ISO 8932-2:2026

International
Standard
ISO 8932-2
First edition
Meteorology — Radiosonde —
2026-05
Part 2:
Laboratory test method for errors
in radiosonde humidity sensor
calibration
Meteorology — Radiosonde —
Partie 2: Méthode d'essai en laboratoire pour les erreurs
d'étalonnage du capteur d'humidité dans la radiosonde
Reference number
© ISO 2026
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ii
Contents Page
Foreword .v
Introduction .vi
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 2
4 Symbols and subscripts . 4
4.1 Symbols .4
4.2 Subscript .4
5 Technical requirements for the laboratory setup . 5
5.1 General .5
5.1.1 Temperature .5
5.1.2 Pressure .5
5.2 Precision hygrometer .6
5.2.1 Type .6
5.2.2 Installation .6
5.2.3 Operation .7
5.3 Humidity generator .8
5.3.1 Type .8
5.3.2 Installation .8
5.3.3 Operation .9
6 Test preparation . 10
6.1 Laboratory environmental conditions .10
6.2 Preparation of the radiosonde .10
6.3 Examination of the laboratory setup . .11
6.3.1 General .11
6.3.2 Examination of the dry gas generator .11
6.3.3 Examination of the liquid bath and climate chamber .11
6.3.4 Examination of the measurement system for the calculation of the reference
relative humidity in the test cell .11
6.3.5 Examination of the radiosonde measurement software and the control software
for the reference relative humidity.11
6.4 Installation of the radiosonde .11
7 Test methods and procedures .11
7.1 Operation .11
7.1.1 Purging the test cell .11
7.1.2 Temperature control of the test cell . 12
7.1.3 Humidity from the humidity generator . 12
7.1.4 Calculation of the reference relative humidity using the humidity generator . 13
7.1.5 Calculation of the reference relative humidity using the precision hygrometer . 13
7.2 Test procedure . 13
8 Data processing . 14
8.1 Calculation of the average values .14
8.2 Calculation of the measurement error .14
9 Evaluation of measurement uncertainty . 14
9.1 General .14
9.2 Uncertainty evaluation for the reference relative humidity, u(h ). 15
ref
9.2.1 Uncertainty of the reference relative humidity . 15
9.3 Uncertainty of the radiosonde relative humidity, uh .17

rad
iii
9.3.1 Uncertainty of the resolution of the radiosonde relative humidity, uh .17

radr_ es
9.3.2 Uncertainty of the repeatability of the radiosonde relative humidity,
�uh .17

radr_ ep
9.4 Calculation of the combined standard uncertainty of the measurement error, uh .17

err
9.5 Calculation of expanded uncertainty .18
10 Method for reporting test results .18
Annex A (informative) Calculation of the reference relative humidity. 19
Bibliography .23

iv
Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards
bodies (ISO member bodies). The work of preparing International Standards is normally carried out through
ISO technical committees. Each member body interested in a subject for which a technical committee
has been established has the right to be represented on that committee. International organizations,
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with the International Electrotechnical Commission (IEC) on all matters of electrotechnical standardization.
The procedures used to develop this document and those intended for its further maintenance are described
in the ISO/IEC Directives, Part 1. In particular, the different approval criteria needed for the different types
of ISO documents should be noted. This document was drafted in accordance with the editorial rules of the
ISO/IEC Directives, Part 2 (see www.iso.org/directives).
ISO draws attention to the possibility that the implementation of this document may involve the use of (a)
patent(s). ISO takes no position concerning the evidence, validity or applicability of any claimed patent
rights in respect thereof. As of the date of publication of this document, ISO had received notice of (a)
patent(s) which may be required to implement this document. However, implementers are cautioned that
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This document was prepared by Technical Committee ISO/TC 146, Air quality, Subcommittee SC 5,
Meteorology.
A list of all parts in the ISO 8932 series can be found on the ISO website.
Any feedback or questions on this document should be directed to the user’s national standards body. A
complete listing of these bodies can be found at www.iso.org/members.html.

