Title of Invention

A NOVAL METHOD OF FABRICATING IMPROVED STANDARD PLATINUM RESISTANCE THERMOMETERS

Abstract

A method of fabricating improved standard platinum resistance thermometers by making a coil of grade platinum wire by known methods; preparing "V shaped leads using platinum wire; fixing the said "V" shaped leads to both ends of the said coil by known methods; cleaning two pieces of quartz capillaries; twisting together the said cleaned capillaries to obtain a former by known methods to provide 3-4 twists; tapering one end of the said former to provide a pen nib shape end; housing the coil obtained in a bifilar form in the former through the nib end in such a manner that the "V" shaped lead ends come out of the reduced capillary ends; passing the lead ends through plurality of quartz discs and sleeves; connecting the said insulated lead ends to an insulated four wire system; encasing the assembly so obtained in a quartz stem, evacuating the quartz stem encased assembly at room temperature, sealing hermetically by known methods at a temperature in the range of 973 to 1273 K the top end of the said gas mixture filled stem containing the said assembly to obtain a Standard Platinum Resistance Thermometer.

Full Text

The present invention relates to a method of fabricating improved standard platinum resistance thermometers. A SPRT is used for precision temperature measurements , calibration and control in R & D Laboratories , Design & Development Institutions, Testing & Calibration Centres and Industries . SPRT is the instrument of interpolation on International Temperature Scale- 1990 (ITS-90) used to define the ITS - 90 from 13.8033 K to 1234.93 K. For low temperature measurements i.e.,from 13.8033K to room temperature a capsule type SPRT having nominal resistance of 25.5 Ω at 273.16 K is used and for temperature between 83.8058 K to 933.473 K stem type SPRT having nominal resistance of 25.5 at 273.16K is used. For temperature above 933.473 K and upto 1234.93 K stem type SPRT with a resistance of 0.25 or 2.5 at triple point of water is used. As per international norm the sensing element of SPRT must be made of pure platinum wire and be strain free. The finished SPRT must meet one of the following criteria: W (melting point of Ga) > 1.11807 or W (triple point of Hg) SPRT acceptable for use up to freezing point of silver i.e., 1234.93 K must meet the following additional criteria. W (freezing point of Ag)) > 4.2844 where W is the ratio of resistance at melting point of Ga, triple point of Hg, freezing point of Ag to the resistance at triple point of water . The temperature -resistance relationship of platinum resistance thermometer is defined with the help of reference function, deviation function and measurement of resistance ratio at fixed points depending upon the temperature range /various subranges specified by the scale (ITS -90). Reference may be made to Metrologia Vol. 27 (199) 3. The reference functions defined are: i) for the range 13.8033K to 273.16 K the following reference function is defined: In[Wr((T90)]= A0 + Σ Ai[{ln(T90/ 273.16)+1.5 } /1.5 ]i (la) An inverse function equivalent to Eq. (la) to within 0. 1mK is (T90/273.16)= B0 + Σ Bi [{Wr(T90))1.6 - 0.65 }/0.35 ]; (Ib) The values of AO , A i ,Bo and B; are given in the ITS -90 text ii) for the range 273K to 1134.78 K the following reference function is defined : Wr(T90)= C0+ ΣCi[{T90/K-754.15}/481 ]' (2a) An inverse function equivalent to Eq. (2a) to within 0.13mK is T90/K-273.15)= D0 + ΣDi [{Wr (T90)-2 64}/l.64}] (2b) The values of the constants Co, Q, DO and Dj are given in theITS-90 text. The deviation functions defined are: (i) for the range 13.8033K to 273.16 K W(T90)-Wr(T90)= a[W(T9o)-l] + b[W(T90)-l]2 + Σci [In W(T90)]i+n 3 with the values of the coefficient a,b and C; being obtained from the measurement at the fixed points and n=2. (ii) The deviation function for the range 273.01 K to 1134.78K is W(T90)-Wr(T90) = a[W(T90)-l] + b[W(T90)-l]2 + c[(T90)-l]3 + d[W(T90)-W(660.323°C)]2. 