US3482199A

US3482199A - Standard thermometer

Info

Publication number: US3482199A
Application number: US699471A
Authority: US
Inventors: Donald J Curtis
Original assignee: Rosemount Engineering Co Ltd
Current assignee: Emerson Process Management Ltd
Priority date: 1968-01-22
Filing date: 1968-01-22
Publication date: 1969-12-02
Anticipated expiration: 1986-12-02
Also published as: DE1902316A1; DE1902316C3; GB1246154A; JPS4925232B1; DE1902316B2

Description

D. J. CURTIS Dec. 2, 1969 S TANDARD THERMOMETER 2 Sheets-Sheet l Filed Jan. 22, 1968 D. J. CURTIS STANDARD THERMOMETER Dec. 2, 1969 2 Sheets-Sheet 2 Filed Jan. 22, 1968 I N VEN TOR. .DOA/gib J.' @UHT/5 United States Patent O 3,482,199 STANDARD THERMOMETER Donald J. Curtis, St. Paul, Minn., assignor to Rosemount Engineering Company, Minneapolis, Minn., a corporation of Minnesota Filed Jan. 22, 1968, Ser. No. 699,471

Int. Cl. H01c 7/08 U.S. Cl. 338--28 9 Claims ABSTRACT OF THE DISCLOSURE A standard thermometer comprising an element and lead wire assembly mounted on a plurality of spaced apart insulator support discs each of which has a plurality of holes which are axially aligned with holes in the adjacent discs. The discs for the element assembly are held together by a central axial wire. The resistance element for the thermometer comprises a plurality of lengths of substantially straight wire threaded through the holes that are radially spaced from the center hole. The lead wires are supported on insulator discs as well. These lead Wire support discs are polished on the side toward the sensing element and are roughened on the opposite side to serve as radiation traps. The tube or well into which the element-lead wire assembly is inserted is also roughened on the exterior by sand blasting to cut down the axial radiation and reections on the inside of the well. The resistance element is wound in a particular manner so that if there is bending of the. end portions of the element Where the straight lengths are 4connected the element will not short out. The central wire serves to hold the assembly of insulator discs for the sensitive element to-gether by having ball members attached to the ends of the wire and bearing against the end discs, with spacers between the adjacent `discs of the resistance. element support. The central :wire also has an external connection and serves as a connection to provide an insulation check to make sure the element is properly insulated for accuracy.

BACKGROUND OF THE INVENTION Field of the invention The invention relates to standard thermometers which are highly precise thermometers used as standards in calibration of other thermometers.

Prior art The National Bureau of Standards has worked in the iield of standard thermometry for several years. Much effort has been made to expand the platinum scale of thermometry up to the gold point which is approximately l063 C. This is substantially higher than the range for which most standard thermometers are designed, which has been up to about 630 C. The National Bureau of Standards built and tested a bird cage type of element that used platinum wire threaded through spaced supporting insulator discs and showed that such a design would work. The present design represents improvements over the prior art because of improvement of and means for determining the insulation properties of the element support and reduction of radiation losses at these high temperatures.

SUMMARY OF THE INVENTION The present invention presents improvements in standard thermometry including the use. of spaced apart parallel element support discs that are held together with a central wire which is also used as a resistance check iwire to check the resistance of the support discs for the resistance element. The thermometer includes lead wires which are sup- ICC ported on support discs that have a polished surface facing toward the. element and a roughened surface on the side away from the element to cut down radiation losses. The outside of the tube section which encloses the leads is sand blasted to cut internal reflections and reduce the radiation transmitted axially along the tube wall in order to make the measurements more accurate. The element is particularly designed for reaching the gold point, o1' approximately 1063 C. The construction has a resistance check lead with an external connection so that the resistance of the supports discs of the element can be checked by measuring the resistance between the check lead and the sensing element.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a side elevational view of a standard thermometer made according to the present invention with parts in section and parts broken away;

FIG. 2 is a side view of the internal construction of the device of FIG. 1 with parts in section and parts broken away;

FIG. 3 is a side elevational view of the element portion of the standard thermometer shown in FIG. 1;

FIG. 4 is an end elevational view of the. element shown in FIG. 3;

FIG. 5 is a top plan view of the device of FIG. 3; and

FIG. 6 is an enlarged top plan view of the element and lead wire junction in the standard thermometer made according to the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT In FIGURE 1, the standard thermometer is shown encased in a quartz outer tube or well 11 which has a closed end 12.