v
Introduction
Temperature and water vapour (i.e., humidity) are two of the basic atmospheric variables and are
important for the initialization of numerical weather prediction and climate modelling. Radiosondes are
widely used to measure atmospheric parameters such as humidity and temperature up to an altitude of
approximately 40 km. A radiosonde is a balloon-borne instrument with several types of sensors for in situ
[1]
profile measurements. A radio transmitter is used to send these data to the observing station. Radiosonde
observations are often used in conjunction with other measurement techniques such as remote sensing
satellites to provide comparative data. For radiosonde-derived data to be useful, the measurement accuracy
of the radio soundings must be known. From a metrological perspective, this measurement accuracy must
be expressed in terms of uncertainty that is traceable to the International System of Units (SI).
Previously, comparative soundings of dew-point hygrometers and radiosondes showed that radiosonde
humidity sensors have a daytime dry bias resulting from solar heating and a time-lag error at low
[2],[3]
temperatures. The dry-bias and time-lag error can be corrected in sounding systems by creating proper
algorithms based on measurements of sensor temperature and response time, respectively. In addition,
polymeric thin-film humidity sensors adopted by most radiosondes are known to produce systematic errors
at the low temperature characteristic of the upper air environment if a calibration curve obtained at room
[4],[5]
temperature only is employed for low temperatures. The capacitance of humidity sensors increases
with adsorbed water vapour in thin films. While the adsorption capacity of the films for water vapour
increases at lower temperatures, it does not fully compensate for the reduced water vapour pressure, leading
to temperature-dependent behaviour in thin-film humidity sensors. Therefore, calibration of these sensors
must be conducted at various temperatures. Thus, an essential prerequisite to resolve the above issues and
to improve the measurement reliability of radiosondes is to calibrate the radiosonde sensors using ground-
[6],[7]
based facilities.
The relative humidity generated by a laboratory setup or factory facility can be traceable to the SI via
temperature, pressure, gas flow rate, and mass measurements related to determining water vapour pressure
from either the humidity generator itself or the measurement of the generated humidity using calibrated
precision hygrometers. The SI traceability of radiosonde humidity sensors must be established through
factory calibration by manufacturers. This document is mainly focused on testing the calibration error of
humidity sensors in radiosondes, which are randomly sampled from a series of products.
The Standing Committee on Measurements, Instrumentation and Traceability (SC-MINT) of the World
Meteorological Organization (WMO) urges users to test selected samples of radiosondes under laboratory
conditions to ensure that the calibrations supplied by the manufacturer are valid. Even if sensors can be
produced in large batches to meet an agreed upon set of standardized performance checks, it is necessary
[8]
for representative sensor samples, selected at random, to be checked more rigorously. This independent
testing would further improve the reliability of radiosonde measurements by verifying the calibration
results applied by manufacturers. While testing is crucial, the guide provided by SC-MINT only provides
limited requirements for the test setup. These requirements describe the need for stability better than
1 %rh and systematic errors less than ±1 %rh at the desired value. More detailed methodologies or test
procedures for the testing of radiosonde humidity sensors in a relevant range of temperatures have not been
reported.
The procedure presented in this document provides the technical requirements for essential laboratory
setups that include a humidity generator, a test cell, a precision hygrometer, a pressure gauge, and a
thermometer. The test procedure, including test preparation, installation of radiosondes in the test cell,
operation of laboratory setups, and comparison between the reference and radiosonde relative humidity,
is presented. The method for evaluating uncertainties associated with both the reference relative humidity
and the radiosonde humidity sensor is also described.
The fundamental technique essential for this test involves the SI-traceable generation and measurement of
water vapour pressure to determine the reference relative humidity and assess its associated uncertainty.
Since humidity generators used in application of this document are capable of producing multiple dew-
point and frost-point temperatures below the test temperature, the testing of sensors at several relative
humidity levels at the test temperature while avoiding condensation can be accomplished. The water
vapour pressure required for the reference relative humidity can be determined from frost or dew point
temperature by either humidity generators or calibrated hygrometers as exemplified in Annex A. Dew-

