Thermal measuring and testing – Thermal testing of a nonthermal quantity
Reexamination Certificate
2000-01-29
2001-08-14
Gutierrez, Diego (Department: 2859)
Thermal measuring and testing
Thermal testing of a nonthermal quantity
C374S029000, C374S043000
Reexamination Certificate
active
06273604
ABSTRACT:
The present invention relates to an analytical instrument for measuring heat evolution in homogeneous fluid samples and, in particular, to a calorimeter for measuring radioactive decay heats in tritium containing gases for the purpose of determining the concentration of tritium therein.
For the avoidance of doubt, the term fluid is intended to include any substance in gas or liquid form, which can be transported through tubes or pipes to fill volumes of suitable shape and size without voids. By the term homogeneous is meant that heat evolution takes place substantially homogeneously throughout a sample volume, i.e. substantially identical amounts of heat per unit volume and time are generated at any location within the fluid sample. Such fluids include:
(a) Gases containing a heat evolving constituent in pure form, as an admixture or chemically bound to other chemical elements, including mixtures thereof. For example, gases containing a radioactive element, such as tritium in pure form as molecular T
2
, in mixtures with the other hydrogen isotopes (H
2
and D
2
), in heterogeneous chemical compounds (HT, DT) or chemically bound in more complicated molecules such as, for example, tritiated (singly or multiply) methane or ethane. Since heat evolved during the radioactive decay of tritium to
3
He is independent of its chemical bonds to other atoms or molecules, the precise measurement of heat evolved from a sample enables the quantity of tritium in the gas (or mixture) per unit volume to be determined.
(b) Liquids containing a heat evolving constituent in pure form, as an admixture or chemically bound to other constituents of the liquid, including mixtures thereof. For example, liquids containing a radioactive element in pure form or in solution. By measuring the heat evolution from such a sample, the concentration of the radioactive element in solution can be determined. Data thus obtained may be used to monitor and control, for example, fuel composition in homogeneous fission reactors (consisting of fissionable materials in the form of aqueous salt solutions) or the concentration of radioactive elements in liquid solutions in fission fuel reprocessing plants, both in product and waste streams.
(c) Liquid suspensions or emulsions containing a heat evolving constituent, homogeneously distributed or otherwise generating heat homogeneously throughout their volume, as a result of, for example, chemical or biochemical reactions. For liquid solutions, suspensions and emulsions which evolve heat due to chemical or biochemical reactions, measurement of the heat evolved may be used for control of (bio-) chemical processes.
In order to measure radioactive gas concentrations, such as tritium concentrations, in gas samples at atmospheric pressure and room temperature with sufficient resolution and accuracy, a magnetic sector mass spectrometer may be used. Such instruments are, however, very large and complex.
The present invention provides an analytical instrument which, in one embodiment, is capable of detecting tritium in gas samples with a resolution and accuracy which matches or exceeds that of magnetic sector mass spectrometers. The analytical instrument is also capable of analysing, for example, tritium concentration in gas samples without the production of radioactive waste; the sample is unadulterated and can be returned to the process after measurement. In contrast, conventional techniques and devices produce radioactive waste streams which have to be reprocessed with some residual contamination ending up as waste.
Accordingly, the present invention provides a device for measuring heat evolution in a homogeneous fluid sample, the device comprising:
(i) a first chamber having a heat capacity C
1
and an internal surface of area A
1
defining a cavity for containing a volume V
1
of the said fluid;
(ii) a second chamber having a heat capacity C
2
and an internal surface of area A
2
defining a cavity for containing a volume V
2
of the said fluid, wherein C
1
is approximately equal to C
2
, A
1
is approximately equal to A
2
and V
1
is greater than V
2
;
(iii) means to detect and measure heat flow between the first and second chambers; and
(iv) means to achieve a temperature equilibration between the two chambers.
The device according to the present invention constitutes a differential micro-calorimeter. The conservation of full symmetry (with the exception of volume difference) as specified in (ii) above is necessary in order to eliminate errors caused by sample lines, surface adsorption and fluctuations in the temperature, which would otherwise cause differences in heat flow to or from either or both volumes other than the desired volume difference effect.
For the avoidance of doubt, a temperature equilibration exists when there is zero or almost zero net heat flow between the two chambers. The means to achieve temperature equilibration preferably comprises heating means to heat the second chamber, which is advantageously responsive to the measured heat flow. This may be achieved by, for example, a feedback control loop.
The means to measure and detect heat flow may be achieved by connecting the first and second chambers by a heat flow sensor. Although a single differential thermocouple may be used, commercially available thermopiles are preferred. Such thermopiles are typically formed from materials having a high thermal EMF, such as p- and n-conducting bismuth telluride semiconductors, which are normally used as thermoelectric heat pumps (Peltier effect). In the range of from 500 to 1500 electrically series-connected thermocouples may typically connect the first and second chambers. Once a heat evolving sample is admitted into the device, it will be appreciated that the larger volume V
1
will receive more heat than the reference volume V
2
. This results in a heat flow through the thermopiles from V
1
to V
2
, which causes an electrical signal to be generated by the heat flow sensor. The surplus heat generated in V
1
may be measured as follows. V
2
is heated by, for example, an electrical heater, such that the heat flow detected by the heat flow sensor is approximately zero, thus forcing temperature equilibration between the two volumes. The power applied to the heater attached to V
2
is then equal to the surplus power in the effective measuring volume V
1
-V
2
. This power can be electrically measured by measuring the current through and the voltage drop across the heating resistor attached to V
2
.
In order to make temperature equilibration automatic, an accurate and sensitive industrial controller with PID (Proportional, Integral, Differential) features is advantageously used. In one embodiment of the invention suitable for tritium gas concentration measurements, the thermopile signal is connected to its ±10 mV input, the set-point is set to zero and its 0-20 mA output is connected to a 5&OHgr; heating resistor attached to V
2
. Heating current and voltage are measured by means of a sensitive Digital Multimeter. The heat evolved by tritium gas of approximately 100% purity in a volume of about 20 cm
3
at a pressure of 100 kPa and a temperature of 300 K is 1.567 mW. The equipment used permits a resolution of approximately
1
W and hence tritium concentration in the sample can be measured with a resolution of better than 0.1%. This is equivalent to a tritium concentration of 0.16 &mgr;gcm
−3
.
Advantageously, the device according to the present invention includes a test feature in the form of an additional heating means to heat the first chamber. This permits the simulation of the admission of a sample by applying a fixed amount of heater power. For example, applying a heating power of 1.567 mW to V
1
would simulate admission of tritium of 100% purity under the conditions given above. Once the control system is correctly tuned and operating, its response will be to apply an approximately equal heater power to the heater attached to V
2
. This feature permits the operation of the instrument to be optimised without the encumbrance of handling radioactive s
Hemmerich Johann Ludwig
Miller Andrew George
Bacon & Thomas
European Atomic Energy Community (EURATOM)
Gutierrez Diego
Verbitsky Gail
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