The tube houses a resistance element assembly illustrated generally at 13, and a lead wire assembly illustrated generally at 14, and shown in FIGURE 2. The resistance element assembly 13 comprises a plurality of (ve as shown) of thin insulator discs 15 having planes at substantially to the longitudinal axis of the tube. These discs, as shown, are. made of insulating material such as quartz. The discs are held spaced with insulator spacers 16 between adjacent discs. Ille discs are each provided with a rst axial opening through which a center wire 17 is threaded. The wire 17 then contacts all of the discs. The spacers 16 are also provided with an opening through which this Wire 17 extends. The wire 17 is perhaps best seen in FIGURE 6, and is an axial wire extending the length of the element assembly, and in addition extends past the end spacer and end disc 18 which is outwardly beyond the actual resistance wire. The wire 17 has a ball or knob 21 fused on the outer end thereof and bearing against the. end disc, at a first end of the assembly. After the spacers 16 and discs 15 have all been threaded over the wire 17, another ball 22 is fused to the wire so it bears against the other end disc 18 at a second end of the assembly. The balls 21 and 22 are fused onto the wire 17 so that they are securely held ou the wire and the second end ball 22 is fused onto the wvire after the. spacers and discs have been threaded in place.

The discs 15 are polished on both sides and are each provided with a plurality of second openings 23 adjacent to the peripheral edges, and spaced substantially equal distances from the center opening through which wire 17 extends. The openings 23 are made so that they will align with the openings in adjacent discs.

A resistance wire 24 that has known temperature Vs. resistance characteristics (platinum or other suitable resistance wire) is formed into a plurality of lengths of straight wire which are joined together at opposite ends of the assembly and are threaded through the aligning openings 23 on adjacent discs. The wire 24 is threaded so that there are bottom lengths 25 which are joined together with a short end length 26 parallel to the face of the end disc at the first end of the assembly and which has very sharp bends at its corners. This forms a square-ended hairpin configuration. These lengths are then joined to lengths 27 adjacent the second end of the element assembly. The junction is made by bending the wires around the spacer 16 at the second end of the assembly and threading the lengths 27 through the openings. The lengths 27 are joined by end member 28 at the first end of the assembly and member 28 is spaced outwardly from the first end disc 15 a distance greater than the end member 26. Lengths 27 are in turn joined to a pair of lengths 29 that are bent around the spacer at the outer end of the element and which are joined together with end member 30 which is spaced outwardly from the end disc 15 substantially the same distance as the end member 28. The

end member

28 and 30 are spaced so that the element lengths 27 and the element lengths 29 are spaced apart further than the element lengths 25. Another pair of lengths of straight wire 31 are joined to the lengths 29 and are threaded through the openings. The lengths 31 are joined by an end member 32 adjacent the end disc 15 at the first end of the assembly. Thus, it can be seen that the wire 24 forms a helical type coil but using straight lengths of wire joined at their ends so the coil section is substantially rectangular. The

end members

26 and 32 are shorter than the

end members

28 and 30, and are closer to the end disc 15 so that if they get bent the

members

26 and 32 can pass through the loops formed by

members

28 and 30 and their connected lengths of wire without shorting out. The element discs 15 are about 1/32 thick and spaced about 7/16 inch apart. The contact area between the hole surfaces and the element wire is thus kept down.

The free ends of the resistance wire 24 are then fused as at 35 and 36 to

jumper lead wires

37A, 38A, 39A and A. The jumper leads of small diameter to reduce conduction of heat along the leads. The jumpers are fused with retainer balls to lead

Wires

37, 38, 39 and 40. The

lead wires

37, 38, 39, and 40 are passed through the end disc 18 of the element assembly and then are threaded through spacers 41 and also through support discs 42 which are used to support the lead wires. The lead wires are much longer than the element assembly and While only three discs 42 are shown in the drawings, there are normally about seven discs 42 such as this for supporting the lead wires in proper orientation with respect to the tubes, keeping them from moving about. The spacers between the discs 42 hold the lead wires properly oriented, and are also made up of insulating material to prevent shorting. The discs 42 are about l@ inch thick as well, but are polished on one side (the side toward the element) and roughened on the other. The discs 42 are spaced about 3 inches apart.

