TR 44-01 - Sanas

TR 44-011. Purpose and ScopeThe purpose of this document is to define the specific environmental, general and technicalrequirements to be met by accredited laboratories in the field humidity measurement and calibration.This document is applicable to South African National Accreditation System (SANAS) AccreditedLaboratories in this field.2. Definitions and ReferencesReferences[1] OHS Act, 1993 Environmental Regulations 1987 - Lighting[2] OHS Act, 1993 General Admin Regulations 1994 – Handling of HazardousChemical Substances[3] NPL, UK 1996 A Guide to the Measurement of HumidityIssued by the NPL and Institute of Measurement and ControlISBN 0-904457-24-9[4] VIM International Vocabulary of Basic and General Terms in Metrology,BIPM, IEC, IFCC, ISO, IUPAC, OIML, 1993.DefinitionsAbsolute HumidityThe mass of water vapour present in a unit volume of moist air of a given temperature andpressure. SI (metric) units are grams of water per cubic metre of air (g.m -3 ). [3]• Dew PointThe temperature at which dew, or condensation forms, on cooling a gas. This is in effect thetemperature at which air becomes saturated in equilibrium with water, expressed in degreesCelsius (ºC) or occasionally other units of temperature.[3]• Response TimeTime interval between the instant when a stimulus is subjected to a specified abrupt change andthe instant when the response reaches and remains within specified limits around its finalsteady value.[4]RepeatabilityThe ability of a measuring instrument to provide closely similar indications for repeatedapplications of the same measurands under the same conditions of measurement. [4]• Relative HumidityRelative humidity is the ratio of the amount of water vapour, e, in the air to the amount of watervapour, e s , that would be in the air if saturated at the same temperature and pressure, and canbe expressedrelative humidity (in %) = e/e s X 100Where e is the water vapour pressure, and e s is the saturation vapour pressure of the prevailingambient temperature [3]• Saturation Vapour PressureMaximum pressure of water vapour that can exist at a given temperature. Expressed in units ofpressure e.g. in pascals (Pa) or in non-SI units such as millibars (mbar) or millimeteres ofmercury (mm Hg). [3]• Water Activity (of a substance)Water activity (a w ) is the relative humidity which is eventually reached in a closed space where ahygroscopic substance, such as a foodstuff, has been placed. It is the same as equilibriumrelative humidity except that it is expressed on a scale of 0 to 1 (no units) [3]©SANAS Page 3 of 7