The present invention relates generally to systems and methodologies for calibrating thermometers. More particularly, the present invention concerns the design and deployment of an improved system based on triple point of water for calibration of platinum resistance thermometers.
Definitions and interpretations
Before undertaking the disclosures below, it may be advantageous to set forth definitions of certain words or phrases used throughout this patent document: the terms "include" and "comprise," as well as derivatives thereof, mean inclusion without limitation; the term "or" is inclusive, meaning and/or; the phrases "associated with" and "associated therewith," as well as derivatives thereof, may mean to include, be included within, interconnect, with, contain, be contained within, connect to or with, couple to or with, be communicable with, cooperate with, interleave, juxtapose, be proximate to, be bound to or with, have, have a property of, or the like; and the term "Triple point" or "Triple point temperature" refers the unique temperature at which solid, liquid and vapour phases of a substance (water, in the present case) co-exist in thermal equilibrium; "TPW" refers Triple point of water; "SPRT" refers Standard Platinum Resistance Thermometer; "HTPRT" refers High Temperature Platinum Resistance Thermometer; "RO" refers reverse osmosis;
Background of the invention and description of related art
Thermometers are widely used in industry, research, and healthcare for determining temperatures or monitoring gradients in temperature of inanimate objects as well as living subjects. Accurate performance of industrial processes, or diagnosis and treatment of illnesses critically depend on precise determination or monitoring of

temperature. Hence it is an acute need to have some means that can reliably ensure the thermometers employed in said use cases provide repeatable correct measurement of temperatures.
Thermometers exist today in many forms ranging from basic mercury in glass type, to more evolved electronic varieties, to industrially-employed infrared, optical or induction temperature sensors. Irrespective of underlying technology, all thermometers are limited, in a way, by that their performances decline or dull over time which can only be remedied by periodic calibration. Hence, there is a pressing need for a robust standard reference that can be used for calibration of thermometers.
As known traditionally in the art, thermometers can be calibrated either by comparing them with other calibrated thermometers or by checking them against known fixed points on the temperature scale. The best known of these fixed points are the melting and boiling points of pure water. However, boiling point of water varies with pressure and hence cannot be a universal standard. Body temperature (of a healthy adult male), the lowest temperature given by a mixture of salt and ice (a Frigorific mixture), which was originally the definition of 0 °F (-18 °C), a thermostat bath or solid block where the temperature is held constant relative to a calibrated thermometer were some of the standards devised in the past, which however have now been replaced by the defining points in the International Temperature Scale of 1990, with melting point of water being more commonly used than its triple point, the latter being more accurate but difficult to manage and thus restricted to critical standard measurement.
TPW is recognized as a defined fixed point reference temperature which finds application in state-of-the-art in form of systems based on TPW for calibration of thermometers. Examples of such systems are Jarrett cells which consist of cylindrical borosilicate glass containers with a reentrant tube serving as a thermometric well. These cells are thoroughly cleaned, filled with high purity gas-free water, and then sealed. The cell is usually provided with a brass or aluminum metal bushing near the bottom of the thermometer well for supporting a thermometer being calibrated or tested in a vertical position so that the bulb is held off the bottom of the thermometer well. A well established equilibrium condition is guaranteed to be within +0.00000° or -0.00015° C. of the triple point of pure ordinary water. However, as known in the art, Jarrett cells are typically fragile as well as cumbersome to use. Prior to use, these

