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The triple point of water
(TPW) is not only the most accurate and fundamental temperature
standard available, it's also one of the least expensive and
simplest to use.
Water cells are essential! Triple point of
water cells fill four critical purposes. First, they provide the
most reliable way to identify unacceptable thermometer drift
between calibrations - including immediately after a calibration
if the thermometer has been shipped. Interim checks are critical
for maintaining confidence in thermometer readings between
calibrations. Second, they provide a critical calibration point
with unequaled uncertainties.
Third, for users who characterize probes
using ratios (that is, they use the ratios of the resistances at
various ITS-90 fixed points to the resistance of the thermometer
at the triple point of water, indicated by 'W'), interim checks at
the triple point of water allow for quick and easy updates to the
characterizations of critical thermometer standards, which can be
used to extend calibration intervals.
And lastly, the triple point of water is
where the practical temperature scale (ITS-90) and the
thermodynamic temperature scale meet, since the triple point of
water is assigned the value 273.16K (0.01°C) by the (ITS-90) and
the Kelvin is defined as 1/273.16 of the thermodynamic temperature
of the triple point of water.
Good triple point of water cells contain only pure water and pure
water vapor. (there is almost no residual air left in them). When
a portion of the water is frozen correctly and water coexists
within the cell in its three phases. The "triple point of water"
is realized. Hart water cells achieve this temperature with
expanded uncertainties of less than 0.0001°C
and reproducibility's within 0.00002°C.
In simple terms, water cells are made from
just glass and water, but there's more to it than that! For
starters, that's not just any water in there.
Heavy Water
Hart cells contain carefully and repetitively distilled ocean
water and are meticulously evacuated and sealed to maintain an
isotopic composition nearly identical to the international
standard. "Vienna Standard Mean Ocean Water," or "VSMOW".
The oxygen atoms found in most water are
predominately comprised of eight protons and eight neutrons (16O).
Some oxygen atoms, however have an extra neutron (17O)
or two (18O). Similarly, the hydrogen atoms in water
normally contain only a single proton (1H), but
sometimes contain a neutron also (2H), resulting in
"heavy" water. These isotopes coexist in varying proportions in
ocean water, and continental water, with ocean water being the
heaviest.
The ITS-90 recommends that water cells be
made from water with "substantially the isotopic composition of
ocean water." Research has shown that TPW errors associated with
isotopic composition can be as large as 0.00025°C. The uncertainty
contribution from VSMOW errors in Hart cells is less than
±0.000007°C. That's seven micro-Kelvin!
Hart offers two options for verifying the
isotopic composition of any water cell, both at nominal costs. We
can submit to a testing laboratory a sample of water taken from
your own cell (after it was completely manufactured, so you
get a valid comparison) and give you the test report. Or we can
send that water sample to you in a sealed ampoule for you to
conduct your own tests. Hart can even provide multiple samples
from the same cell (virtually as many as you'd like) so you
can check for changes over time.
Impurities
Further, the potential for errors due to water impurity is even
greater than errors from isotopic composition. Hart cells undergo
multiple distillation processes and utilize special techniques to
retain water purity. Among other things, their primary standards
scientists are able to connect quartz cells directly to the glass
distillation system without using coupling hardware that may
invite contamination.
Glass Vs. Quartz
Most Hart water cells may either be purchased with borosilicate
glass or with fused silica ("quartz") housings. What's the
difference? Glass is less expensive than quartz, but it's also
more porous, allowing impurities to pass through it over time.
Research indicates that cells made from glass typically drift
about 0.000006°C per year.
Many Sizes
Hart cells come in four general sizes. Models 5901A, 5901C, and
5901D each come in either quartz or glass shells and include 265mm
of thermometer immersion depth. The primary difference between
these models (other than the arm on the 5901A) is the inside
diameter of the probe well. Note that the inside diameter of the
5901C cells varies with the shell material. A variety of baths is
available, which can maintain the triple point within these cells
for many weeks. Accredited (NVLAP) test certificates are available
with any cell under Harts Model 1904-TPW.
The 5901A cells include an arm that can be
used as a handle, a hook, or a McLeod gauge to demonstrate how
much residual air is trapped in the cell. Carefully developed
manufacturing process at Hart keep the air bubble in a quartz cell
as small as the air bubble in glass cells.
A fourth size, the 5901B cell, comes in a
glass version and is significantly smaller than the other cells.
It is designed for use with Harts 9210 Maintenance Apparatus,
which automates the realization and maintenance of the TPW. The
Model 9210-5901B combination is perfect for both calibrating
thermometers and providing checks of sensor drift.
Accessories
For simplest realization of the TPW in the Hart larger cells, the
model 2031 "Quick Stick" Immersion Freezer uses dry ice and
alcohol to facilitate rapid formation of an ice mantle within the
cell without requiring constant intervention while the mantle
forms. Insurance is also available for each water cell purchased
from Hart. Water cells are not difficult to handle nor is the TPW
difficult to realize, but they are delicate and accidents do
happen.
For a nominal fee, Hart will insure your cell
in one-year increments. If something goes wrong, just let Hart
know and they will replace cell. No questions asked. |