4 The values of constants a,b,c and d being determined from measurements at freezing points of tin, zinc and aluminium. C. W. Siemen, in 1871 as described in Precision resistance thermometry,Temperature- Its measurements and control in science and industry, pp162 - 179 (Reinhold Publishing corp. New York, N.Y.1941), was first to use a platinum resistance thermometer as a "commercial device" for temperature measurement. Callender, in 1891 as described in Phil Mag, 3 2(1891), 104) proposed a design of PRT as a scientific instrument. This design consisted of platinum wire bifilarly wound on mica strip inserted in to a hard glass/quartz sheath. Mica disks uniformly spaced along the stem separated the four thin, silver/copper lead wires, effectively minimising the conduction error. In 1932, C.H.Meyer (Meyer,C.K, J.Res.NBS,9(1932),807) described an improved and compact design of sensing element for the PRT. Here the platinum wire of 0.1 mm diameter is converted in to a helical coil which is further bifilarly wound over mica cross of 5 mm diameter. The two ends of the wire forming the sensing element are fused to thicker platinum leads of about 0.3 mm dia and short length-two leads from each end-thus forming a four lead resister. These platinum leads are fused together to long gold leads insulated by glass/quartz sleeves and mica spacers. The entire assembly is housed in a glass / quartz sheath and hermetically sealed at the top, enclosing helium gas alongwith a trace of dry air to keep the impurities in platinum in the oxidised state. He also proposed a design in which platinum wire is wound directly and bifilarly on mica cross with close spacing. Consequently reducing the heating effect due to measuring current. This design become quite popular. A simple design of sensing element was proposed by C.R. Barber, as described in J. Sci.Instr. 27, (1950) 47 in order to reduce the size of sensing element. In this design, the platinum helical coil of 1mm of outer diameter made of platinum wire of 0.05 mm diameter is housed in a thin glass tubing . Two thicker platinum leads are fused at each end of the coil. The tube is double bent at the middle to form ,a sensor .The SPRT before gas filing and hermetically sealing is baked at 450° C for 12 hours and dry air is admitted and carefully pumped out again. This process is repeated many times before finally filing the SPRT with dry air. J H Hall of National Physical Laboratory Teddington UK has described in J.Sci. Instrum. (1933), Vol 10,page 4, a quartz former for constructing PRTs for use at high temperature i.e. upto 904K, the upper temperature limit of the use of SPRT on standard as defined in EPTS-68 Donald J. Curtis of Rosemount Engineering Company, Minneapolis, Minnesotta has reported in his paper entitled "Platinum Resistance Interpolation Standards,Range -200 C to 650 C", (ft. S Patent No.3,296,572, Standard Thermometer, issued to Lowell A. Kleven and assigned to Rosemount Engineering Company(1967) that for stabilisation of SPRT 500 hours annealing at a temperature of 500° C is required before taking any readings. H.Tinsley & Co.. of England also manufactures the similar type of SPRT's,in the range 90 K to 904 K . They are using the technique of sealing the platinum leads with quartz former. The drawbacks observed are : 1. The pitch of the coil in the vicinity of the platinum quartz seal is more. 2. The self heating effect at triple point of water of these thermometers is greater than 2 mK i.e., between 2.6 mK to 3.3 mK at triple point of water. At National Physical Laboratory, New Delhi, a constituent laboratory of Council of Scientific and Industrial Research , several types of PRTs with sensors of Meyer's and Barber's design have been constructed as described by K D Baveja and Ram Kishan in J.of Sci. & Indus. Research vol 40(1981) 492. Attempt has been made to improve the design by C.R.Barber in which some central turns of coil tend to get fused in glass during the bending. To overcome this difficulty, two or three turns at the middle of the coil were carefully straightened. So one gets 6 mm to 9 mm of straight wire of platinum in the portion at which the glass / quartz tube is double bent keeping the platinum wire free at the bend as well. Further, to improve the circulation of helium exchange gas around the sensor, 3 to 4 vents in each half of the tubing were provided instead of one each in the original design. Further improvements were made with a view of convenience in the construction of the sensors, by making a former of two thin glass / quartz tubes lying adjacent and joined together by fusing at two points. Lower end of these two tubes are open and tapered. The platinum coil with spot welded thicker platinum leads, is housed suitably in the above former. The platinum leads are fused with glass / quartz at the top ends. In this design the platinum coil remains in the strain free state after adequate annealing. The drawbacks observed are: 1. The minimum condition of 0.003925, as per IPTS-68/ ITS-90 could not be achieved. 