Throughout 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.
Recently, the popularity of the Ice Point Reference has been replaced by the development of the more accurate Triple Point of Water Cell.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.
However, 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.
The 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
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.
The 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.
Water 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 temperature.