GB2457993A

GB2457993A - Improvements in or relating to the calibration of thermometers

Info

Publication number: GB2457993A
Application number: GB0804308A
Authority: GB
Inventors: John P Tavener
Original assignee: Isothermal Technology Ltd
Current assignee: Isothermal Technology Ltd
Priority date: 2008-03-07
Filing date: 2008-03-07
Publication date: 2009-09-09
Anticipated expiration: 2028-03-07
Also published as: GB0804308D0; GB2457993B

Abstract

A conductive block 20 has an elongate bore 24 extending therethrough. The block 20 also has a recess 22 which is arranged to receive one end of a fixed point cell 12. When the fixed point cell 12 is received within the recess 22 the elongate bore 24 is aligned with, and extends, a thermometer receiving bore 14 in the fixed point cell. The fixed point cell 12 is heated to attain a known temperature and the conductive block 20 is heated to the same temperature by heater and control means 26. Thus, a thermometer which extends through the elongate bore 24 and within the thermometer receiving bore 14 is brought accurately to the temperature of the fixed point cell without immersion effects. This enables accurate calibration of the thermometer.

Description

-1-2457993

IMPROVEMENTS IN OR RELATING TO THE CALIBRATION OF

THERMOMETERS

The present invention relates to a method of calibrating a thermometer, to an immersion compensating apparatus for use with a fixed point cell when calibrating a thermometer, and to an immersion compensating apparatus in combination with a fixed point cell.

Fundamentally a thermometer measures its own temperature. If, therefore, a thermometer is to be used to measure the temperature of a location, it must be immersed in that location sufficiently so that it reaches the site temperature.

Generally, a thermometer will be immersed for a certain amount at a location. The non immersed stem of the thermometer will act to conduct heat away from the location.

To ensure accurate calibration of a thermometer it is necessary to immerse it sufficiently at a site to ensure that the thermometer has reached the site temperature. Thus, steps need to be taken to ensure that the thermometer is not leaking heat away from the site. Put another way, it is necessary to ensure that stem conduction is eliminated.

In co-pending British patent application No. 0717714.0 the applicants describe a fixed point cell which has an excellent heat conduction performance and is able to attain the known change of state temperatures of pure substances with great accuracy. Such a fixed point cell can be used for thermometer calibration, but the immersion issues discussed above still arise.

The present invention seeks to reduce immersion effects during calibration of a thermometer.

According to a first aspect of the present invention there is provided a method of calibrating a thermometer utilising a fixed point cell having a thermometer receiving bore therein, the fixed point cell being arranged such ( that its thermometer receiving bore is aligned with, and extended by, an elongate bore in a conductive block, the method comprising the steps of: heating the fixed point cell such that the temperature within the thermometer receiving bore accurately attains a known temperature, heating the conductive block to the known temperature, and arranging a thermometer to be calibrated such that it extends within the thermometer receiving bore and within the aligned elongate bore whereby the thermometer is reliably heated to the known temperature for calibration.

In embodiments of the invention, the fixed point cell provides the accurate temperature to which the thermometer is to be brought for calibration, and the conductive block eliminates or reduces stem conduction.

Preferably, the fixed point cell contains a pure substance which changes state at a single known temperature. The method steps defined above are repeated a number of times using a different pure substance, and hence a different known temperature, for each repetition.

The length of the conductive block can be selected as required.

Obviously, the longer the length of the conductive block, the longer the length of the elongate bore. It would be possible to provide for the conductive block to be variable in length, but generally a block of an appropriate length will be selected to meet circumstances.

In an embodiment, the method comprises the step of controlling the temperature to which the conductive block is heated such that it is within 0.1°C of the known temperature attained by the fixed point cell.

According to a further aspect of the present invention there is provided an immersion compensating apparatus for use with a fixed point cell when calibrating a thermometer, the apparatus comprising: a conductive block having an elongate bore extending therethrough, and a recess formed therein and arranged to receive one end of the fixed point cell, c the arrangement being such that when one end of the fixed point cell is received in the recess the elongate bore is aligned with, and extends, a thermometer receiving bore extending within the fixed point cell, and the apparatus further comprising means for heating the conductive block to a required temperature.

Preferably, the conductive block is made of a thermally conductive metal, for example, of aluminium, copper or aluminium bronze.

io Any appropriate heating means may be used to heat the conductive block. For example, the heating means may be a band heater, which may be electrically powered, and which encircles the conductive block.

