PRESSURE TRANSDUCERS FOR TREAT SODIUM LOOPS. Page: 8 of 17
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6
A solution satisfying the above design criteria was realized by an
adaptation of commercially available transducers in wide use on conventional
high-temperature systems.
The strain-gauge, bridge-connected pressure transducer was chosen
as the sensor, a final choice being made between three competing and opera-
tionally equivalent devices. The transducer selected had been used in high-
radiation fields at steady state with good success and has a maximum
operational temperature rating of 600 F. It is about 1.5 cm in diameter by
about 2 cm long. Each transducer is equipped with an integral mounting
flange having a cross section about 1.5 mm square. The gauges have
natural frequencies of 17-20 kHz and a frequency response that is essen-
tially flat to 3000 Hz. In the as-received condition, they exhibit no detectable
departure from linearity over the full range. These gauges can be excited by
a constant 5-V dc voltage applied to the Wheatstone bridge composed of
two active and two inactive unbonded strain wires, supported on a star-spring
mounting. The preferred method of excitation for TREAT tests is by an
ac carrier wave, using a demodulator-to-analog dc output amplifier. This
method tends to clarify the output of the transducer by eliminating spurious
electrical noise and reducing the transducer sensitivity to the high transient
radiation levels in TREAT.6 With either method of excitation, the full-range
output of the transducer is 21.5-24.5 mV of bridge unbalance. This is more
than adequate for amplifier transmission to a distant recorder and can be
applied directly to millivolt indicators,
III. DESIGN FEATURES
The transducer selected was adapted to the sodium system by incor-
porating it into an all-welded construction of Type 304 stainless steel, the
material of the loops. Although such a transducer had never been success-
fully installed by welding, and the manufacturer stated that such treatment
would render the transducer completely inoperative, a technique for
installing the transducer by welding was developed. A combination of
electrical monitoring of the transducers for bridge strain due to heating,
the use of specially shaped water-cooled copper chills at the weld zone,
and tungsten-inert-gas weldments of short segments was the key to
successful weldments. This technique has been used successfully on
more than three dozen transducers.
The all-welded assembly has provision for isolating the stainless
steel diaphragm of the transducer from the molten sodium of the loop by a
pressure-transmitting column of NaK. The NaK, in turn, is separated from
the loop sodium by a multiconvolution welded metal bellows with a very low
spring rate. This bellows serves as a thermal-expansion volume compen-
sator for the NaK column and is fabricated of AM-350 stainless steel with
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Robinson, L. E. & Purviance, R. T. PRESSURE TRANSDUCERS FOR TREAT SODIUM LOOPS., report, January 1, 1969; Illinois. (https://digital.library.unt.edu/ark:/67531/metadc1035203/m1/8/: accessed April 25, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.