MSL Technical Guide 1
The Ice Point

Introduction
The melting point of water is a very simple, effective
and inexpensive temperature reference. Figure 1 showsthat the melting point of water at atmospheric pressure isnear 0.0025 °C. However, this is not the ice point as it isused as a temperature reference. The ice point is defined as the equilibrium temperature of ice and airsaturated water, which occurs at the lower temperatureof 0.0 °C almost exactly. The 0.0025 °C difference iscaused by dissolved air in the water and ice.
Historically the ice point was the defining point for
many temperature scales until the more precise water
triple-point cells were developed. It still has a major rolein thermometry since it is a fixed point that can be readily
achieved by almost any laboratory with a minimal outlay
of resources. It is essential for people who take theirtemperature measurements at all seriously. Whether theaccuracy required is ±100 °C or ±0.01 °C, the ice point is
an invaluable aid for ensuring that a thermometer isfunctioning correctly.
The main advantage of the ice point is that it can bemade very simply and extremely cheaply and, so long as
the basic principles are followed, it is relatively easy torealise an accuracy of ±0.01 °C. The ice point can beused to achieve uncertainties of the order of ±2 mK but
very close adherence to the procedure is needed.
100

LIQUID

101SOLID

Critical point(373.99 °C)

Normal melting point(0.0025 °C)

0.1
Normal boiling point(99.974 °C)

0.01
0.001
0.00010.00001
200

Triple point(0.01 °C)

300

VAPOUR

400

500

600700Temperature / K

Figure 1. The phase diagram for pure water showing the different temperatures and pressures at which water exists as liquid,solid (ice), and vapour.

The Equipment
To assemble an ice point you will need:
An insulating container, such as a vacuuminsulated flask or expanded polystyrene flask, approximately 300 mm to 400 mm deep and 80 mm to 100 mm
in diameter is ideal. The flask retards the melting of theice by its insulating properties. It should be deep enoughto hold the full length of the thermometer below its ice
point with 50 mm to 100 mm extra depth to accumulate
melt-water. If a metal-sheathed thermometer is being
checked it will need to be immersed to a minimum ofabout 300 mm.
A siphon is placed in the flask to enable the removalof excess water as the ice melts. Because the definitionof the ice point is the equilibrium of melting ice with airsaturated water, air must be allowed to circulate throughthe melt water on the surface of the ice. In addition, water has its maximum density at about 4 °C. If a large volume of water is allowed to gather at the bottom of theflask, it is possible for the water to become warm. Thus,
the water level should never be allowed to rise to reachthe bottom of the thermometer.
Clean, shaved ice that is free of impurities and ideally made from distilled or de-ionised water. Because
freezing is also a purification process, food-grade ice
made in freezers that employ a washing process is alsosatisfactory. Good, clean tap water is often satisfactory
but should be avoided as it will occasionally be contaminated or have a high concentration of additives from thewater treatment process. If tap water must be usedcheck its electrical resistivity; at 10 °C its resistivity
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should be higher than 0.5 × 10 Ω.m. Some tap water
will be completely unsatisfactory in this respect and it
pays to check.
The ice must be shaved or crushed, ideally into
small chips measuring less than 1 mm across. For liquidin-glass thermometers, which have a poor thermal conductivity, larger chips up to 5 mm will be satisfactory.
However, for steel sheathed probes, such as platinum
resistance thermometers, fine ice is essential if accuracies of ±0.01 °C are to be achieved. The ice may be
shaved using commercial ice shavers ranging fromcheap plastic bar accessories to professional ice shavers. A low cost alternative, which is satisfactory for infrequent use, is a food processor with a grating disc. Notethat discs with blades or knives are not suitable because
they do not cut ice very effectively and the processor will
be quickly damaged.
Approximately 300 mL of clean water is required.
Distilled water or de-ionised water is ideal, as is the melt
water from the ice.
A clean rod of a similar diameter to the thermometeris used to make a hole in the ice.

The Procedure
First, one-third fill the flask with clean water. Freshly
shaved ice is quite often colder than 0 °C. By wetting theice we ensure that it is melting. The difference in thecondition of the ice is readily visible since cold ice
freezes water vapour from the atmosphere giving it awhite frosty appearance. By comparison the wet ice, at
0 °C, is quite translucent (see Figure 2).

Measurement Standards Laboratory of New Zealand • fax: +64 (0)4 931 3003 • e-mail: msl@irl.cri.nz • website: http://msl.irl.cri.nz

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be demonstrated by an independent measurement of theice-water temperature with a calibrated thermometer.
If electrical insulation from the water is required, anoil-filled thermowell may be inserted into the ice.
The ice-point can also be adapted to suit some lowtemperature radiation thermometers. This is described inMSL Technical Guide 2 – “The Infrared Thermometry IcePoint.”

MetalRodSiphon
TubeVacuum
FlaskFigure 2. Shaved ice, with frosty ice on the left and ice afterslushing on the right.

Add the shaved ice to a sufficient depth. For liquid-inglass thermometers, the container should be filled to the
top to allow the thermometer to be read without parallax
errors. For other thermometers, there must be sufficient
ice to ensure good immersion.
Siphon off any excess water, and compress the remaining ice to form a tightly packed slush.
Immerse the thermometer. For liquid-in-glass thermometers use the clean rod to make a hole beforehand
to prevent breakage and undue stress on the bulb (seeFigure 3)
Wait approximately 15 to 20 minutes for thermal
equilibrium to be reached before reading the thermometer. Read the thermometer several times at intervals of afew minutes to be sure that equilibrium has beenreached. For steel-sheathed thermometers, it may benecessary to compress the ice quite firmly to achieve an
accuracy of 0.01 °C.
Periodically it will be necessary to add ice to the topof the container and siphon off the melt water to preventthe level rising to the bottom of the thermometer.

Notes
This procedure is not suitable for general thermocouple use. Although it is suitable for a single thermocouple reference junction, it will not cope with the large
heat input from many thermocouples or one heavy thermocouple. To ensure good thermal contact with the reference junction a well-stirred ice-water mixture or acommercial ice-point apparatus may be more suitable.
The ice-water mixture is, however, susceptible to temperature stratification, i.e. ice at 0 °C floating on top of
the water and water at 4 °C (the temperature at whichwater is most dense) sinking to the bottom of the container. For this reason the ice-water mixture is not considered a temperature reference and its traceability must

Ice slush

WaterBeaker
Water

Figure 3. An ice-point apparatus for calibrating thermometers
or for checking their stability.

ReferencesJ V Nicholas and D R White, Traceable Temperatures:
An Introduction to Temperature Measurement and Calibration, 2nd Ed., John Wiley & Sons, Chichester, 2001.
Figures reprinted, with permission, from Traceable Temnd
peratures 2 Ed., J V Nicholas and D R White, John
Wiley & Sons, Chichester.
Prepared by D R White, June 2002.

MSL is New Zealand’s national metrology institute, operating within Industrial Research Limited under the authority of theNew Zealand Measurement Standards Act 1992.

Measurement Standards Laboratory of New Zealand • fax: +64 (0)4 931 3003 • e-mail: msl@irl.cri.nz • website: http://msl.irl.cri.nz

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