2. Heating effect of SPRT in this design is found to be more than 0.0002 at triple point of water with 1 mA current wherein ideally it should be very close to zero. In Another design of sensor going up to 933.473 K, quartz former is given double bend/twist in the middle so as to provide support to the two halves of the coil. The four platinum leads of the sensor are joined to the four gold leads by spot welding. The long gold leads are insulated from one another by quartz sleeves and quartz disks. The assembly is housed in quartz sheath forming the stem of the thermometer. Finally the thermometer is sealed at the top of the head, enclosing spectroscopically pure argon at 20cms to 25cms of mercury pressure along with trace of dry air. The above design has also some drawbacks as given below: 1. A single twist in the middle of the quartz former can not provide sufficent support to the two halves of the platinum coil. It sagged badly, when used for long hours at 904 K or higher. 2. In this design the standard temperature coefficient of resistance (TCR) of 0.003925 could not be achieved. Mostly it was around 0.003920, which indicates that platinum coil gets contaminated while fusing platinum leads with quartz and also the coil gets strained. SPRTs thus formed could not fulfil the condition of IPTS-68 i.e., of SPRT 0.003925, which indicates the purity of platinum and strain free state of the sensor coil. 3. Many sensors were rejected at the fusing stage, due to leads burning. The pitch of the coil in the capillaries of the former at the fused end increased due to uncontrolled heating during fusing. Also devitrification of quartz former occurred during sealing of platinum leads with quartz. 4. Heating effect of the SPRT at 1 mA current while measuring resistance at triple point of water was more. It was of the order of 0.0004 instead of ideally being zero. . 5. The thermometer after completion had to be annealed for several hours to stabilise it at triple point of water. The main object of the present invention is to provide a novel method of fabricating improved Standard Platinum Resistance Thermometers (SPRT), which obviates the drawbacks as detailed above. Another objective of the present invention is to provide a novel method which is economical, simple & less time consuming. Yet another object of the present invention is to provide a novel method of fabrication, which minimises the heating effect of Standard Pltinum Resistance Thermometers. Still another objective of this invention is to provide a novel method to fabricate SPRT resulting in a better thermometer acceptable for use for precision temperature measurement in the range 273.15 K to 904 K, which fulfils the criteria of ITS - 1990 i.e., W (melting point of Ga) > 1 11807 Another object is to provide an improved Standard Platinum Resistance Thermometer made by the novel method of the present invention. In fig. 1 of the drawing accompanying this specification the schematic diagram of an embodiment of the standard paltinum resistance thermometer is shown, wherein : (1) represents the platinum coil, (2) represents the quartz former, (3) shows the platinum coil welded to 'V shaped platinum leads, (4) shows the quartz discs,(5 ) shows the quartz sleeves and (6) pen nib shaped end of former. Accordingly the present invention provides a method of fabricating improved standard platinum resistance thermometers which comprises: (i) making a coil (1) of grade platinum wire having a minimum temperature coefficient of resistivity of 0.003925 by known methods; (ii) preparing 'V shaped leads (3) using platinum wire; (iii) fixing the said 'V shaped leads to both ends of the said coil by known methods in such a manner that the apex of the said 'V shaped lead is attached to each of the ends of the said platinum coil; (iv) cleaning two pieces of quartz capillaries(2) by known methods; (v) twisting together the said cleaned capillaries (2) to obtain a former by known methods to provide 3-4 twists; (vi) tapering one end of the said former by known methods to provide a pen nib shape end; (vii) reducing the capillaries diameters of the other end of the said former by known methods in such a manner as to hold the joints of the said 'V shaped leads and coil; (viii) housing the coil obtained in step (iii) in a bifilar form in the former through the nib end in such a manner that the 'V shaped lead ends come out of the reduced capillary