Control means may be provided for controlling the heating means in response to the temperature attained by the conductive block such that the conductive block can be heated to the required temperature. The control means preferably comprise a temperature sensor.

The length of the conductive block, and hence of the elongate bore, can be chosen as appropriate. Fixed point cells may be 180-200mm long and preferably the conductive block will have a length in the range 1-150mm, preferably of the order of 100mm.

The invention also extend to an immersion compensating apparatus in combination with a fixed point cell, where the immersion compensating apparatus is as defined above, and wherein the fixed point cell received within the recess has a cylindrical crucible surrounding the thermometer receiving bore, a pure substance being held within the crucible, a sealable metal container enclosing the crucible, and an annular metal tube surrounding the container, wherein an inner, substantially cylindrical wall of the metal tube is formed by the outer peripheral wall of the container.

Preferably, both the crucible and its enclosing container are substantially annular in cross-section to define the longitudinal thermometer receiving bore, which is closed at one end, and which extends along a central longitudinal axis of the fixed point cell.

I

Embodiments of the present invention will hereinafter be described, by way of example, with reference to the accompanying drawings in which: Figure 1 shows graphically the loss of heat from a thermometer by S stem conduction, Figure 2 shows an isometric view of a fixed point cell for use when calibrating a thermometer, and Figure 3 shows an embodiment of an immersion compensating apparatus of the invention indicating its combination with a fixed point cell of Figure 2.

Figure 1 shows graphically the immersion of a thermometer T into a site S. The site is at a temperature T5. It will be seen that flow of heat from the site S along the part of the thermometer not immersed in the site causes a temperature profile as shown in Figure 1. As a result, the thermometer attains a temperature which is different from that of the site. Obviously, if a thermometer which is being calibrated has a temperature profile along its length of the type shown in Figure 1, errors will arise.

Figure 2 shows a fixed point cell as described and illustrated in co-pending British patent application No. 0717714.0. Such a fixed point cell, generally indicated at 12, can be heated to accurately attain a known temperature. The fixed point cell 12 has a thermometer receiving bore 14, closed at the lower end as illustrated, extending along its central longitudinal axis. This bore 14 is defined by and surrounded by an annular crucible, not visible, which is enclosed within a sealable metal container 3. The outer peripheral wall of the container 3 is received within a metal tube 1. The metal tube 1 holds a vaporizable fluid which, upon heating, is arranged to provide an isothermal environment around the crucible.

There is a pure substance contained within the crucible which may be, for example, mercury, water, gallium, indium, tin, zinc, aluminium, silver or copper. When heated each of those substances will attain a fixed, known temperature on changing state. For example, when water changes state from liquid to solid it is at 0°C.

A fixed point cell as shown in Figure 2 may be, for example, 180- 200mm long. A thermometer to be calibrated using the fixed point cell may require an immersion of the order of 300mm so that the effects of stem conduction can be eliminated. Figure 3 illustrates an embodiment of an s immersion compensating apparatus for use with a fixed point cell as shown in Figure 2.

The apparatus of Figure 3 comprises a conductive block 20 which is made of a thermally conductive material, for example of aluminium, copper or aluminium bronze. This conductive block 20 has a recess 22 therein which is shaped and sized to receive one end of the fixed point sensor 12, which is indicated in Figure 3. It will be seen that the block 20 has an elongate bore 24 extending therethrough. The bore 24 opens into the recess 22 such that when a fixed point cell 12 is in the recess 22, the bore 24 is aligned with, and extends, the thermometer receiving bore 14 of the fixed point cell 12.

The conductive block 20 is provided with a heater and control means schematically indicated at 26. The heater, for example, may be a band heater arranged to encircle the block 20 and to be electrically powered. The control means may include a temperature sensor to ensure that the block 20 is brought to a temperature substantially the same as that of the fixed point cell 12 during a calibration process.

When calibrating a thermometer the fixed point cell 12, containing an appropriate pure substance, is received within the recess 22 of the block 20.

The fixed point cell 12 is heated by any appropriate means to bring the pure substance therein to the point at which it changes state. At this point, the temperature of the fixed point cell is known to a high degree of accuracy.

Generally, the fixed point cell 12 is heated from underneath using any available simple heater. The block 20 is heated by its heater 26 to a temperature substantially the same as that attained by the fixed point cell.

