Why Use Fluke Calibration Metal Freeze-Point Cells?

By now you’re aware of the ITS-90 temperature scale and you’ve probably been preparing budgets for the new primary standards you need in your lab. Well, you’ve come to the right place because we’ve got all of the primary standards, including a complete line of the best metal freeze-point cells you can buy.

All Fluke Calibration cells meet the rigid standards of ITS-90. The table below lists the cells Fluke Calibration manufactures with their freezing points and other critical data. The hydrostatic head pressure in a cell causes a small change in the equilibrium temperature that should be applied to the measurements as part of the calibration process.

The table also states the reproducibility and expanded uncertainty of each type of cell.

Fluke Calibration scientists have designed and tested metal freeze-point cells for many years. Not only do we manufacture all the major freeze-points, our metrologists have written extensively on the theory and use of cells and have created new designs covering a range of applications no other company can match. They can answer every question you have on freeze-points and explain how we have handled the radiation losses along the well, minimized possible stem error and made pressure corrections for the highest possible measurement accuracy.

Pure metals melt and freeze at a unique temperature through a process involving the absorption, or liberation, of the latent heat of fusion. If 100% pure metals were available, each cell of a particular type would melt and freeze at its exact theoretical temperature, but since 100% pure metal is not possible, cells vary slightly from their theoretical absolute.

The best freeze-point cells are the ones that get very close to their theoretical freezing temperature and provide a temperature plateau that’s stable and long lasting for calibration work. Changes as small as 0.01 mK (0.00001°C) are measurable, so cell uncertainty is definable. As shown in the table, Fluke Calibration cells come very, very close to the absolute theoretical limits possible with today’s material science. We use 99.9999% pure metal. Six 9s purity is as good as it gets. Some manufacturers may use five 9s. Make sure they specify purity on their quotations.

In addition to the data in the table, another measure of the quality of a freeze-point cell is its ability to maintain a freeze plateau. The longer the plateau, the better the cell and the more cost-effective it is to use. Fluke Calibration fixed-points have plateaus that range from 14 hours to 14 days. Call us and we’ll share our knowledge of creating the longest and most stable freeze plateaus in the industry. No other producer of freeze-points beats the performance of a Fluke Calibration cell.

All freeze-point cells have to be used and maintained in specially-designed freeze-point furnaces. Furnaces come in a variety of temperature ranges, and each one must have a very uniform temperature region for the freezing and melting process. High accuracy is achieved by the formation of two liquid-solid interfaces during freezing. One interface should be adjacent to the inner surface of the crucible, and the other should be on the outer surface of the central well, which is the closest point to the thermometer.

A very thin solid shell should form on the central well, and the concentration of impurity in this thin shell should be much lower than the average impurity in the core metal. The temperature on the second liquid-solid interface should be very close to that found in a 100% pure metal under ideal conditions.

Fluke Calibration’s freeze-point furnaces provide the proper temperature, stability and uniformity to create the longest, most stable temperature plateau possible.

The immersion depth of a thermometer in the liquid metal (the distance from the surface of the liquid metal to the middle point of the thermometer’s sensor) is approximately 180 mm in cells made by Fluke Calibration. The outside diameter of each cell is 48 mm, the thermometer well is 8 mm and cell length is 290 mm.

Cells are delicate and must be handled with extreme care. Fluke Calibration provides special storage cases, gloves for handling and complete care instructions. Cells are hand carried rather than shipped to ensure complete integrity of measurements.

The performance of every Fluke Calibration cell is guaranteed. When you call, ask about our temperature calibration school where you can learn more about the theory and operation of all metal freeze-point cells. We’ll answer every question you have and help solve any problem you encounter. We use metal freeze-point cells in our lab every day, and our metrologists have decades of experience with them.