vi
[9]
point is the temperature, where water vapour is in equilibrium with liquid water at the same pressures.
An example for obtaining multiple dew-point and frost-point values is presented using a saturator-based
humidity generator. This approach is chosen because of its traceability, and its documented validation by
[4]-[8]
metrological and meteorological experts in testing radiosonde humidity sensors.
Since calibration is valid at the time of calibration, this test can incorporate additional sources of uncertainty
related to transportation and storage of the sensors, which can introduce additional uncertainty in
the results. It's important to note that when considering uncertainty in soundings, other factors such as
radiation dry bias and time-lag should also be considered, as summarized in Table 5 of Reference [3]. While
all uncertainty terms affecting the results should be considered, the procedure in this document primarily
focuses on testing a subset of the uncertainty factors outlined in Reference [3], specifically those related to
testing temperature-dependent calibration corrections. In uncertainty analysis, the uncertainty of the test
setup, reference devices, and other potential uncertainties affecting radiosonde measurements under static
conditions are included in this test.
[10]-[12]
It is worth noting that while the saturator-based technique is founded on Korean and US patents, the
patent holder has granted a license, free of charge, to an unlimited number of applicants globally, without
discrimination, and under reasonable terms and conditions. This license allows for the creation, use, and
sale of implementations based on this ISO document. However, it's important to state that this document is
not solely based upon this patented technique. Alternative equivalent techniques, utilizing either different
types of humidity generators or precision hygrometers, can also be employed for determining reference
relative humidity if they are SI-traceable and meet the specifications required under this document.
Due to potential limitations in the number of test setups or laboratories available for conducting this test,
peer-reviewed reports or papers published online or offline resulting from research activities conducted by
academia or meteorological institutes can be utilized as a test report when following this test procedure.

vii
International Standard ISO 8932-2:2026(en)
Meteorology — Radiosonde —
Part 2:
Laboratory test method for errors in radiosonde humidity
sensor calibration
1 Scope
The document specifies testing procedures for determining calibration error for radiosonde humidity
sensors sampled from mass production batches based on varying the levels of relative humidity at
atmospheric upper-air temperatures using a laboratory setup. This document provides:
a) technical requirements for a laboratory setup to evaluate the calibration errors of radiosonde humidity
measurement;
b) a test procedure for evaluating calibration error of radiosonde humidity sensors for a temperature
1)
range of −90 °C to 35 °C and for a relative humidity of 1 %rh to 100 %rh. Note, this document, is based
upon relative humidity calculated by the percentage of water vapour pressure divided by saturation
water vapour pressure over liquid water, not over ice, even at temperatures below 0 °C; hence, the
maximum relative humidity is less than 100 %rh below 0 °C;
c) a method for evaluating the uncertainty for the measured radiosonde humidity calibration errors.
2 Normative references
The following documents are referred to in the text in such a way that some or all of their content constitutes
requirements of this document. For dated references, only the edition cited applies. For undated references,
the latest edition of the referenced document (including any amendments) applies.
IEC 60050-713:2021, International Electrotechnical Vocabulary (IEV) - Part 713: Radiocommunications:
transmitters, receivers, networks and operation
IEC 60068-3-6:2018, Environmental testing – Part 3-6: Supporting documentation and guidance – Confirmation
of the performance of temperature/humidity chambers
IEC 60068-3-11:2007, Environmental testing – Part 3-11: Supporting documentation and guidance –Calculation
of uncertainty of conditions in climatic test chambers
ISO/IEC Guide 98-1:2024, Guide to the expression of uncertainty in measurement — Part 1: Introduction
ISO/IEC Guide 98-3:2008, Uncertainty of measurement — Part 3: Guide to the expression of uncertainty in
me a s ur ement (GUM: 1995)
ISO/IEC Guide 99:2007, International vocabulary of metrology — Basic and general concepts and associated
terms (VIM)
WMO No.182, 1992, International Meteorological Vocabulary
1) Currently, the lowest possible temperature of commercially-available climate chambers is approximately -75 °C. The
temperature range can be adjusted based on the capability of the climate chamber used.

