TR 44-01 - Sanas

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TR 44-01• Water Vapour PressureThat part of the total pressure contributed by the water vapour. Expressed in units ofpressure e.g. pascals (Pa). [3]3. Abbreviationsa wwater activityBMCBest Measurement CapabilityMSDS Material Safety Data SheetsrhRelative HumiditySANAS South African National Accreditation System4. Environmental RequirementsSignificant changes in relative humidity may occur with even slight variations in temperature and withoutchange in absolute humidity. For example, a 1ºC variance in temperature at 20ºC and 50% rh canintroduce an error of ± 3,2 %rh.[3].Therfore an accredited laboratory working in the field of humidity measurements shall operate under thefollowing environmental conditions:4.1 The laboratory should be maintained at a temperature of 23 ° C ± 3 ° C. Where measurements areperformed outside this specified requirement, the laboratory must ensure that appropriatecorrections are applied.4.2 Rates of change and gradients of the ambient temperature and humidity must be kept to below1 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 beprovided.[1]4.4 Localized isolation in the form of chambers, cabinets and enclosures may be utilized in order tominimize the effects of thermal currents and temperature gradients and in so doing provide astable and homogeneous environment conducive to reduced measurement uncertainties.4.5 The laboratory shall consider and include the stability of the environmental conditions on therelative humidity when estimating the uncertainty of measurement.5. General Requirements5.1 Laboratories operating in the field of humidity may be accredited for the calibration ofdewpoint hygrometers, relative humidity hygrometers, humidity generators, water activitymeters, saturated and unsaturated salt solutions, environmental chambers, and limitedtemperature calibration.(Refer to the Schedule of Accreditation – Appendix 1)5.2 Laboratories performing the calibration of the temperature indicating or recording component ofthermo-hygrometers or thermo-hygrographs respectively are limited to a BMC of ±1 ºC andrange 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 eachspecific parameter. Generally, the units will be °C ( dew point), °C (frost point), %rh relativehumidity, a w (water activity).6. Technical Requirements6.1 The laboratory is required to have MSDS for all standards fluids, and chemicals, includingsaturated and unsaturated salt solutions, and cleaning materials used within the laboratory.[2]6.2 The laboratory is required to have procedures in place for the storage and usage of standards inorder to ensure their ongoing integrity.©SANAS Page 4 of 7
TR 44-016.3 The laboratory shall have and apply procedures for all calibrations.6.4 The laboratory shall have the necessary standards and equipment required to perform thecalibration measurements in accordance with their accreditation schedule.6.5 All measurements shall be taken only after adequate stabilization time. This period shall beaddressed in the calibration procedures, taking in to account possible differences in responsetime due to different time constants of the reference standard and the hygrometers or saltsolutions under test.6.6 The calibration of environmental and climatic chambers requires the availability of suitableinstrumentation capable of data storage. Laboratory procedures shall address the number oftemperature and relative humidity sensors required, and the spatial distribution thereof, and thisshall 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 overwhich the measurements are to be performed6.7 It is recommended that the humidity be measured at each point in the chamber where thetemperature is measured. The application of corrections based on spatial temperaturemeasurements, will result in humidity values with a larger uncertainty.6.8 Laboratories should take cognisance of effects of barometric pressure on humiditymeasurements6.9 Laboratories shall record and report the temperature at which the measurement of water activityor relative humidity has been made.7. The Accreditation ScheduleThe accreditation schedule shall follow the following format; the functions will be determined by thelaboratory application, ranges and uncertainties are approximate.See Appendix 1©SANAS Page 5 of 7
Page 2 and 3: TR 44-01CONTENTS:1. Purpose and Sco
Page 6 and 7: TR 44-01APPENDIX 1.S C H E D U L E

TR 44-01 - Sanas

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