cells need to be frozen with dry ice for refrigerating the inside of the reentrant thermometer well so that the water freezes from the thermometer well outward to form a mantle of ice around the well. Great care and expertise is required to prevent accidentally rupturing the cell. The art hence needs some simplified, robust and inexpensive means that retain the resolution of TPW cells, but whose implementation is simplified, robust, and not prone to easy damage / breakage.
As seen above, conventional TPW cells require a central thermowell and the delicate process of generating an ice mantle. Also, these cells are extremely fragile and must be carefully handled and stored to prevent breakage. The vertical technique employed in conventional TPW cell results in a tendency, as the ice melts around the thermowell, to cause the ice mantle to come loose and float up and away from the thermometer tip. This tendency of losing the ice mantle from the vicinity of the thermometer tip results in a decrease of the useful time for the conventional cell since the whole equilibrium condition is based on achieving the condition of pure water, ice, and water vapor in thermal equilibrium with the temperature of an assigned value of +0.01° C. Hence, there is a pressing need for more practical, rugged, TPW devices to replace the delicate, awkward, and expensive-to-maintain TPW devices currently available in the field.
In the brief prior art study undertaken, examples of TPW cell-based calibration devices are seen to be recited in US5219225A (assigned to the US Secretary of Army), CN101078656A (assigned to Covidien AG), US20070206653A1 (assigned to National Institute of Advanced Ind Science and Tech AIST), US6939035B2 (assigned to the United Kingdom Secretary of State). However, these systems are not without deficiencies that prevent true accomplishment of the needs voiced here above.
Reference to thermometer calibration apparatuses can also be found in, for example, Japanese Patent No. 3465402; Japanese Patent Laid-Open No. 2004-317193; Japanese Patent No. 2990276; P. Bloembergen, G. Bonnier and H. Ronsin, "An International Intercomparison of Argon Triple Point Calibration Facilities, Accommodating Long-stem Thermometers" Metrologia 27 (1990) pp. 101-106; G. Furukawa, "Argon triple point apparatus with multiple thermometer wells" in Temperature: Its Measurement and Control in Science and Industry, Vol. 6, Part 1, American Institute of Physics, (1992) pp. 265-299; and S. L. Pond, "Argon Triple-Point Apparatus for SPRT Calibration", in Temperature: Its Measurement and Control

in Science and Industry, Vol. 7, Part 1, American Institute of Physics, (2002) pp. 203-208. These apparatuses use a cooling liquid such as alcohol or silicone oil and a low-temperature freezing medium such as liquefied nitrogen. However, the techniques involved disadvantageously limit the calibration temperature to the one determined by the properties of the freezing medium used.
Background art therefore does not list a single effective solution embracing all considerations mentioned hereinabove, thus preserving an acute necessity-to-invent for the present inventor who, as result of focused research, has come up with novel solutions for resolving all needs once and for all. Work of the presently named inventor, specifically directed against the technical problems recited hereinabove and currently part of the public domain including earlier filed patent applications, is neither expressly nor impliedly admitted as prior art against the present disclosures.
A better understanding of the objects, advantages, features, properties and relationships of the present invention will be obtained from the following detailed description which sets forth an illustrative yet-preferred embodiment.
Objectives of the present invention
The present invention is identified in addressing at least all major deficiencies of art discussed in the foregoing section by effectively addressing the objectives stated under, of which:
It is a primary objective to provide easy and reliable testing and/or calibration of thermometers.
It is another objective further to the aforesaid objective(s) that the easy and reliable testing and/or calibration of thermometers is made possible by reference to a substance, or compound which has a fixed known melting point or triple point and thus provides a known temperature to which a thermometer can be tested and / or calibrated.
It is another objective further to the aforesaid objective(s) that the easy and reliable testing and/or calibration of thermometers is made possible using an improvised TPW cell based system.