3 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO/IEC Guide 99:2007, WMO No.182,
IEC 60050-713:2021, IEC 60068-3-6:2018, and IEC 60068-3-11:2007 and the following apply.
ISO and IEC maintain terminology databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https:// www .iso .org/ obp
— IEC Electropedia: available at https:// www .electropedia .org/
3.1
radiosonde
instrument intended to be carried by a balloon through the atmosphere, equipped with devices to measure
one or several meteorological variables (such as pressure, temperature, humidity), and provided with a
radio transmitter for sending this information to the observing station
Note 1 to entry: A series of battery-powered telemetry devices that are suspended high in the atmosphere using a
weather balloon, which measure atmospheric parameters, such as temperature and humidity, and transmit data to a
ground system using radio frequencies.
Note 2 to entry: To ensure the traceability of radiosondes as a product under WMO control, the WMO code (BUFR code
0-02-011) must be obtained. This WMO code shall be unique for each radiosonde, and a single code shall not be used
for more than one type of radiosonde.
3.2
radiosonde body
housing of a radiosonde (3.1) comprising a circuit board with measurement chips and a data transmission
section with antennae and batteries
3.3
radiosonde sensor boom
arm connected to the radiosonde body (3.2) to which the temperature and humidity sensors are attached
3.4
humidity generator
device that can produce the desired water vapour pressure
Note 1 to entry: The water vapour pressure of the generator output together with the controlled temperature can be
converted into the reference relative humidity in the test cell (3.5).
3.5
test cell
chamber in which radiosonde humidity sensors can be installed for testing
Note 1 to entry: The test cell is installed inside a space within which the temperature can be controlled [e.g. a climate
chamber (3.7)] to evaluate the temperature-dependency of the measurements from the radiosonde humidity sensors.
3.6
precision hygrometer
humidity measurement instrument that can accurately measure the humidity from low frost-points
(−100 °C) to high dew-points (35 °C)
Note 1 to entry: It is used to check the reference relative humidity in the test cell (3.5) generated by the humidity
generator (3.4). The measurements from a precision hygrometer can also be used as the reference relative humidity
in the test cell (3.5) if the precision hygrometer is calibrated, and correction and uncertainty are employed. In general,
chilled-mirror precision hygrometers are used.
Note 2 to entry: A thermometer and a vapour pressure equation are also needed to obtain relative humidity values.
Manufacturers may use a different vapour pressure equation for the factory calibration.

3.7
climate chamber
enclosed space where the internal temperature and humidity can be controlled within specified limits
Note 1 to entry: The climate chamber can be used to control the temperature and, thus, the relative humidity as well
of the test cell (3.5).
3.8
liquid bath
equipment in which liquid is present in a container, with a specific volume, to maintain a constant
temperature
Note 1 to entry: A liquid bath can be used to control the water vapour saturation temperature of a saturator-based
humidity generator (3.4).
3.9
dry gas generator
device that produces a gas with negligible water vapour content
Note 1 to entry: It can be used to lower the humidity by mixing of its output with a humid gas with known water
vapour pressure in a humidity generator (3.4).
Note 2 to entry: It can be used to purge the water adsorbed onto the test cell (3.5).
3.10
platinum resistance thermometer
PRT
temperature-responsive device consisting of a sensing resistor within a protective sheath, internal
connecting wires and external terminals to permit connection of electrical measurement devices
3.11
reference thermometer
instrument used to measure the temperature in the test cell (3.5) for the calculation of the reference relative
humidity
Note 1 to entry: Generally, calibrated PRTs (3.10) are used.
3.12
reference pressure gauge
instrument used to measure the pressure in the test cell (3.5) for the calculation of the reference relative
humidity
Note 1 to entry: Generally, calibrated capacitance diaphragm gauges are used.
3.13
measurement error
measured quantity value minus a reference quantity value
3.14
measurement uncertainty
non-negative parameter characterizing the dispersion of the quantity values being attributed to a
measurand, based on the information used
Note 1 to entry: Measurement uncertainty includes components arising from systematic effects, such as components
associated with corrections and the assigned quantity values of measurement standards, as well as the definitional
uncertainty. Sometimes estimated systematic effects are not corrected for but, instead, associated measurement
uncertainty components are incorporated.
Note 2 to entry: The parameter may be, for example, a standard deviation called standard measurement uncertainty
(or a specified multiple of it), or the half-width of an interval, having a stated coverage probability.

