Description
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. Hart's 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 reference Hart cell.
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 ultra-clean,
state-of-the-art lab, using high-density, high-purity graphite
crucibles containing metal samples with purity of at least
99.9999 % (six 9s) and, in many cases, 99.99999 % (seven 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 precise pressure of the argon
gas to ensure the most accurate corrections for pressure.
Once manufactured, all Hart cells are tested and supplied with
an assay of metal-sample purity. Every traditional size ITS-90
cell further undergoes more rigorous testing in our primary
standards lab where we realize melt-freeze curves and perform a
detailed “slope analysis” to confirm cell purity. If you want more
data, we’ll give you an optional intercomparison with our own
reference cells.
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’s 5943 Gallium Cell is sealed in a stainless steel
envelope. High purity gallium (99.99999 %) is enclosed in a
plastic and metal shell. The stainless steel container is then
filled with pure argon gas at one standard atmosphere at the
melting-point temperature.
Gallium expands by 3.1 % when it freezes 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. Realization and maintenance of the cell
is automated with our 9230 Maintenance Apparatus. This apparatus
will provide melting plateaus up to eight days and a convenient
control to automatically 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 and can last for up to two months when
maintained in our 7012 or 7312 baths. |