ends; (ix) passing the lead ends through plurality ef quartz discs(4) and sleeves (5); (x) connecting the said insulated lead ends to an insulated four wire system; (xi) encasing the assembly so obtained in a quartz stem, evacuating the quartz stem incased assembly at room temperature to attain a vacuum in the range of 10-5 mbar followed by continuous evacuation in the range of 10-5 to 10-8 mbar and annealing at a temperature raised in steps of 50 -100 K upto 973 K for a period of 50 to 100 hours at each temperature, maintaining the temperature and vacuum followed by filling the said stem with a mixture of inert gas and oxygen in a ratio in the range of 80% to 95% 20% to 5%, repeating the evacuation ,annealing and gas filling step (xii) sealing hermetically by known methods at a temperature in the range of 973 to 1273 K the top end of the said gas mixture filled stem containing the said assembly to obtain a Standard Platinum Resistance Thermometer. In an embodiment of the present invention the reference grade platinum wire used may be such as having a minimum temperature coefficient of resistivity of 0.003925. In another embodiment of the present invention the twisting of capillaries may be done to obtain an internal diameter in the range of 1 .0mm to 0 6.0mm. In yet another embodiment of the present invention the inert gas used may be such as argon ,nitrogen, helium of 99.999% purity. In still another embodiment of the present invention the oxygen gas used may be of a purity of 99.995%. In still another embodiment of the present invention the mixture of the inert gas and oxygen gas used may be at a pressure of in the range of 30%-50% of atmosphere. Accordingly the present invention provides an improved standard platinum resistance thermometer fabricated by the novel method as described above. A bifilar coil of 0.7mm to 1.3mm diameter made from 0.075 mm dia. reference grade platinum wire is housed in the twisted quartz capillaries former. The ends of the coil are spot welded to a thicker V shaped leads. The coil is anchored at the top open end of the former in such a way that it does not require platinum -quartz seal. Quartz former : Two pieces of quartz capillaries 1.5mm to 2.5mm outer diameter and 0.85 to 1. 5mm internal diameter and of length in the range of 40 to 80mm are cleaned in pure nitric acid having a concentration in the range of 50-80% for a period in the range of 5 to 30 minutes and, then washed with hot clean water such as double distilled , triple distilled of deionflised^water, several times to remove the last traces of acid. These capillaries are given 3-4 twists together without changing their internal diameter. The quartz former is then subjected to following operations: i. the lower end is tapered using diamond file/cutter and oxy hydrogen flame , to make it in the pen nib shape, ii. at the top end of the former, the internal diameter of the capillaries is reduced to about 0.5mm -0.7mm for a length of about 1mm to 2mm. It is again cleaned with nitric acid and then washed with clean hot to remove the acid as described earlier. Platinum coil: About 130 cms of reference grade (99.999+% pure), 0.075 mm diameter platinum wire is taken and coiled on a 0.4mm-0.8mm o.d stainless steel wire using a simple coil winding machine. About 4cm to 6 cm long platinum coil thus formed is removed from stainless steel wire and was washed with pure alcohol and dried. Then this coil is stretched to about 6 cm to 9 cm length uniformly to separate the turns to avoid shortening of turns. The coil resistance is measured at room temperature in a constant temperature box to correct its nominal resistance at 273 16 K to 25.5+0.2 . Platinum wire 8 cm long dia. 0.3 mm is bent in the middle to make it in 'V shape. Its lower end is then (used in oxy-hydrogen flame to make a small ball of 0.5 mm to 0.7 mm dia The ends of the coil are spot welded to these small balls of 'V' shaped platinum leads. The platinum coil is parted in two halves at the middle of the coil and 2-3 turns of coil are carefully straightened. The straighened portion of wire is made to 'IJ' shape. Now this element is carefully inserted in the quartz capillaries through the lower opening of former The leads are taken through the constriction at the top and anchored in such a way that no quartz-platinum seal is required. The four thicker leads are passed through quartz disk of 4.5 mm diameter having four holes of 0.35 mm diameter. Ten mm to fifteen mm quartz sleeves cover the