ITS-90 Fixed-Point Cells

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Hart scientists have designed and tested ITS-90 fixed-point cells for many years. Not only do we manufacture all the major fixed 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.  (Why Use Hart Metal Freeze-Points?)

Our testing of fixed-point cells is also unmatched. The scope of our  accreditation includes the testing of ITS-90 fixed-point cells. Each cell may be purchased with this intercomparison option, which includes comparing the equilibrium value of your cell against that of a Hart cell that has been intercompared with a cell at NIST.

Traditional Freeze-Point Cells

If you want true primary temperature standards capability, you want metal freeze-point cells that are very close to the theoretical freezing temperature and provide plateaus that are both stable and long lasting.

Hart’s metal freeze-point cells are the culmination of more than 20 years of primary standards experience.  No other company has as much experience in the development of metal fixed-point cells as Hart. That’s why you’ll find Hart cells in many national metrology institutes around the world.

Each Hart cell is carefully constructed in our state-of-the-art lab, using high-density, high-purity graphite crucibles containing metal samples with purity of at least 99.9999% (six 9s).  The crucible is enclosed within a sealed quartz glass envelope that is evacuated and back-filled with high-purity argon gas.  A special sealing technique is used to seal the cell at the freezing point.  We measure and record for you the exact pressure of the argon gas to ensure the smallest corrections for pressure.

Every Hart cell undergoes thorough testing in our accredited lab where we realize melt-freeze curves and perform a detailed “slope analysis” to confirm cell purity.  This data is supplied with every cell.  If you want more data, we’ll give you an optional intercomparison with our own reference cells that have been tested at NIST.

Gallium Cells

Gallium cells are a great reference for validation of instruments subject to drift, like SPRTs, and they’re important for calibrating sensors used near room or body temperatures in environmental monitoring and in life sciences applications.

Hart makes two traditional-size gallium cells. The Model 5903 is sealed in a borosilicate glass envelope. Pure gallium (99.99999%) is enclosed in a plastic and glass shell. The borosilicate container is then filled with pure argon gas at one standard atmosphere at the melting-point temperature.

Gallium expands on freezing by 3.1%, requiring the cell to have flexible walls. Unlike some manufacturers’ cells, which are made from PTFE enclosure materials, our cells don’t need pumping and refilling because they’re not gas permeable. In fact, we guarantee our cells will maintain their uncertainty of 0.1 mK for at least five years. When maintained in a Hart Model 7012 or 7312 Bath, your melting plateaus will last as long as 14 days-that’s days, not hours.

Hart’s new Model 5943 Stainless Steel Gallium Cell uses similar manufacturing techniques and provides the same low uncertainty as our glass-enclosed version, and its use is automated with our Model 9230 Gallium Cell Maintenance Apparatus. This device allows users to achieve a new melt plateau each week with an investment of just five minutes. Never has the maintenance of a world-class gallium cell been easier.

Water Cells

While simple ice baths are often used as a calibration point at 0°C, their limitations include gradients, purity problems, repeatability issues, and variances in construction and measurement techniques. Triple point of water cells not only solve these problems, they represent the most used temperature on the ITS-90, and they’re inexpensive to own and use.

Hart makes three traditional size TPW cells that have been repeatedly proven in national labs to surpass their published uncertainty specification of ±0.0001°C. Ice mantles may be formed using dry ice, LN2, or immersion freezers (we’ll teach you how) and can last for up to two months when maintained in our Model 7012 or 7312 Baths.

Open Cells

Made from the same materials and with the same manufacturing techniques as their sealed counterparts, Hart’s new series of “open” fixed-point cells include a valve for connecting to a precision pressure-handling system within your lab. Using such a system, the cell can be evacuated, charged and purged several times with a pure inert gas, then charged again to a regulated pressure level while measurements are made with the cell.

Because open cells allow users to measure the pressure within the cell, uncertainties due to pressure corrections may be minimized. Open cells can be used for demanding temperature-versus-pressure applications as well as precision SPRT calibrations.

See how Hart's metal fixed-point cells tested at NIST.

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