The lead wires are held to prevent them from straining the joints where they attach the element wire by the balls 45 which are onto the lead wires on a side of the disc 18 opposite that from the ball 22. The balls 45 then prevent the leads from moving outwardly and breaking the fused

joints

35 and 36. The balls 45 are attached to each of the lead wires so that each of the lead wires is held. The central wire 17 is also fused at 46 to a lead 47. This lead 47 is then carried down through the supportdiscs 42 (an opening is provided for this lead and also for the other lead wires in the discs 42). Note that as shown the element wire actually terminates on a side of end disc 18 opposite from the lead Wires.

Adjacent to the outer end of the tube 11, a housing 48 is sealingly mounted. This housing includes an outwardly extending tube 49 which goes laterally out from the side of the housing and the lead 47 is passed through this tube. This leaves the center wire 17 available so that it can be used for resistance check. .'Ihe other leads are mounted in a suitable manner for connection to instrumentation from the housing 48. The instrumentation used, of course, is any desired readout equipment well known in the art. The sensor is sealed and backfilled with dry air, dry nitrogen or dry argon. The check lead is also sealed with respect to tube 49.

This thermometer is made specially so that it can be used up to the vgold point which is approximately l063 C. The element resistance has been standardized at .25 ohms at the ice point which gives approximately 1 ohm at the gold point. This has been a standard design consideration. In addition, the present unit has been made so that the insulation materials are adequate at this temperature. One of the great difficulties in expanding the range of standard thermometry to the gold point is due to the inadequacy of insulators. The decreased resistance of the insulators with temperature in conventional thermometers is sufficient to cause an error which is intolerable for laboratory standard work. Another problem is radiation losses which destroy accuracy. In order to cut down the radiation losses through the element, the discs 42 supporting the lead wires are polished on the side toward the element, and are roughened on the opposite side. These discs then act as radiation traps or shields. The outside of the tube 11 which surrounds the lead Wire section is sandblasted to cut down internal reflections, consequently reducing the radiation transmitted axially along the tube.

In addition, the conduction error has been reduced in the present device by using the jumper wires connecting the sensing element to the lead wires. The jumpers are of smaller diameter wire than the lead Wires themselves.

The central wire 17 is used` not only to hold the element assembly together 'but for. making an insulation check at will. The Wire 17 contacts the surfaces defining the first opening through which the wire passes in the discs 15 so that there is a contact with these discs. Likewise, the element wire 24 contacts the discs 15 on the surfaces delining the holes through which the wire passes. The resistance of the insulation discs 15 can be checked by measuring the resistance between the leads for the central wire 17 and one end of the sensing element. A measurement of this resistance bears a known relation to the error in the resistance of the sensing element due to the measurable resistance of the insulating material. If a resistance breakdown should occur in any one disc it would be detected by a rather large change in measured resistance between the lead wire for the center wire 17 of the element assembly, and one end of the element wire itself. The reliability of this insulation check is enhanced by the fact that the electric field effect on the center wire is fairly uniform, because it is approximately equal distance from each length of the sensing element Wire.

The insulation material acts as a shunting resistance with the element, and so the actual resistance of the shunt (the insulation discs in this instance) is at some ratio to the measured resistance when using the check lead and one end of the resistance element. This ratio will change slightly with different insulation materials, different diameter wire and diameters of the holes. Because the shunting resistance is related to the contact area between the element wire and the hole surface supporting the wire it is important that the check lead wire 17 and the element wire 24 have the same transverse dimension and have the same characteristics. In ordinary use, the wires 17 and 24 are about .010| diameter. The holes in the discs 15 are only slightly larger.

The ratio of the measured resistance to the actual shunt resistance can be determined experimentally and this ratio used to determine the actual shunt resistance at any temperature of operation. This knowledge can be used in turn to determine the error due to the insulation resistance changes at higher temperatures. However, in many cases the error will be a limiting effect (if the measured resistance between the center wire 17 and the element wire 24 is below a certain value, the percentage of error will increase by a known percentage) rather than part of the calibration of the thermometer across a range of temperatures.

This ratio of measured resistance to actual shunt resistance was experimentally derived for the assembly described above by constructing a model of discrete known value resistors representing each length of resistance wire between discs and electrically conductive paperboard of known resistance representing the insulator discs. The resistors were supported by their connecting wires on the paperboard discs. A central wire was also used in the model. The model was constructed since the errors in the actual temperature sensor are so small as to be extremely difficult to determine experimentally with known measuring techniques at the high temperatures involved. A resistance measurement was made between the central wire and element of an actual temperature sensorand also the resistance' was measured between the two halves of the resistance-element of the actual sensor after theelement had been opened at its mid-point. These resistance measurements had a certain ratio. The ratio of,- these measurements on the actual sensor was reproduced in the model by physically adjusting the contact area between the paperboard discs and the connecting wires leading from the resistors. Then, since the resistance values of the resistors and paperboard discs of the model were precisely known, the ratio of the measured resistance between the central wire of the model and the resistors representing the element to the actual shunt resistance was found. This same ratio can be applied to the actual sensor with assurance. The ratio for the assembly described herein was found to be 1:2, i.e. the measured resistance between the check lead and one end of the element is actually about onehalf the value of the true shunt resistance.