Note 3 to entry: Measurement uncertainty comprises, in general, many components. Some of these may be evaluated
by Type A evaluation of measurement uncertainty from the statistical distribution of the quantity values from series
of measurements and can be characterized by standard deviations. The other components, which may be evaluated
by Type B evaluation of measurement uncertainty, can also be characterized by standard deviations, evaluated from
probability density functions based on experience or other information.
Note 4 to entry: In general, for a given set of information, it is understood that the measurement uncertainty is
associated with a stated quantity value attributed to the measurand. A modification of this value results in a
modification of the associated uncertainty.
3.15
standard uncertainty
measurement uncertainty (3.14) expressed as a standard deviation
3.16
coverage factor
k
number larger than one by which a combined standard measurement uncertainty is multiplied to obtain an
expanded measurement
Note 1 to entry: Coverage factor is usually symbolized.
3.17
expanded uncertainty
product of a combined standard measurement uncertainty and a factor larger than the number one
Note 1 to entry: The factor depends upon the type of probability distribution of the output quantity in a measurement
model and on the selected coverage probability.
Note 2 to entry: The term “factor” in this definition refers to a coverage factor (3.16).
4 Symbols and subscripts
4.1 Symbols
For the purposes of this document, the following symbols apply.
E
saturated water vapour pressure at a given T and P
e
water vapour pressure
f
enhancement factor
k
coverage factor
P
pressure
h
relative humidity
T
temperature
U
expanded uncertainty
u
standard uncertainty
v
gas flow rate
4.2 Subscript
Within the document, the following subscripts are used.

cal
calibration
DUT
device under test
g
generator
grad
gradient
h
hygrometer
s
saturator
t
test cell
w
water
ref
reference
rep
repeatability
res
resolution
5 Technical requirements for the laboratory setup
5.1 General
5.1.1 Temperature
The temperature of the test cell can be varied between −90 °C and 35 °C using a climate chamber or a liquid
bath. A climate chamber is more desirable as it minimizes handling of the radiosonde.
The reference temperature of the test cell should be measured inside the test cell using calibrated PRTs to
calculate the reference relative humidity.
If the radiosonde humidity sensor operates using a heating element, both the reference thermometer and the
radiosonde temperature sensor can be affected by the humidity sensor heater. The temperature measured
by the reference PRT should not be impacted by the radiosonde heater.
NOTE The impact of the heater can be reduced by increasing the air ventilation rate in the test cell to around
-1 -1 [13]
5 m∙s to 6 m∙s to facilitate the convective cooling of sensors as experienced during soundings.
The temperature stability of the test cell should be within ±0,1 °C. A change in the test cell temperature of
0,1 °C corresponds to a change in the relative humidity of roughly 0,6 %rh to 0,8 %rh in the temperature
range of –90 °C to 30 °C.
The temperature gradient around the radiosonde humidity sensor should be measured using a calibrated
thermometer to evaluate the uncertainty due to the spatial gradient of the reference relative humidity.
Measurement points of two reference PRTs should be above and below the humidity sensor and the distance
between PRTs be 10 mm to 50 mm. When testing multiple sensors, the temperature distance between these
sensors may be larger than for two sensors resulting in a corresponding increase in uncertainty.
5.1.2 Pressure
The pressure of the test cell should be measured using a calibrated pressure gauge to calculate the saturation
water vapour pressure and the reference relative humidity in the test cell.
The pressure gradient between the test cell and the precision hygrometer should be measured using a
calibrated pressure gauge to evaluate the uncertainty when a calibrated precision hygrometer is used to
determine the reference relative humidity in the test cell instead of a humidity generator.