Generally, the temperature of the block 20 can be brought to within about 0.1°C of the temperature of the fixed point cell.

If then a thermometer, not shown, is arranged to extend within the ( thermometer receiving bore 14 of the fixed point cell 12, and along the elongate bore 24 of the conductive block 20, the thermometer can accurately be brought to the temperature of the fixed point cell without stem conduction.

As the thermometer is accurately at the known temperature, it can be calibrated. In this respect, because the block 20 is of a thermally conductive metal, there is no temperature gradient along the length of the block.

As shown in Figure 3, the conductive block 20 has a length L. This can be chosen as necessary to provide sufficient length to compensate for the immersion effects of different designs of thermometer. For example, the length L may be from 1-150mm, preferably 100mm.

It will be appreciated that variations in and modifications to the embodiments as described and illustrated can be made within the scope of the appended claims. (

Claims

CLAIMS1. A method of calibrating a thermometer utilising a fixed point cell having a thermometer receiving bore therein, the fixed point cell being arranged such that its thermometer receiving bore is aligned with, and extended by, an elongate bore in a conductive block, the method comprising the steps of: heating the fixed point cell such that the temperature within the thermometer receiving bore accurately attains a known temperature, io heating the conductive block to the known temperature, and arranging a thermometer to be calibrated such that it extends within the thermometer receiving bore and within the aligned elongate bore whereby the thermometer is reliably heated to the known temperature for calibration.
2. A method as claimed in Claim 1, wherein the fixed point cell contains a pure substance which changes state at a single known temperature, the method comprising repeating the method steps defined in Claim I a number of times using a different pure substance, and hence a different known temperatUre, for each repetition.
3. A method as claimed in Claim I or Claim 2, comprising selecting the length of the conductive block, and hence selecting the length of the elongate bore.
4. A method as claimed in any preceding claim, further comprising the step of controlling the temperature to which the conductive block is heated such that it is within 0.1°C of the known temperature attained by the fixed point cell.
5. An immersion compensating apparatus for use with a fixed point cell when calibrating a thermometer, the apparatus comprising: a conductive block having an elongate bore extending therethrough, and a recess formed therein and arranged to receive one end of the fixed point cell, (-B-the arrangement being such that when one end of the fixed point cell is received in the recess the elongate bore is aligned with, and extends, a thermometer receiving bore extending within the fixed point cell, and the apparatus further comprising means for heating the conductive block to a required temperature.
6. An immersion compensating apparatus as claimed in Claim 5, wherein said conductive block is made of thermally conductive metal.
7. An immersion compensating apparatus as claimed in Claim 6, wherein said conductive block is made of aluminium, copper or aluminium bronze.
8. An immersion compensating apparatus as claimed in any of Claims 5 to 7, wherein said heating means comprises an electrically powered band heater encircling the conductive block.
9. An immersion compensating apparatus as claimed in any of Claims 5 to 8, further comprising control means for controlling the heating means in response to the temperature attained by the conductive block such that the conductive block can be heated to the required temperature.
10. An immersion compensating apparatus as claimed in Claim 9, wherein said control means comprises a temperature sensor.
11. An immersion compensating apparatus as claimed in any of Claims 5 to 10, wherein the length of the conductive block is in the range 1-150mm, preferably of the order of 100mm.
12. An immersion compensating apparatus in combination with a fixed point cell, wherein the immersion compensating apparatus is as claimed in any of Claims 5 to 11, and wherein the fixed point cell received within the recess has a cylindrical crucible surrounding the thermometer receiving bore, a pure substance being held within the crucible, a sealable metal container enclosing the crucible, and an annular metal tube surrounding the container, wherein an inner, substantially cylindrical wall of the metal tube is formed byCthe outer peripheral wall of the container.
13. An immersion compensating apparatus in combination with a fixed point cell as claimed in Claim 12, wherein both the crucible and its enclosing container are substantially annular in cross-section to define the longitudinal thermometer receiving bore, which is closed at one end, and which extends along a central longitudinal axis of the fixed point cell.
14. A method of calibrating a thermometer utilising a fixed point cell substantially as hereinbefore described with reference to the accompanying drawings.
15. An immersion compensating apparatus for use with a fixed point cell when calibrating a thermometer substantially as hereinbefore described with reference to the accompanying drawings.
16. An immersion compensating apparatus in combination with a fixed point cell substantially as hereinbéfore described with reference to the accompanying drawings.