It is pointed out that standard thermometers have to be extremely accurate. As operating temperatures rise obtaining this accuracy gets more and more diiiicult. By cutting down radiation losses along the lead wires (also conduction losses) and also by being able to easily check the insulation characteristic of the supports, accuracy and dependability is increased further.

What is claimed is:

1. In a resistance thermometer for use as a standard over a wide temperature range and comprising a plurality of connected lengths of straight element wire having known resistance-temperature characteristics and supported in a strain free manner in an element assembly, having first and second ends, the improvement comprising:

a plurality of first spaced apart insulator support discs each having a -first hole located substantially in the center of the disc and second holes located adjacent to the periphery thereof and substantially equal distance from the first hole and substantially in axial alignment with the holes of adjacent discs, said holes being defined by interior surfaces,

a first* length of substantially straight element wire threaded through the first holes in each of said first discs and supported by the surfaces defining the first hole, said first length passing through said discs and terminating at a connection point adjacent the second end of said assembly,

a plurality of second lengths of substantially straight element wire, each length being threaded through aligning second holes in said first discs for support by the surfaces defining said holes and connected to adjacent lengths at opposite ends to form a continuous element winding of several lengths connected together and terminating at first and second connection points adjacent the end disc on the second end of said element assembly.

2. The resistance thermometer as specified in claim 1 and means to permit the connection of resistance measur- Cir ing equipment between the connection point of said first length of substantially straight element wire and one of the connection points of said second lengths of element wire.

3. The resistance thermometer as specified in claim 1 wherein said first length of element wire is substantially the same diameter as the second length of element wire, and said first holes have the same diameter of said second holes in said discs.

4. The resistance therometer as specified in claim 3 wherein said second lengths of element wire are positioned symmetrically with respect to the axis of said first length of element wire.

5. The resistance thermometer as specified in claim and a plurality of insulator tubes positioned between adjacent discs, said insulator tubes surrounding said first length of element lwire only, and separate means fastened to said first length of element wire at spaced locations thereon and bearing against the outer surfaces of the end discs at the first and second ends of the element assembly to hold the insulator tubes and discs onto the first length of wire assembly. s

6. The resistance thermometer as specified in claim and lead wire means attached to said first length of element wire and the first and second connection points of said connected second lengths of element wire, said lead wire means being of substantial length, second insulator disc means having holes therethrough to form surfaces supporting said lead wire means, a tubular housing surrounding said assembly of second lengths of element wire and said first lengths of element wire and said lead wire means, said yfirst and second discs fitting within said tubular housing, and wherein the second discs supporting said lead wire means have a polished surface on a side thereof facing said second lengths of element wire and a roughened surface on the side thereof opposite from the second lengths of element wire.

7. The resistance thermometer as specified in claim 5 and jumper wire connectors between said lead wire means and the connection attached to the second lengths of element wire points to said second lengths of element wire, said jumper wires being of smaller diameter than the element wires and the lead wire means to reduce conduction from said element wire to said lead wire means.

8. The combination as specified in claim 5 wherein the element wires and lead wire means are inserted within a tubular member made of quartz and the outer surface of said tubular member is roughened in the portion of the tube surrounding the lead wire means.

9. The sensor of claim 8 wherein the second lengths of straight wire are joined in pairs adjacent a first end of the assembly and adjacent an outer face of an end disc by separate lengths of wire parallel to the plane of the end disc and substantially parallel to each other, a first pair of second lengths being spaced apart less than a second pair of second lengths, and the separate length of wire joining said first pair of second lengths being positioned closer to the end disc than the separate length of wire joining the second pair of second lengths.

References Cited UNITED STATES PATENTS 2,516,672 7/1950 Broekman 338-25 X 3,296,572 1/ 1967 Kleven 338-28 3,308,666 3/1967 Anderson et al 73--362 RODNEY D. BENNETT, JR., Primary Examiner H. C. WAMSLEY, Assistant Examiner U.S. Cl. X.R.

73-362; SSS-273, 290