5.2 Precision hygrometer
5.2.1 Type
A chilled-mirror hygrometer (CMH) measures the dew or frost point temperature at which a constant
condensation layer is kept on a mirror surface. This condensation layer is monitored with using a
photodetector. A CMH can be employed to measure frost-point and dew-point with an accuracy of ±0,1 °C.
However, two CMHs can be needed to cover the entire range of frost-point and dew-point (−100 °C to 35 °C)
using this document because the upper measurement limit of some CMHs measuring low frost points is a
dew-point of 20 °C. For the calibration of CMHs, the method given in ISO/TR 12148:2009 should apply.
Cavity ring-down spectroscopy (CRDS) measures the optical extinction due to water vapour in gases that
scatter and absorb near-infrared light. CRDS can measure low water vapour concentrations from 1 part per
billion (ppb) to 50 parts per million (ppm) with an accuracy in the ppb-level. However, this technique cannot
generally be employed to measure water concentrations higher than 100 ppm.
Tunable diode laser absorption spectroscopy (TDLAS) using near-infrared light wavelength determines the
concentration of water vapour in the gas phase. TDLAS can be employed to measure a wide range of frost-
point and dew-point from −70 °C to 20 °C with an accuracy of ±0,2 °C.
Other technologies can be used provided they can meet measurement goals and data quality requirements.
5.2.2 Installation
It is desirable to minimize the path (tubing) between the humidity generating system and the test cell
because water adsorption and desorption occurs slowly on surfaces and may impact the humidity in the test
cell especially at low temperatures. The precision hygrometer should be installed downstream of the test
cell to measure the frost-point and dew-point in the test cell as shown in Figure 1.

Key
1 dry air generator
2 gas flow for humidity generation
3 gas flow for purging of water
4 humidity generating system
5 heat exchanger
6 climate chamber
7 temperature sensor
8 pressure gauge
9 test cell
10 radiosonde sensor boom
11 precision hygrometer
12 gas out
13 chilled mirror hygrometer
14 cavity ring-down spectrometer
15 tunable diode laser absorption spectroscopy
16 other hygrometer
17 SI traceable
Figure 1 — Example of the test configuration using an SI-traceable precision hygrometer as a
reference
When a precision hygrometer measurement is used as the reference relative humidity for the test cell, the
hygrometer shall be within calibration to maintain SI traceability. In this case, the humidity generating
system depicted in Figure 1 may not be SI-traceable. Consequently, any type of humidity generating
system (such as a dry air cylinder) can be utilized as long as the stability of the generated relative humidity
in the test cell remains within 1 %rh. A pressure gradient along the tubing between the test cell and the
hygrometer may be present, which results in the water vapour pressure measured by the hygrometer to be
slightly different from that of the test cell. To compensate for this, the pressure in the hygrometer should
also be measured using a calibrated pressure gauge to calculate the reference relative humidity in the test
cell using the hygrometer measurement.
5.2.3 Operation
The measurement range for the frost-point and dew-point temperature using a precision hygrometer should
be from −100 °C to 35 °C. Two precision hygrometers can be needed to cover the entire range of frost-point

