SERIES 4000 Precision Dewpointmeter (Remote, Climatic, RS and TRS)Users Guide
1 INTRODUCTION
1.1 General
Michell Instruments manufacture a wide range of on-line and portable instruments for
the determination of moisture content in air and gases. In addition, the company
manufactures Humidity Calibration Systems for Industrial and Scientific users and
undertakes custom design-and-build contracts for large scale humidity monitoring
and control applications.
The Series 4000 Precision Dewpointmeter is a precision instrument which may be
used in Calibration and Standards Laboratories and in exacting process monitoring
and control applications where wide humidity range and high accuracy is a prime
requirement.
A direct indication of dew point to a resolution of 0.01 °Celsius (°C) is provided by the
instrument display, the dew point may also be displayed in a number of other
engineering units including °Farenheit (°F), Parts Per Million by Volume (ppm(v)),
Parts Per Million by Weight in SF6 (ppm(w) in SF6), grams per cubic metre (g/m3)
and grams per kilogramme (g/kg). Additionally, the instrument can measure and
display the gas temperature using an external remote P.R.T. probe (supplied as
standard). This temperature may be displayed in °C or °F. Relative Humidity (% rh)
may then be calculated from the fundamental gas dew point and temperature at
atmospheric pressure. The instrument may also be configured to display measured
gas pressure (kPa) using an external pressure transducer, and use this measured
pressure to calculate pressure compensated (ppm(v), (ppm(w))SF6, g/m3 and g/kg.
Experiments conducted by the National Physical Laboratory (NPL) have indicated
that, by reference directly to the instrument's resistance thermometer, accuracies of
thousandths of a °C. dew point can be observed and repeated.
NPL (Teddington, UK) use a Series 4000 as a Transfer Standard instrument. Also,
the National Measurement Accreditation Service (UKAS) use one as a portable
Transfer Standard to audit the performance of Humidity Calibration Laboratories.
The Series 4000 is based on the cooled mirror principle and is derived from Michell
Instruments' highly successful Transfer Standard Dew-point Hygrometer. This
Transfer Standard was developed during the early 1980's in collaboration with the
EEC Bureau of Reference as a European link to the National Institute of Standards
and Technology (NIST) humidity standard in Washington, USA.
Dew points over the range -100 °C to +80 °C may be measured with the Series 4000
Remote instrument with the relevant auxilliary heating / cooling system, this is
equivalent in terms of absolute humidity at atmospheric pressure to a range of 10
ppb(V) to almost 50% V/V (percentage moisture by volume).
In order to measure very low humidities a cooling system may be required,
depending on the model this may be a single (RS) or twin (TRS) refrigeration system.
In order to measure high humidities the sensor needs to be heated above the dew-
point temperature, this is usually achieved by placing the sensor in a heated
chamber (climatic sensor).
The use of the single or twin refrigeration systems on the S4000 RS and S4000 TRS
models, together with the use of electronic PID temperature control, enables sensor
unit operating temperatures as low as -80 °C to be maintained. The low ambient
operating temperatures of these refrigerated sensors minimises the uncertainties of
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