It is another objective further to the aforesaid objective(s) that the improvised TPW cell based system so provided meets stipulated conditions of the International Temperature Scale.
5 It is another objective further to the aforesaid objective(s) that the improvised TPW cell based system so provided is maintenance-free and can be used reproducibly the world over for calibration of thermometers.
It is another objective further to the aforesaid objective(s) that the improvised TPW 10 cell based system is robust and inexpensive yet has accurate and precise operability within skills of even an ordinary layman in the field.
The manner in which the above objectives are achieved, together with other objects and advantages which will become subsequently apparent, reside in the detailed 15 description set forth below in reference to the accompanying drawings and furthermore specifically outlined in the independent claims. Other advantageous embodiments of the invention are specified in the dependent claims.
Brief description of drawings
20 An embodiment of the present invention is described below, by way of example only, with reference to the accompanying drawing, in which-FIGURE 1 is a schematic cross-sectional view of the sealed module (mini variety) of the present invention.
25
FIGURE 2 is a schematic perspective view of the sealed module (mini variety) of FIGURE 1.
FIGURE 3 is a photograph showing the double distillation apparatus used for 30 preparing water to be filled in the TPW cell of the present invention.
FIGURE 4 is a schematic layout of the PID controller circuit of the present invention.
FIGURE 5 is a photograph of the standard temperature source enclosure of the 35 present invention.

FIGURE 6 is a schematic representation of placement of heating and cooling components in the standard temperature source enclosure of FIGURE 5.
FIGURE 7 is a photograph of the user interface of the standard temperature source 5 enclosure of FIGURE 5.
FIGURE 8 is a schematic view of the sealed borosilicate glass tube used in assembly of the TPW cell of the present invention.
10 FIGURE 9 is another schematic view of the borosilicate glass tube used in assembly of the TPW cell of the present invention.
FIGURE 10 is a perspective view of the heat sink used in the standard temperature source enclosure of the present invention. 15
FIGURE 11 is a graph explaining experimental validation of the TPW cell of the present invention in comparison to a nationally accredited laboratory standard.
The above drawings are illustrative of particular examples of the present invention but 20 are not intended to limit the scope thereof. The drawings are not to scale (unless so stated) and are intended for use solely in conjunction with their explanations in the following detailed description. In above drawings, wherever possible, the same references and symbols have been used throughout to refer to the same or similar parts. Though numbering has been introduced to demarcate reference to specific 25 components in relation to such references being made in different sections of this specification, all components are not shown or numbered in each drawing to avoid obscuring the invention proposed.
Attention of the reader is now requested to the detailed description to follow which 30 narrates a preferred embodiment of the present invention and such other ways in which principles of the invention may be employed without parting from the essence of the invention claimed herein.
Summary of the present invention 35 The present invention is directed to the construction and operations of a TPW cell and its maintenance enclosure for calibration of thermometers. The TPW cell is a

enclosure made of borosilicate glass filled partially with highly pure gas-free water, and the maintenance enclosure is a dry block furnace with peltier element for heating and fan for air-assisted cooling under accurate control of a PID circuit. User interface is provided for allowing user to set or alter temperature of the enclosure to allow 5 reaching the priorly known triple point of water.
Detailed description
Principally, general purpose of the present invention is to assess disabilities and
shortcomings inherent to known systems comprising state of the art and develop new
10 systems incorporating all available advantages of known art and none of its disadvantages. Accordingly, the disclosures herein are directed towards construction and operations of a TPW cell for calibration of thermometers, which is intended to encompass various embodiments, among which a few are explained below with reference to certain examples that illustrate generically the manner in which principles
15 of the present invention may be employed.
With reference to the accompanying drawings, FIGURE 1 and FIGURE 2 depict the construction of a cylindrical TPW cell (001) included in the system proposed herein. As seen here, construction comprises a cylindrical chamber made of high-quality, 20 specially treated borosilicate glass which has a centered well / thermowell and is partially (or nearly) filled with 99.999% pure distilled water, and sealed under vacuum by flame sealing means / procedures common to art using a brazing torch.
Special treatment for borosilicate glass cited in the preceding paragraph refers that 25 the cylindrical chamber is handled with care, and cleaned 7 to 8 times with water vapour, hot water, hydrochloric acid and placed on a stand for drying out. This process is repeated twice a day at least for 15 consecutive days. Details of this process are outlined later in this paper.
30 According to another aspect of the present invention, the TPW cell provisioned as per the foregoing narration differs from those available conventionally, as it is comprised of high quality borosilicate glass and made available in single size 165mm length, 32mm OD, 8mm ID. The material of construction ensures no impurities to pass through, therefore no drawbacks associated with use of glass in making TPW cells
35 including drifts otherwise experienced in thermometry over time.