exposed portion of the lead wires. Leads are then passed through another quartz disk as shown in Fig. 1. These four platinum leads are further spot welded to long Gold/platinum lead system. Fifty to sixty cms long, 0.25mm-0.30rnm dia., gold / platinum leads are insulated by quartz sleeves and quartz disks. The entire assembly is housed in 5mm i.d . / 7 mm o.d., 500 mm long, cleaned quartz sheath. The leads are taken out at the top end are hermetically sealed using a low vapour pressure resin (Torr Seal). The assembly is then connected in annealing/evacuating/gas filling system, comprising of turbo molecular pump, capable of giving a vacuum of 10 -7 mbar to 10-8 mbar. The assembly is first evacuated at room temperature for few hours to attain a vacuum better than 10-5 mbar. Then it is heated to a temperature of 323 K in a tubular furnace for 10 to 20 hours and continuously evacuated. Again the temperature is raised in steps of 50 - 100K up to 973 K and thoroughly evacuated for 50 to 100 hours at each temperature. At a temperature of 973 K and vacuum of the order of 10-5 - 10-7 mbar, mixture of 99.999% argon (as exchange gas) and 99.995% oxygen (to oxidise the impurities) at a pressure of one atmosphere is admitted in the thermometer assembly and heated to about 973K for 20 to 50 hours. This step is reaped twice and then the assembly is evacuated to a vacuum of 10-5 - 10-7 mbar. Finally before hermetically sealing the SPRT, it is filled with a mixture of argon and oxygen ( purity as stated above) at a pressure of 30% to 50% of atmosphere at a temperature of 973 K . After gas filling the assembly is sealed using oxy-hydrogen flame. The four gold / platinum leads are soft soldered to four Teflon insulated flexible wires about two meters long. The aluminum handle covers the top head of SPRT and teflon wires come out of the aluminum handle. The following example is given by way of illustration and should not be construed to limit the scope of the invention. Example -1 Two pieces of quartz capillaries ,2mm o.d,l mm i.d of 70 mm length are taken and cleaned in 50 % HNO3 and washed many times with hot deionised water. These capillaries are given three twists together without changing their i.d. One end of these twisted capillaries is tapered using diamond file and oxy-hydrogen flame in the pen shape. The internal diameter of capillaries of the other end is reduced to 0.7 mm for a length of about 1mm. Then, about 130 cms. of reference grade (99.999% pure) 0.075 mm diameter wire is taken and coiled on a 0.55 mm o.d stainless steel wire using a simple coil winding machine. Coil thus formed is removed and washed in pure alcohol and dried. Then this coil is stretched uniformly to about 8 0 mm length. The coil is measured at room temperature to correct its R0 ≈ 25.5 Ω + 0 2Ω 80 mm of 03 mm thick platinum wire bent in the middle to make it in 'V' shape Its lower end is fused to make a small ball of 0.8 mm diameter The ends of the coil are spot welded to these 0 8 mm diameter of 'V' shaped platinum leads The platinum coil is parted in two halves at the middle and 3 turns of the coil are straightened This straightened portion is make in the 'U' shape Now this element is inserted in the quartz capillaries through the pen nib shaped end of the former and free ends of 'V' shaped leads are given two 90° turns to effect the anchoring and leads are passed through quartz disk of 4.5 mm diameter having 4 holes of 0.35 mm diameter, 10 mm long, 0.5 mm i.d quartz sleeves are then put to cover the exposed platinum leads and then leads are passed through another quartz disk. These four platinum leads are spot welded to long gold/ platinum leads system. Gold/ Platinum leads are insulated by quartz sleeves and quartz disks / separators. This assembly is housed in 5 mm i.d and 7 mm o.d , 500 mm long , cleaned quartz sheath. The leads are hermetically sealed at the top using a low vapour pressure resin ( Torr Seal). The above assembly is then connected to evacuating/annealing/gas filling system . It is evacuated for few hours to a vacuum of 10-6 mbar. Then it is heated at a temperature of 50 °C for few hours and is continuously evacuated The process is repeated in steps of 100°C upto 700°C and thoroughly evacuated for many hours at each temperature. At a temperature of 700°C and vacuum of about 10-7 milibar, a mixture of 90% Ar and 10% Oxygen is admitted in the thermometer assembly and heated at 700°Cfortwo days. The above step is repeated twice and assembly is