the history of temperature measurement 0 °C, the freezing point of water, has
been the defining point for an accurate temperature reference. Building an Ice Point Reference has been a
simple, inexpensive, and effective way to quickly calibrate many different
types of thermometers and temperature sensors.
the popularity of the Ice Point Reference has been replaced by the development
of the more accurate Triple Point of Water Cell. However, Triple Point of Water
Cells can cost over a thousand dollars and some laboratories do not need the
measurement accuracies associated with them.
A properly made Ice Point Reference can obtain accuracies of ±0.01 °C or better and is a useful alternative where it meets the accuracy needs of the laboratory. Because an Ice Point Reference can be readily created by almost any laboratory with a minimal outlay of resources, it will always have a place in temperature measurement and will likely be used for many years to come.
the Ice Point Reference is susceptible to temperature gradients as the ice
melts and the water accumulates at the bottom of the container. For this reason,
the Ice Point Reference is not considered a traceable reference and a
calibrated thermometer must be used to measure the temperature of the water and
provide the measurement traceability.
Creation of the Ice
Cleanliness is important
as small amounts of contaminants or other impurities can cause the temperature
to deviate from 0°C. All equipment and utensils should be rinsed with distilled
water prior to use and gloves should be worn to keep the contents clean.
most efficient way of making ice for the Ice Point Reference is to make ice
cubes in plastic ice cube trays using distilled or de-ionized water that meets
a minimum of Reagent Grade IV per ASTM Specification D1193 ‘Standard
Specification for Reagent Water’. Tap water should never be used, as the level
of contaminants is variable and unknown or have a high concentration of
additives from the water treatment process.
The ice trays should be
filled with the distilled water and put inside a freezer overnight to produce
an adequate supply of ice prior to the preparation process. Any
plastic bins or attachment that will contain the ice should be pre-chilled in a
freezer prior to use. An adequate amount of liquid distilled water to prepare
the Ice Point should also be cooled in a refrigerator.
Preparation of the Ice
is prepared by either shaving or crushing the ice into a fine, snow-like
texture. Shaved ice is
preferred, but crushed ice is acceptable as long as the particles are small. Fine
ice particles less than 1 mm in diameter are essential if accuracies of ±0.01
°C are required. Larger ice chunks can produce local temperature depressions if
used in the Ice Point Reference.
best units for preparing the ice are commercial ice shavers or food processors
with a grating disc. Blenders or other equipment that use blades or knives
typically do not cut the ice into the sizes required effectively and damage to
the blades is likely through frequent use.
purity is a major contributor to Ice Point Reference accuracy. The individual
ice cubes should be inspected before being shaved for a clear, colorless
appearance. Contaminants may concentrate at the center of the ice cubes and may
appear as translucent, milky or discolored areas. Any ice cubes should be discarded
if any are found with observable impurities as the Ice Point accuracy will be compromised.
Constructing the Ice Point Reference
The Ice Point Reference should be prepared in an insulated
beaker or flask of sufficient size to incorporate the temperature sensors being
calibrated. The beaker or flask should also be inserted into a larger, outer receptacle
to assist in the thermal insulation of the Ice Point Reference. The outer
receptacle can be filled with additional ice or contain insulation materials.
The Ice Point container should
be filled approximately one-third full with the cooled, distilled water. The
shaved ice should be added until the container is full. Stir the ice in the
container and compress the ice to form a tightly packed slush to ensure that
there are no air bubbles. Add additional shaved ice as required until the
container is full. Drain any excess water from the top of the container by
tilting the flask allowing for the top layer of the ice to remain moist.
Use any remaining shaved
ice and distilled water to create a secondary chilled bath to pre-cool the
device to be calibrated.
Once the Ice Point Reference
has been prepared, cover the container and allow the contents to stabilize for
at least fifteen minutes before use. Once stabilized, the Ice Point Reference
should maintain its temperature for approximately 30 minutes.
It will be necessary to
occasionally add ice to the top of the container and drain off the melted water
in the bottom of the Ice Point Reference. Water is most dense at 4 °C, so any
melted ice will accumulate at the bottom of the container as it warms to that