According to alternative embodiments of the present invention, the TPW cell (001) is intended to be manufactured by the applicant named herein in two sizes - mini and standard, as per table 1 below.

Dimension Embodiment 1 (mini) Embodiment 2 (standard)
1) Outer diameter 32mm 50mm
2) Total length 165mm 450mm
3) Material for shell Borosilicate glass Borosilicate glass
4) Immersion depth 150mm 265mm
5) Insert construction 1 hole of 8mm diameter 1 hole of 12mm diameter
6) Uncertainty 2mK 2mK
7) Reproducibility <1mK <1mK
5 Table 1
According to one aspect of the present invention, the TPW cell proposed herein is partially filled with highly pure, gas-free water of isotropic composition. River water is used in the preferred embodiment.
10
Accordingly, pure / clean river water to fill in the TPW cell is first distilled 6 times using a double distillation chamber. Here, water is heated up in the first chamber so that water vapor collects in other chamber. Then the vapor again heated up at high temperature and passed into another chamber surrounded by cold water for
15 condensation and thus obtaining the distilled water, the process being repeated successively as mentioned before. According to another embodiment of the present invention, water to fill the TPW cell is prepared by distillation of RO water 4 times in double distillation chamber. A clean TPW cell is filled with distilled water prepared as per the either of the processes mentioned earlier, while being heated by a spirit lamp.
20 It shall be evident to the reader that sealing makes the resultant construction independent of pressure changes and therefore more reliable for obtaining accurate standard reference temperatures.
Resultant TPW cell obtained as per the foregoing narration is thus arranged to 25 contain 20% pure vapor 80% pure water. This combination of water vapor, pure water and ice embodies the triple point of water.

It shall be worth mentioning that the TPW cell is subjected to an special treatment being an elaborate cleaning process before receiving the distilled water. Accordingly, the TPW cell is rinsed for at least a week, 7 to 8 times in a day with distilled water in order to remove all the impurities, followed by cleaning 2 times by steam and then by 5 warm water. Optionally, a dilute Hydrochloric Acid (HCl) wash is given 2-3 times in case of any contaminants are present.
According to a further step in implementation of the present invention, the TPW cell is sealed at one end first using a brazing torch. Then the water in the TPW cell is boiled
10 using sprint lamp until steam is released from the other open end, at which juncture the other end is sealed using the brazing torch. During sealing process, heat is provided by sprit lamp to TPW cell till 20% water vaporized and 80% remaining in cell. By using this method, it is ensured that very good stability and reproducibility are attained, while making sure that only pure water and steam remains in the cell, and
15 the TPW cell is ready to use.
For the reader, brazing is a metal joining process in which two metals are joined together by melting. Brazing is applied on glass cell. At the time water is boiled and pure water, water vapor are remaining in the cell, open area is sealed by brazing
20 torch. Both the open tubes are sealed. First, one open tube is sealed first by brazing torch and then water in the TPW cell is boiled using sprint lamp. When water starts boiling and steam is released, this open tube is sealed using brazing torch, therein ensuring that only pure water and steam remains in the cell. When hot gas torch flame touches the surface of glass, it melts fast, aiding in the sealing process and
25 therefore properly closing the open tubes wherein the surface of glass cools down and becomes hard again. This method is implemented via a hand-held, fixed position torch and requires expert human skill to accurately accomplish the same.
Construction of the TPW cell recited above helps the temperature of inner ice mantle 30 to remain constant within 1 mK and remain preserved for long time when equilibrium is reached.
The TPW cell recited above is intended to be maintained using a 230V AC portable
dry block furnace shown at FIGURE 3, which is dimensioned 430mm (Height) x
35 170mm (Width) x 188mm (Depth) and has a weight of 14 kg. Said dry block furnace
is designed to have necessary stability and uniformity and provide sufficient