evacuated to a vacuum of 10-7 mbar. Finally before sealing the SPRT , it is filled with 90% Argon and 10% Oxygen to a pressure of about 40% of atmosphere at a temperature of 670°C. The assembly is hermetically sealed. The four Gold/Platinum leads are soft soldered to 2 meters long Teflon insulated flexible wires. The Aluminium handle covers the top head of the SPRT and Teflon wires come out of the top of Aluminium handle. The novelty of the invention is in achieving the temperature coefficient of resistivity greater than 0.003925 using the method of present invention, a requirement for the use of any PRT as an international primary standard for calibration of thermometers. The inventive steps in the process of the present invention are: 1 . Anchoring of the platinum coil through a constriction thus eliminating the use of making of Pt to quartz seal. 2. The annealing is carried out under continuous evacuation with the temperature being raised in steps. 3. The filing of a mixture of inert gas and oxygen is done at a temperature up to which the SPRT is to be used. The advantages of the invention are following: 1. It is comparatively easy to fabricate the SPRT. Any reasonably skilled person can fabricate the SPRT. 2. Self heating effect is less. 3. No quartz-platinum seal is required. 4. Almost no rejection of sensor during fabrication. 5. It takes less time to initially stabilise the SPRT at triple point of water. 6. No wastage of material and time. We claim: 1. A method of fabricating improved standard platinum resistance thermometers which comprises: (i) making a coil (1) of grade platinum wire having a minimum temperature coefficient of resistivity of 0.003925 by known methods; (ii) preparing 'V shaped leads (3) using platinum wire; (iii) fixing the said 'V shaped leads to both ends of the said coil by known methods in such a manner that the apex of the said 'V shaped lead is attached to each of the ends of the said platinum coil; (iv) cleaning two pieces of quartz capillaries(2) by known methods; (v) twisting together the said cleaned capillaries (2) to obtain a former by known methods to provide 3-4 twists; (vi) tapering one end of the said former by known methods to provide a pen nib shape end; (vii) reducing the capillaries diameters of the other end of the said former by known methods in such a manner as to hold the joints of the said 'V shaped leads and coil; (viii) housing the coil obtained in step (iii) in a bifilar form in the former through the nib end in such a manner that the 'V shaped lead ends come out of the reduced capillary ends; (ix) passing the lead ends through plurality of quartz discs(4) and sleeves (5); (x) connecting the said insulated lead ends to an insulated four wire system; (xi) encasing the assembly so obtained in a quartz stem, evacuating the quartz stem encased assembly at room temperature to attain a vacuum better than 10-5 mbar followed by continuous evacuation in the range of 10-5 to 10-8 mbar and annealing at a temperature raised in steps of 50 -100 K upto 973 K for a period of 50 to 100 hours at each temperature, maintaining the temperature and vacuum followed by filling the said stem with a mixture of inert gas and oxygen in a ratio in the range of 80% to 95% and 20% to 5%, repeating the evacuation ,annealing and gas filling step if required, (xii) sealing hermetically by known methods at a temperature in the range of 973 to 1273 K the top end of the said gas mixture filled stem containing the said assembly to obtain a Standard Platinum Resistance Thermometer. 2. A method as claimed in claim1 wherein the twisting of the capillaries is done to obtain an internal diameter in the range of 1.0 mm to 6.0 mm. 3. A method as claimed in claims 1-2 wherein the inert gas used is argon, nitrogen, helium of 99.999% purity. 4. A method as claimed in claims 1-3 wherein the oxygen gas used is of 99.995% purity. 5. A method as claimed in claims 1-4 wherein the mixture of inert gas and oxygen used is at a pressure in the range of 30%-50% of atmosphere. 6. A method of fabricating improved standard platinum resistance thermometers substantially as herein described with reference to the example and drawing accompanying this specification.

Documents:

732-del-1999-abstract.pdf

732-del-1999-claims.pdf

732-del-1999-correspondence-others.pdf

732-del-1999-correspondence-po.pdf

732-del-1999-description (complete).pdf

732-del-1999-drawings.pdf

732-del-1999-form-1.pdf

732-del-1999-form-19.pdf

732-del-1999-form-2.pdf