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TR 44-02

CRITERIA FOR LABORATORY ACCREDITATION IN THE FIELD OF HUMIDITY METROLOGY

Approved By:

Chief Executive Officer: Ron Josias Senior Manager: Mpho Phaloane

Revised By: Field Manager: Neville Tayler

Date of Approval: 2014-11-21

Date of Implementation: 2014-11-21

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CONTENTS:

1. Purpose and Scope ............................................................................................................... 3

2. Definitions and References .................................................................................................... 3

3. Abbreviations ......................................................................................................................... 4

4. Environmental Requirements ................................................................................................ 4

5. General Requirements ........................................................................................................... 5

6. Technical Requirements ........................................................................................................ 5

7. The Accreditation Schedule ................................................................................................... 5

Appendix 1: Accreditation Schedule........................................................................................... 6

Addendum 1: Amendment Record ............................................................................................... 7

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1. Purpose and Scope

The purpose of this document is to define the specific environmental, general and technical requirements to be met by accredited laboratories in the field humidity measurement and calibration. This document is applicable to South African National Accreditation System (SANAS) Accredited Laboratories in this field.

2. Definitions and References

References [1] OHS Act, 1993 Environmental Regulations 1987 - Lighting

[2] OHS Act, 1993 General Admin Regulations 1994 – Handling of Hazardous Chemical Substances

[3] NPL, UK 1996 A Guide to the Measurement of Humidity Issued by the NPL and Institute of Measurement and Control ISBN 0-904457-24-9 [4] ISO/IEC Guide International Vocabulary of Basic and General Terms in 99-12:2007 Metrology, (VIM) 3

rd Edition BIPM, IEC, IFCC, ISO, IUPAC, OIML,

2007.

Definitions

� Absolute Humidity The mass of water vapour present in a unit volume of moist air of a given temperature and pressure. SI (metric) units are grams of water per cubic metre of air (g.m

-3). [3]

• Dew Point The temperature at which dew, or condensation forms, on cooling a gas. This is in effect the temperature at which air becomes saturated in equilibrium with water, expressed in degrees Celsius (ºC) or occasionally other units of temperature.[3]

• Step Response Time Duration between the instant when an input quantity value of a measuring instrument or measuring system is subjected to an abrupt change between two specified constant quantity values and the instant when a corresponding indication settles within specified limits around its final steady value. [4]

� Repeatability Condition of measurement repeatability condition condition of measurement, out of a set of conditions that includes the same measurement procedure, same operators, same measuring system, same operating conditions and same location, and replicate measurements on the same or similar objects over a short period of time. [4]

� Repeatability repeatability measurement precision under a set of repeatability conditions of measurement. [4]

• Relative Humidity Relative humidity is the ratio of the amount of water vapour, e, in the air to the amount of water vapour, es, that would be in the air if saturated at the same temperature and pressure, and can be expressed

relative humidity (in %) = e/es X 100

Where e is the water vapour pressure, and es is the saturation vapour pressure of the prevailing ambient temperature [3]

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• Saturation Vapour Pressure Maximum pressure of water vapour that can exist at a given temperature. Expressed in units of pressure e.g. in pascals (Pa) or in non-SI units such as millibars (mbar) or millimeteres of mercury (mm Hg). [3]

• Water Activity (of a substance) Water activity (aw) is the relative humidity which is eventually reached in a closed space where a hygroscopic substance, such as a foodstuff, has been placed. It is the same as equilibrium relative humidity except that it is expressed on a scale of 0 to 1 (no units) [3]

• Water Vapour Pressure That part of the total pressure contributed by the water vapour. Expressed in units of pressure e.g. pascals (Pa). [3]

3. Abbreviations

aw water activity CMC Calibration and Measurement Capability MSDS Material Safety Data Sheets rh Relative Humidity SANAS South African National Accreditation System

4. Environmental Requirements

Significant changes in relative humidity may occur with even slight variations in temperature and without change in absolute humidity. For example, a 1ºC variance in temperature at 20ºC and 50% rh can introduce an error of ± 3,2 %rh. [3] Therfore an accredited laboratory working in the field of humidity measurements shall operate under the following environmental conditions:

4.1 The laboratory should be maintained at a temperature of 23°C ± 3°C. Where measurements are performed outside this specified requirement, the laboratory must ensure that appropriate corrections are applied.

4.2 Rates of change and gradients of the ambient temperature and humidity must be kept to below 1 deg C per hour, and 10% rh per hour.

4.3 A high degree of cleanliness shall be maintained in the laboratory and adequate lighting shall be

provided. [1] 4.4 Localized isolation in the form of chambers, cabinets and enclosures may be utilized in order to

minimize the effects of thermal currents and temperature gradients and in so doing provide a stable and homogeneous environment conducive to reduced measurement uncertainties.

4.5 The laboratory shall consider and include the stability of the environmental conditions on the

relative humidity when estimating the uncertainty of measurement.