immersion depth for the TPW cell proposed herein, specifically via an insertion block having diameter 32mm x 180mm long with 4 x 6mm hole of 150 mm insertion depth for TPW cells (mini variety). Insert construction is dimensioned to 33mm diameter X 220mm length. Said dry block furnace is provided with Peltier elements to generate 5 stable temperature in the well, ranging between -15°C to 110°C with temperature resolution of 0.1°C and stability of 0.05°C and thus form a highly stable standard temperature source for calibrating RTD / thermocouples.
According to another aspect of the present invention, temperature of the calibrator is 10 set and controlled by a self tuned PID controller circuit of which is shown in FIGURE 7 with automatic super fine adjustment. Using the interface provided (press buttons), the user can press “UP” or “DOWN” buttons to change the temperature set-point value. RS-232 communication port is provisioned in the calibrator for serial communications to computational devices in charge of the user, say for example a 15 personal computer and the like.
Basic operations of PID controller used in dry block furnace may be explained with
reference to the underlying aspects-
1) The Temperature Controller - The controller has a dual display, the upper
20 display indicates the measured temperature, and the lower display indicates
the desired temperature or set point.
2) Altering the Set point - To change the set point of the controller use the UP
and DOWN keys to raise and lower the set point to the required value. The
lower display changes to indicate the new set point.
25 3) Monitoring the Controller Status
A row indicate the controllers status as follows OP1 Heat Output OP2 Cool Output
REM This beacon indicates activity on the PC interface
30 ALM this indicates when PV (Present value) is more than 50°C.
4) Units - Momentary pressing of the Scroll key will show the controller units °C or °F by using SCROLL key & UP & DOWN key unit can be change.
Self-tuned PID controller used for increased or decreased temperature. As seen in 35 the accompanying drawings, an isothermal enclosure (Aluminum basket) is provided

in which TPW cell is placed and the thermocouple / RTD can be calibrated against the temperature of the calibrator.
The calibrator controller uses a precision RTD as a controlling sensor and controls 5 the well temperature with thermoelectric cooler (peltier cell). To obtain and maintain a required temperature the controller varies the power to the heater via solid-state relay. There is one electricity driven fan which is situated under the heating chamber for cooling the heater. A heat sink, shown at FIGURE 10 is provisioned for dissipation of heat. Said heat sink includes a pair of plate-type heat exchangers of aluminum, 10 each of which is dimensioned to have a height 200mm, width 100mm, and depth 42mm
The CALsys -15/110 dry block calibrator was designed for portability, moderate cost and ease of operation. Design of inner chamber and peltier assembly shall be
15 appreciated from the accompanying FIGURE (). As shown, the TPW cell is placed in the basket which is placed between peltier cell and heat sink. Both sides of the basket peltier are assembled as shown in figure. peltier is placed in between heat sink and basket. Fan is placed below heat sink. For use, the TPW cell is inserted in the dry block furnace which has uniformity of 0.15 oC. The cell is put into the bath and
20 temperature of the controller is set to -4.7 oC.
Temperature calibrator Calsys -15/110 used as maintenance apparatus for realization of TPW cell. This process is repetitive because TPW cell is realized on the particular temperature. There is no other furnace used for realization
25
The maintenance apparatus used is Calsys -15/110 dry block furnace. No other bath is involved in this process. For use, the TPW cell is first washed with methanol, and placed in center well of the dry block furnace. Center well of cell is filled with methanol for better thermal contact. Then, temperature in controller is set for 0oC
30 (internal nomenclature: “Supercool”) the cell and achieve freezing point of cell. When set temperature is realized, the TPW cell is initialized by removing gently, shaking it, and replacing it back into the well, so that the cell is maintained at the triple point temperature while it is being used, taking care that said cell is not dropped, struck, or stressed at any time. The TPW cell may be held upright and shaken from side to side
35 to observe the water suddenly freeze uniformly throughout the cell, which indicates that triple point is reached and the TPW cell is ready to be used as a temperature

