellow protective cap on the HMP155A, and insert the sensor into the shield.
3. Insert the sensor up into the gland as far as it will go. See FIGURE 7-5 (p. 14) (right).
4. Using an adjustable wrench, tighten down the nut on the gland until the sensor is held
firmly in place. Do not overtighten.
7.3.3 Mount the shield
1. Attach the radiation shield to the tripod mast, crossarm, or tower leg using the supplied U-
bolt. See FIGURE 7-5 (p. 14) and FIGURE 7-6 (p. 15) for examples of shield mounting.
2. Route the cable to the data logger, and secure the cable to the mounting structure using
cable ties.
FIGURE 7-5. HMP155A and 41005-5 14-plate radiation shield (left) and
RAD14 (right) on a tripod mast
HMP155A Temperature and Relative Humidity Probe 14

FIGURE 7-6. HMP155A and 41005-5 14-plate radiation shield on a CM200-series crossarm
8. Operation
8.1 Measurement
The HMP155A uses a 100Ω PRT to measure temperature and a Vaisala HUMICAP®180 capacitive
sensor to measure relative humidity. Campbell Scientific data loggers measure the analog
voltage outputs of the HMP155A Temperature and Relative Humidity Probe with either the
VoltSE() or VoltDiff() measurement instruction.
8.2 Long cable lengths
This section describes the error associated with measuring the HMP155A with a single-ended
measurement when the probe has a long cable. To avoid these problems, Campbell Scientific
recommends measuring the HMP155A using a differential analog measurement when long cable
lengths are required. Generic data logger connections for measuring the HMP155A using a
differential measurement are given in Table 7-2 (p. 9).
Understanding the details in this section is not required for the general operation of the
HMP155A with Campbell Scientific data loggers.
The signal reference (white) and the power ground (black) are connected inside the HMP155A.
When the HMP155A temperature and relative humidity are measured using a single-ended
analog measurement, both the signal reference and the power ground are connected to ground
HMP155A Temperature and Relative Humidity Probe 15

at the data logger, and both serve as the return path for 12V. The voltage will drop along those
wires because thewires have resistance.
The HMP155A draws approximately 4mA when it is powered. The wire used in the HMP155A has
resistance of 27.7Ω/1000ft. Since the signal reference and the power ground are both connected
to ground at the data logger, the effective resistance of those wires together is half of
27.7Ω/1000ft, or 13.9Ω/1000ft. Using Ohm’s law, the voltage drop (Vd), along the signal
reference/power ground, is given by Eq. 1 (p. 16).
Eq. 1
This voltage drop will raise the apparent temperature and relative humidity because the
difference between the signal and the signal reference wire at the data logger 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.
8.3 Absolute humidity
The HMP155A measures relative humidity. Relative humidity is defined by the following equation:
Eq. 2
where RH is the relative humidity, e is the vapor pressure in kPa, and esis 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 dewpoint temperature. The saturation vapor pressure is the
maximum amount of water vapor that air can hold at a given air temperature. The relationship
between dewpoint 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 (p. 16) 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 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,
HMP155A Temperature and Relative Humidity Probe 16

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 online by the data logger (see example program
available at www.campbellsci.com/downloads/hmp155a-example-programs ).
9. Troubleshooting and
maintenance
9.1 Troubleshooting
Symptom: Relative Humidity is reported as –9999, NAN, –80°C, or 0%
1. Check that the sensor is wired to the correct analog input channels as specified by the
measurement instructions.
2. Verify the range code for the single-ended or differential measurement instruction is
correct for the data logger type.
3. Verify the red power wire is correctly wired to the 12V or SW12V channel. The channel the
wire is connected to will depend on the data logger program.
Connect the red wire to a 12V terminal to constantly power the sensor for troubleshooting
purposes. With the red wire connected to 12V, 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*0.14–80.0; relative humidity %=mV*0.1).
Symptom: Incorrect temperature or relative humidity
1. Verify the multiplier and offset parameters are correct for the desired units (Data logger
programming (p. 10)).
9.2 Maintenance
The HMP155A 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
contaminates.
HMP155A Temperature and Relative Humidity Probe 17