and dew-point temperatures. At an air pressure of 1 000 hPa, a frost-point of −100 °C corresponds to a water
vapour concentration of approximately 0,014 ppm.
The measurement stability of the precision hygrometer should be better than ±0,1 °C for the frost-point and
dew-point temperature. A frost-point and dew-point change of 0,1 °C corresponds to a relative humidity
change of 0,6 %rh to 0,8 %rh in the temperature range of –90 °C to 35 °C.
The uncertainty of the precision hygrometer should be better than 0,3 °C to 0,15 °C for a frost-point and
dew-point temperature range of −100 °C to 35 °C. In general, the calibration uncertainty of hygrometers
increases as the frost-point of a gas decreases because the humidity standard uncertainties and hygrometer
reproducibility uncertainty also increase with frost-point decrease.
5.3 Humidity generator
5.3.1 Type
[13]
5.3.1.1 Saturator-based frost-point and dew-point generator
This generator controls the saturated water vapour pressure of the input gas flowing through the saturator
at a specific temperature and pressure. In general, the saturator is submerged in a liquid bath to control the
temperature, and to control the range of humidity generation from –100 °C to 35 °C in frost-point and dew-
point.
[14]
5.3.1.2 Diffusion-tube humidity generator
This generator controls the diffusion of water vapour into a dry gas stream (for which the humidity is known)
through a tube connected to a water reservoir under controlled temperature and pressure. Generally, frost-
point and dew-point temperature in the range –100 °C to –70 °C can be achieved.
[14]
5.3.1.3 Coulometric humidity generator :
The coulometric humidity generator produces a controlled amount of hydrogen and oxygen through the
electrolysis of water governed by Faraday’s law, followed by the recombination of the generated hydrogen
and oxygen back into water and then the addition of the generated water into the dry gas stream, for which
the humidity is known. The humidity generation can cover –100 °C to –70 °C in frost-point temperature.
[15]
5.3.1.4 Divided-flow humidity generator
This type of generator controls humidity by diluting the humidity produced by generators mentioned above
through mixing with an extremely dry gas stream.
5.3.1.5 Other SI-traceable and characterized methods
Other SI-traceable and characterized methods to generate humidity can be used provided they meet the
required humidity range and necessary uncertainty.
If the uncertainty of a humidity generator is not determined, it cannot be considered SI-traceable. In such
cases, the humidity generator can be calibrated against SI-traceable precision hygrometers to establish
traceability to the SI.
5.3.2 Installation
The humidity generator should be installed upstream of the test cell containing the radiosonde so that the
gas with a known water vapour pressure flows into the test cell (see Figure 2).
The temperature of the tubing connecting the humidity generator and the test cell shall be higher than the
frost-point and dew-point temperature of the generator to prevent the formation of frost or dew within the
tubing.
If dry gas is needed for the generation of the desired humidity, the dry gas generator should be upstream of
the humidity generator. It is also recommended to flow the dry gas directly into the test cell and bypass the
humidity generator to efficiently purge water that has adsorbed onto the surface of the test cell.
5.3.3 Operation
The generated frost-point and dew point temperature ranges from −100 °C to 35 °C. At an air pressure of
1 000 hPa, a frost-point of −100 °C corresponds to a water vapour concentration of approximately 0,014 ppm.
The stability of the generated humidity should be better than ±0,1 °C of the frost-point and dew point
temperature. A frost-point and dew point change of 0,1 °C corresponds to a relative humidity change of
maximum 0,8 %rh in a temperature range of −90 °C ~ 35 °C.
It can take several hours for the generated relative humidity to equilibrate. The test cell shown should be
located downstream of the humidity generator.
Only the sensor boom should be inserted into the test cell, with the remaining parts located outside the test
cell. This is to minimize water desorption from these surfaces in the test cell which can add water vapour
during testing.
If the size of the test cell is too large compared to the radiosonde sensor boom to test multiple sensors at the
same time, the gradient of the relative humidity and temperature inside the test cell should be evaluated. The
effect of water adsorption and desorption on the surface of the sensor boom can also impact the humidity in
the test cell, especially at cold temperatures.

Key
1 dry air generator
2 generation
3 purging
4 humidity generator
5 heat exchanger
6 climate chamber
7 temperature sensor
8 pressure gauge
9 test cell
10 radiosonde sensor boom
11 hygrometer
12 gas out
13 saturator-based generator
14 diffusion-tube generator
15 coulometric generator
16 divided flow generator
17 other humidity generator
18 SI traceable
Figure 2 — Example of the test setup using an SI-traceable humidity generator as a reference
6 Test preparation
6.1 Laboratory environmental conditions
Tests should be conducted in a laboratory with a room temperature of 23 °C ± 3 °C and a relative humidity of
50 %rh ± 20 %rh.
6.2 Preparation of the radiosonde
Radiosonde measurements can be conducted via wired or wireless communication. The communication
setup of the radiosonde should be evaluated for suitability before the test.
The acceptable operation of the data acquisition software recording the measurement data from the
radiosonde should be verified before starting testing.

If the measurements are taken via wired communication, it should be ensured by the manufacturer that the
measured relative humidity is the same as that obtained by wireless communication.
Before taking
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