Isotech Evaluation User Manual

Isothermal Technology Ltd

Evaluation

Fixed Points for the Thermal Calibration Laboratory

(Celebrating 10 years of Slim Cell Production)

Introduction

The advantage of fixed point calibration is the small uncertainties associated with the
method.

The very pure fixed point substance can be melted or frozen to produce an absolute
temperature for calibrating temperature sensors.

The figure below shows the changes in slope and absolute temperature that occur for
various purities.

Typically a 6N pure cell will melt 80% of its contents over 2mK and its contents will
freeze 50% in between 0.1 and 0.2mK. The above is the accepted method of specifying
the purity of a fixed point – see references 1 and 2 for more details.

A 5N pure cell will melt 80% of its contents over 20mK and 50% of its freeze will occur
over 1 to 2mK.

Typical Melting & Freezing Curves of Metals of Three Levels of Purity

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For all but Primary Laboratories, it is quicker, simpler and easier to use fixed point cells
during their melt. Isotech’s Slim Cells are all 6N+ pure so that the melt curve is very flat.

Small or ‘Slim’ Fixed Point Cells

These cells contain less of the metal contained in cells designed to fully realize the ITS­90 Scale. By reducing the size (both diameter and length) the slim cells can be fitted into
much smaller pieces of apparatus. The negative side of such a design concept is that
thermometers being calibrated are not so deeply immersed. This may, or may not be a
problem, as we shall show later in this evaluation.

The melt plateau has the following advantages over the freeze:-

1. It can be automated. A simple timer switches on the apparatus 1 to 2 hours
before it is needed its controller set to 1°C above the melt. The cell then
automatically comes onto its melt, which will last all day. Over night the timer re­freezes the cell ready for the next day. To freeze a cell means melting it first and
most of the day is lost.

2. As thermometers are calibrated (at the rate of about 1 per 20 minutes) each one
re-freezes a little of the melted cell causing the melt to lengthen i.e. the more
calibration is performed the longer the plateau.

3. Stem conduction is minimized. This is because the sensor being calibrated
passes through the apparatus, which is 1°C above the cell’s temperature before it
comes out into ambient air.

Summary Chart of the Results Obtained in this Evaluation

Slim cells in small apparatus compared to large cells in large apparatus
Slim Mercury Triple Point Cell in Europa ±0.1mK

Slim Water Triple Point Cell in Europa or Venus ±0.3mK
Slim Gallium Cell in Europa, Venus or Calisto ±0.3mk
Small Indium Cell in Calisto ±0.5mK
Slim Tin Cell in Medusa ±0.6mK
Slim Zinc Cell in Medusa ±1.4mK
Slim Aluminum Cell in Oberon ±5mK

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Experimental Results

The following results at various fixed points were all performed in the same way.
A Standard Platinum Resistance Thermometer was calibrated in a large cell in a large

apparatus. The thermometer was then transferred to the smaller cell in a large apparatus
to check out the cell’s purity.

Lastly the smaller cell was placed in the portable apparatus and the difference (if any)
gives a measure of the stem conduction error due to the portable apparatus.

For convenience I will group the cells into 3.
Group 1 comprises the Triple Point of Mercury, the Triple Point of Water and the Gallium

Cell.
1 piece of apparatus can create the conditions to melt or freeze any of group 1 cells.

Called Europa, this piece of apparatus can create temperatures around the cells from –
45°C to +140°C.

The most complex temperature to create is the Triple Point of Water, and so this will be
described first.

In 1982 a paper was presented at the 5th Conference of Temperature by Cox & Vaughn
in which was described a slush method for creating the Triple Point of Water. Briefly the
method comprised of supercooling the water cell to –7°C and then giving it a shake.
Shaking the cell initiated nucleation and sufficient (about 30%) water turned to small ice
crystals to bring the cells temperature up to +0.01°C.

This method has been adapted for use in the Europa, or Venus.
A Small Triple Point of Water Cell was placed inside the Europa apparatus the

temperature adjusted to cool the water cell to –6°C or –7°C it was then shaken to create
a slush of ice and water. After a further 30 minutes at –7°C the Europa was reset at 0°C.
To gauge the accuracy of this approach a 25.5ohm quartz sheathed Standard Platinum
Resistance Thermometer was calibrated in a Large Triple Point of Water Cell. Next it
was transferred to the small cell in Europa. A plateau lasting longer than 16 hours was
obtained with the thermometer reading within 0.3mK of its calibrated value (Graph 1).

Such a system, using RS232 link can be automated (with the exception of the shake) to
provide a economically priced Triple Point of Water temperature all day every day.

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The Gallium Point

Either by changing cells, or by having a second Europa apparatus the melting
temperature of Gallium can also be created as described below:-

A small gallium melt point cell is placed inside the Europa, the temperature was set so
that the block was 2 to 3°C above the gallium melt point. A thermometer was placed in
the reentrant tube of the cell and the warm-up and arrival on the plateau are observed.
Once the gallium begins to melt, 5cc of warm water was introduced in the re-entrant tube
to melt a sheath around the reentrant tube, and the temperature of the Europa was
reduced to 0.5°C above the melt temperature.

The thermometer read within 0.3mK of the calibrated gallium point after 20 minutes and
remained within 0.3mK of the expected melt value for over 48 hours, (Graph. 2)

Unlike the gallium apparatus, the Europa is not self-protecting, its flexibility precludes
this. Thus after melting, it is necessary to remove the cell and freeze the gallium from the
bottom up. This is because gallium expands 3% as it freezes. Freezing is easily
accomplished by placing the cell in 30 to 50mm of cold water, or onto a bed of ice cubes.

The Europa can also be used with a Slim Mercury Triple Point Cell as follows:­After placing the cell into the well of the Europa its control temperature is set to –45°C

the cell is cooled, supercooled, nucleates and quickly comes onto its freeze plateau. At
this time the set point is raised to 0.5°C below the freeze temperature and calibration
can begin. Again the more sensors are calibrated, the longer the freeze plateau.

Thus with 1 small piece of apparatus and 3 slim cells, 3 of the most fundamental points
of ITS-90 can be created and maintained for a working day or longer to an accuracy of

0.0003°C or better.

Isothermal Technology | Pine Grove | Southport | Merseyside | PR9 9AG | United Kingdom

Tel: [phone] / 544611 | Fax: [phone] | [e-mail] | www.isotech.co.uk