standard for calibration / testing purposes. It shall be understood that at this stage, since there is uniform distribution of liquid water and ice in the TPW cell its temperature will remain at 0.01°C. Industrial applicability 5 The TPW cell system in accordance with the foregoing narration has been reduced to practice by the inventor named herein, and resultant system has been found to serve as an able facility for fixed point calibration of SPRTs, HTPRTs and standard thermocouple probes as per international calibration standard for thermometers, that is, ITS-90.
10
The TPW cell made according to the protocol mentioned above has been tested by the inventor named herein, in comparison to a quartz SSPRT as per TPW Cell Realization Test protocol. Accordingly, the TPW cell was realized (calibrated) in maintenance apparatus as per test conditions (table 2), observations and results of
15 which are provided in tables 3 and 4 respectively.

Cell no.
Cell size (small/big) M-01 Small
Material used Borosilicate glass
Water used Raw water (eight pass, distilled water)
Realization sensor Sprt metal (quartz)
Realization measuring equipment Asl bridge(61/2 digit)
Maintanance apparatus Dry block calsys -15/110(tempsens make)
Temperature require at freezing -4.7 deg c

Observation duration 60 min Table 2: test conditions
SR. NO. TIME TPW cell of the present invention (Ohms) NPL Certificate Ohms
1 10:31 AM 25.13355 25.133705
2 10:32 AM 25.13355 25.133705
3 10:33 AM 25.13355 25.133705
4 10:34 AM 25.13355 25.133705
5 10:35 AM 25.13355 25.133705

SR. NO. TIME TPW cell of the present invention (Ohms) NPL Certificate Ohms
6 10:36 AM 25.13355 25.133705
7 10:37 AM 25.13355 25.133705
8 10:38 AM 25.13355 25.133705
9 10:39 AM 25.13355 25.133705
10 10:40 AM 25.13355 25.133705
11 10:41 AM 25.13355 25.133705
12 10:42 AM 25.13355 25.133705
13 10:43 AM 25.13355 25.133705
14 10:44 AM 25.13355 25.133705
15 10:45 AM 25.13355 25.133705
16 10:46 AM 25.13355 25.133705
17 10:47 AM 25.13355 25.133705
18 10:48 AM 25.13355 25.133705
19 10:49 AM 25.13355 25.133705
20 10:50 AM 25.13355 25.133705
21 10:51 AM 25.13355 25.133705
22 10:52 AM 25.13355 25.133705
23 10:53 AM 25.13355 25.133705
24 10:54 AM 25.13365 25.133705
25 10:55 AM 25.13365 25.133705
26 10:56 AM 25.13365 25.133705
27 10:57 AM 25.13365 25.133705
28 10:58 AM 25.13365 25.133705
29 10:59 AM 25.13365 25.133705
30 11:00 AM 25.13365 25.133705
31 11:01 AM 25.13365 25.133705
32 11:02 AM 25.13365 25.133705
33 11:03 AM 25.13365 25.133705
34 11:04 AM 25.13365 25.133705
35 11:05 AM 25.13365 25.133705
36 11:06 AM 25.13365 25.133705
37 11:07 AM 25.13365 25.133705
38 11:08 AM 25.13365 25.133705