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5. General Requirements

5.1 Laboratories operating in the field of humidity may be accredited for the calibration of

dewpoint hygrometers, relative humidity hygrometers, humidity generators, water activity meters, saturated and unsaturated salt solutions, environmental chambers, and limited temperature calibration.

(Refer to the Schedule of Accreditation – Appendix 1) 5.2 Laboratories performing the calibration of the temperature indicating or recording component of

thermo-hygrometers or thermo-hygrographs respectively are limited to a BMC of ±1 ºC and range of 15 to 50ºC, unless the laboratory is accredited as a temperature calibration laboratory.

5.3 The laboratory will measure and report the measured value in the units applicable for each

specific parameter. Generally, the units will be °C (dew point), °C (frost point), %rh relative humidity, aw (water activity).

5.4 The laboratory is required to have MSDS for all standards fluids, and chemicals, including

saturated and unsaturated salt solutions, and cleaning materials used within the laboratory. [2]

6. Technical Requirements

6.1 The laboratory is required to have procedures in place for the storage and usage of standards in

order to ensure their ongoing integrity. 6.2 The laboratory shall have and apply procedures for all calibrations. 6.3 The laboratory shall have the necessary standards and equipment required to perform the

calibration measurements in accordance with their accreditation schedule. 6.4 All measurements shall be taken only after adequate stabilization time. This period shall be

addressed in the calibration procedures, taking in to account possible differences in response time due to different time constants of the reference standard and the hygrometers or salt solutions under test.

6.5 The calibration of environmental and climatic chambers requires the availability of suitable

instrumentation capable of data storage. Laboratory procedures shall address the number of temperature and relative humidity sensors required, and the spatial distribution thereof, and this shall be dependant on the volume of the chamber. Guidance may be obtained from IEC 68-2-66:1994 ‘Environmental testing’. The calibration procedure shall address the time period over which the measurements are to be performed

6.6 It is recommended that the humidity be measured at each point in the chamber where the

temperature is measured. The application of corrections based on spatial temperature measurements, will result in humidity values with a larger uncertainty.

6.7 Laboratories should take cognisance of effects of barometric pressure on humidity

measurements 6.8 Laboratories shall record and report the temperature at which the measurement of water activity

or relative humidity has been made.

7. The Accreditation Schedule

The accreditation schedule shall follow the following format; the functions will be determined by the laboratory application, ranges and uncertainties are approximate. See Appendix 1

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APPENDIX 1: Accreditation Schedule

S C H E D U L E O F A C C R E D I T A T I O N

Permanent Address of Laboratory: SuperLab 25 Popular Street Tree Park Port Southern Africa Postal Address: P O Box 22222 Port Southern Africa X000 Tel : (055) 321-4567 Fax : (055) 321-6789 E-mail: dolittle@mweb.co.za

Technical Signatories Nominated Representative Issue No. Date of issue Expiry date

: Dr I. Dolittle Mr D Green : Mrs M Brown : 01 : October 200X : October 200X

ITEM

MEASURED QUANTITY, TYPE OF

GAUGE OR INSTRUMENT

RANGE OF MEASURED

QUANTITY

CALIBRATION AND MEASUREMENT

CAPABILITIES EXPRESSED

AS AN UNCERTAINTY (±)

1

Dew-point Hygrometer

- 75 ºC to - 60 ºC - 60 ºC to 17 ºC

0,4 + (0,073* │t + 60│) °C 0,4 °C

2

Salt Solution: Saturated & unsaturated 15 °C to 30 °C (note the rh may also be expressed as a range)

0 % rh 11 % rh 33 % rh 53 % rh 75 % rh 90 % rh 95 % rh

2 % rh 2 % rh 2 % rh 2 % rh 2 % rh 2 % rh 2 % rh

3

Digital Hygrometers / Thermo hygrometers 15 °C to 30 °C

10 to 90 % rh 15 ºC to 50 ºC

2 % rh 1ºC

4 Analogue RH Indicators 10 to 90 % rh 5 % rh

5

Humidity Recorders Thermo-Hygrograph Data Loggers

10 % rh to 90 % rh 15 ºC to 50 ºC

10 % rh to 90 % rh

15 ºC to 50 ºC

5 % rh 1ºC

3 % rh

1ºC

6

Water Activity

0,100 aw to 0,750 aw 0,750 aw to 0,950 aw

0,020 aw 0,025 aw

7 Environmental Chambers 5 % rh to 95 % rh 4 % rh

8 On-site calibration of item 7 above

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Addendum 1: Amendment Record

Proposed By: Section Change

FM / STC References Updated reference to the GUM FM Definitions Updated added defintions for Repeatability (conditions), Step response time FM / STC Abbreviations BMC changed to CMC FM / STC General Added 5.4 FM / STC Technical Moved 6.1 to 5.4 FM Appendix 1 Updated schedule to new format