Figure 1, A water manometer mounted on a vertical travelling microscope used for the slope determination.
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Figure 2 Typical pressure plot from Standard Calibration Procedure
Figure 3 Typical pressure plot from Temperature Calibration Procedure
Meteormetrics' Instruments are specified, designed, manufactured and calibrated to the high standards required for scientific research, precision metrology, fine process control and exact environmental monitoring. They currently find applications in health and safety studies, in marine instrumentation, in environmental surveying, in industrial manufacturing, in scientific laboratories, in the automotive industry, and in meteorological measurement and standardisation.
In all these applications, reliability, accuracy, precision, stability, readability and convenience are important factors which attract users to Meteormetrics' Instruments. It is thus important that users can obtain dependable verification of the instruments' performance at the time of purchase and at intervals during their working life. In addition to the extensive testing of each instrument prior to despatch, Meteormetrics Limited provides three levels of calibration and certification to meet users' requirements.
This is carried out by comparing the reading of the instrument with the pressure measured using a water
manometer equipped with a travelling microscope. One example of this instrument is shown in figure 1. The
instrument under test is connected to the manometer, a pressure ballast and a means of adjusting the
pressure. A series of calibration points are obtained across the required pressure range and the results
plotted and the slope determined by a least-squares regression. Typical results for a Meteor2000WXDL
are shown in figure 2.
This calibration method has three important features:
(a) It is an primary method: in principle it requires only measurements of mass, length and time in the form
of the manometric height, the density of water and the acceleration due to gravity. This is true when the atmospheric pressure does not change during the calibration
process, which is independently checked. If the atmospheric pressure does change in the course of the calibration,
the results can be corrected for the change.
(b) It is a transparent method: The calibrating technician has visual assurance that the pressure
applied to the instrument is correct. This is most useful in the avoidance of errors. A weakness of
electronic measuring devices is that they are opaque in this sense: the user has no certain
visual method of ascertaining whether the instrument is operating correctly. This is even more significant
where opaque instruments are calibrated against standards which are themselves opaque in this sense.
(c) It is a relative method: It only calibrates the slope of the instrument's response. For many
applications of barographs this is all that is required especially where the offset will be adjusted to
read the atmospheric pressure corrected to sea level at a particular location.
This calibration is applied routinely, without extra charge, to all Meteormetrics' barographs intended
for precision applications. Additionally the instrument will be set to read local or sea level pressure
and a specified altitude upon request.
In addition to the above calibration a Meteormetrics' Precision Barograph can be compared to a standard
instrument which has been calibrated by the National Physical Laboratory (UKAS accredited) and which carries an NPL certificate
of calibration. The instrument supplied has its own calibration certificate and a copy of the certificate for
the NPL calibrated instrument with which it has been compared. A 10 point calibration is carried out.
The instrument supplied with this level of calibration has also the facility to be restored to the calibrated offset
even if it has been subsequently adjusted to read sea-level pressure values or pressures from some other offset.
The long term drift of the Meteor2000 series of instruments is not greater than 0.27mB per year. This means that
recalibration every three years with result in measurements normally within a millibar of the correct value throughout
the calibration period. There is a charge for this level of calibration on supply of the
instrument and for subsequent recalibrations.
For the best fully independent UKAS accredited calibration, Meteormetrics Limited recommends its instruments be calibrated by the National Physical Laboratory. The NPL offers 4 or 10 point calibration procedures across any range of pressure specified by the user of the instrument. After taking instructions from the client, Meteormetrics Limited will arrange for the calibration to be carried out by the NPL, and will provide an NPL certificate with the instrument. This service will be carried out at cost of the NPL calibration fee plus a service charge.
For instruments which contain an on-board thermometer, temperature calibration is provided where required to 0.1 deg C precision
by comparison with a chromel alumel thermocouple with a cold junction of melting ice, read by a precision Time Electronics
Potentiometer with a resolution of 1 microvolt. This is equivalent to 0.025 degree C. A typical calibration graph
is shown in Figure 3.
Meteormetrics Limited invites enquiries on any aspect of their instruments and their calibration. If you have particular requirements in terms of precision for a barographic instrument, please get in touch. We may well be able to help.
Kevin Scott
Meteormetrics Limited.