SR. NO. TIME TPW cell of the present invention (Ohms) NPL Certificate Ohms
39 11:09 AM 25.13365 25.133705
40 11:10 AM 25.13365 25.133705
41 11:11 AM 25.13365 25.133705
42 11:12 AM 25.13365 25.133705
43 11:13 AM 25.13365 25.133705
44 11:14 AM 25.13365 25.133705
45 11:15 AM 25.13365 25.133705
46 11:16 AM 25.13365 25.133705
47 11:17 AM 25.13365 25.133705
48 11:18 AM 25.13365 25.133705
49 11:19 AM 25.13365 25.133705
50 11:20 AM 25.13365 25.133705
51 11:21 AM 25.13365 25.133705
52 11:22 AM 25.13365 25.133705
53 11:23 AM 25.13365 25.133705
54 11:24 AM 25.13365 25.133705
55 11:25 AM 25.13365 25.133705
56 11:26 AM 25.13365 25.133705
57 11:27 AM 25.13365 25.133705
58 11:28 AM 25.13365 25.133705
59 11:29 AM 25.13365 25.133705
60 11:30 AM 25.13365 25.133705
Table 3: Observations
Average readings 25.13361
Plateau duration ~ 60 min
SSPRT reading 25.13370
Table 4: Results

As per above results it can be seen that the TPW cell of the present invention has same values to realize in different maintenance apparatus. The reproducibility (means changing in our maintenance apparatus for realization TPW cell reading and values having some mili Kelvin change i.e. 1-2mk (mili Kelvin changes). These results shows that the TPC cell proposed herein has good reproducibility. The readings are in an hour and time period above, so it also shows very good stability
The present invention is identified in having the following salient features-
1) Based on TPW, which is the most accurate and fundamental temperature standard available but also least expensive and simplest to use
2) Provides the most reliable way to identify unacceptable thermometer drift between calibrations, and also variation in sensor readings
3) As phase equilibrium temperatures of pure water is extremely stable and will not change with location and time, the resultant system is robust and not susceptible to differences between diverse user environments.
4) Quick and easy updates to the characterizations of critical thermometer standards, which can be used to extend calibration intervals
5) Can easily measure drift of sensors per month or year which can be used for adjustments in calibration.
6) Automatic and easy to calibrate as it negates the need of long probe, complex connections like secondary calibration, and reference thermometers
7) Can be used for primary as well as secondary calibration
8) Has built-in programming for fast and easy operation
As will be realized further, the present invention is capable of various other embodiments and that its several components and related details are capable of various alterations, all without departing from the basic concept of the present invention. Accordingly, the foregoing description will be regarded as illustrative in nature and not as restrictive in any form whatsoever. Modifications and variations of the system and apparatus described herein will be obvious to those skilled in the art. Such modifications and variations are intended to come within ambit of the present invention, which is limited only by the appended claims.
It shall be generally noted that at least a major portion of the foregoing disclosures of this patent document contains material that is subject to copyright protection. The copyright owner has no objection to the facsimile reproduction by anyone of the

patent document or the patent disclosure, as it appears in files or records of the receiving Patent Office(s), but otherwise reserves all copyright rights whatsoever.

We claim,

A system for calibration of a thermometer, comprising-
a) a sealed module containing a substance of which triple point is known, to serve as a reference standard for the thermometer to be calibrated;
b) an enclosure for receiving the sealed module, therein serving as a highly stable standard temperature source for said sealed module to thereby allow a user to calibrate the thermometer in reference to the triple point of the substance contained in the sealed module; and
c) means for controlled heating and cooling integrated in the enclosure for receiving the sealed module and therein accurately bringing the substance contained in said sealed module to its triple point.
The system for calibration of a thermometer as claimed in claim 1, wherein the sealed module is a cylindrical chamber made of borosilicate glass having a centered well descending internally along its length for receiving the thermometer to be calibrated and therein enclosing a sealable cavity between the cylindrical chamber and the centered well for receiving the substance of which triple point is known.
The system for calibration of a thermometer as claimed in claim 2, wherein the sealed module is dimensioned to have a total length of 165mm, an outer diameter of 32mm, an inner diameter of 8mm, therein enclosing a sealed cavity holding the substance of which triple point is known to a level of 133 mm around the centered well having a total immersion depth of 150mm and an inner diameter of 8mm.
The system for calibration of a thermometer as claimed in claim 2, wherein the sealed module is dimensioned to have a total length of 450mm, an outer diameter of 50mm, an inner diameter of 12mm, therein enclosing a sealed cavity holding the substance of which triple point is known to a level of 360

mm around the centered well having a total immersion depth of 265mm and an inner diameter of 12mm.
The system for calibration of a thermometer as claimed in any one of the claims 2, 3 or 4, wherein before receiving the substance of which triple point is known, the cylindrical chamber is subjected to a daily cleaning protocol including 7 to 8 rinses with distilled water, followed by 2 washes each with steam and warm water for 15 days.
The system for calibration of a thermometer as claimed in any one of the claims 2, 3, 4 or 5, wherein the cylindrical chamber is additionally subjected to 2 to 3 rinses with dilute hydrochloric acid in the event of impurities are present in said cylindrical chamber.
The system for calibration of a thermometer as claimed in claim 1, wherein the substance contained in the sealed module is 99.999% pure, distilled gas-free river water of isotropic composition which is distilled six times using a double distillation chamber.
The system for calibration of a thermometer as claimed in claim 1, wherein the substance contained in the sealed module is 99.999% pure, distilled gas-free water of isotropic composition obtained from reverse osmosis treatment of tap water, which is further distilled four times using a double distillation chamber.
The system for calibration of a thermometer as claimed in any one of the claims 7 or 8, wherein water is so that final contents of the sealed module are 20% pure vapor and 80% pure water.
The system for calibration of a thermometer as claimed in any one of the claims 1, 2, or 3, wherein the enclosure is a dry block furnace having portable dimensions of 430mm height, 170mm width, and 188mm depth, and a weight of 14 kg, in which the sealed module is receivable via an insertion block of 33mm diameter and 220mm length having an insertion depth of 150mm.
The system for calibration of a thermometer as claimed in any one of the claims 1, 2, or 4, wherein the enclosure is a dry block furnace having portable dimensions of 430mm height, 170mm width, and 188mm depth, and a weight

of 14 kg, in which the sealed module is receivable via an insertion block of 33mm diameter and 220mm length having an insertion depth of 150mm.
The system for calibration of a thermometer as claimed in claim 1, wherein means for controlled heating and cooling are a thermoelectric Peltier element and a fan disposed below said thermoelectric Peltier element respectively, both being under control of a self-tuned proportional-integral-derivative controller circuit of FIGURE 0 being connected to a user interface to allow a user to set and alternatively alter temperature as desired of the enclosure.
The system for calibration of a thermometer as claimed in claim 12, wherein the self-tuned proportional-integral-derivative controller circuit controls heating and cooling via varying electrical supply to the Peltier elements through a solid-state relay and combinatorial switching ON and OFF of the fan disposed below said thermoelectric Peltier element.
The system for calibration of a thermometer as claimed in any one of the claims 12 or 13, wherein a heat sink comprising a pair of common art plate type heat exchangers dimensioned to have 200mm height, 100mm width, and 42mm depth is provisioned for dissipation of heat from the system.
The system for calibration of a thermometer as claimed in claim 12, wherein the user interface includes a visual display and a press button panel disposed on surface of the enclosure at a position facing and thereby accessible to the user.
The system for calibration of a thermometer as claimed in any one of the claims 12 to 14, wherein the enclosure serves as a highly stable and controlled standard temperature source capable of temperatures ranging between -15°C to 110°C with temperature resolution of 0.1 °C and stability of 0.05°C.
The system for calibration of a thermometer as claimed in the claims 12 and 14, wherein the sealed module is received in the enclosure between the Peltier element and heat sink to therein being accurately brought to triple point of the substance contained in said sealed module.

The system for calibration of a thermometer as claimed in any one of the claims 7 or 8, wherein triple point of water is the temperature at which uniform distribution of liquid water, vapour and ice is obtained, being particularly 0.01 °C.
The system for calibration of a thermometer as claimed in claim 1, wherein the thermometer is selected among standard platinum resistance thermometers, high temperature platinum resistance thermometers and their equivalents.