GB2155238A - Temperature sensing device with in-built calibration arrangement - Google Patents

Abstract

The temperature sensing device comprises a temperature sensor, such as the welded measurement junction (10) of a thermocouple, which produces an electrical signal representative of a sensed temperature. The sensor is contained within a ceramic insulator (13, 14). A ceramic housing (15) contains the ceramic insulator and a miniature melting or freezing point cell into which the sensor, contained within the insulator, extends. The cell is closed at one end by a plug of ceramic paste (16) and at the other end by a ceramic cap (18). The cell contains a mass of material, preferably a pure metal or alloy (19), of which the melting or freezing point is known. The housing is mounted in a protection tube (22), preferably of ceramic material, having an integral closed end adjacent to the ceramic cap of the housing. The mass of material acts as a miniature version of a laboratory melting or freezing point cell so that the device can be calibrated without removing it from a temperature sensing position. <IMAGE>

Description

SPECIFICATION A temperature sensing device This invention relates to a temperature sensing device and particularly but not exclusively to a thermocouple.

A thermocouple is normally removed from its temperature sensing position and taken to a laboratory for calibration. The most accurate known method of calibration is to use a freezing point cell.

This, known method relies on the fact (see the "Manual on the use of Thermocouples in Temperature Measurement" at page 118 published by The American SocietyforTesting and Materials, 1916 Race Street, Philadelphia, PA 19103, U.S.A.) that the EMF developed by a homogeneous thermocouple at the freezing point of a metal is constant and reproducible if the following conditions are satisfied: (1) The thermocouple is protected from contamination (2) The thermocouple is immersed in the freezing-point sample sufficiently far to eliminate heating or cooling of the junction by heat flow along the wires and insulator.

(3) The reference junctions are maintained at a constant and reproducible temperature.

(4) The freezing point sample is pure.

(5) The metal is maintained at essentially a uniform temperature during freezing.

The EMF of a thermocouple at the melting point of a metal may be determined in a similar way, but under laboratory conditions it is possible usually to obtain more accurate results using the freezing point.

However, in order to calibrate a thermocouple in this manner the thermocouple must be removed from its temperature sensing position and it has always been a problem that the errors introduced in returning the thermocouple to its temperature sensing position invalidate the calibration (see pages 123 and 124 of the "Manual on the use of Thermocouples in Temperatue Measurement").

According to the invention, there is provided a temperature sensing device comprising a temperature sensor for producing an electrical signal representative of a sensed temperature and contained within a ceramic insulator, a ceramic housing containing the ceramic insulator and a miniature melting or freezing point cell into which the sensor, contained within the insulator, extends, the cell being closed at one end by a plug of ceramic paste interposed between the housing and the insulator and at the other end by a ceramic cap, and containing a mass of material of which the melting or freezing point is known and a protection tube having an integral closed end, the housing being mounted in the protection tube with the ceramic cap of the housing adjacent to the integral closed end of the protection tube, whereby the device can be calibrated without removing it from a temperature sensing position.

Preferably, the material is a pure metal, which substantially surrounds the temperature sensor, e.g.

the measurement junction of a thermocouple.

In practice, it is better to make use of the melting point rather than the freezing point of the material because of the tendency in non-laboratory conditions for the mterial to supercool. In this case the material is chosen to have a melting point close to but below the temperature or temperature range the device is to sense so that each time the temperature at the sensing position of the device is raised from below the melting point of the material the recorded EMF will exhibit a plateau at the melting point with the effect that the device is self-calibrating. Hence, the temperature sensing device contains its own miniature version of a laboratory melting orfreezing point cell which does not in any way impede the normal operation of the device.

One embodiment of the invention will now be more particularly described with reference to the accompanying drawing which is a sectional view of the temperature sensing end of a thermocouple constructed in accordance with the invention, on a enlarged scale.

Referring to the drawing the thermocouple shown therein comprises two wires 11 and 12 of dissimilar metals welded together to form a measuring junction 10. The wires 11 and 12 and junction 10 are contained in a ceramic electrical insulator 13 of very pure recrystallised alumina. The insulator 13 has two bores in which the two wires 11 and 12, respectively, are located and is closed at its end adjacent to the junction 10.

In practice the above arrangement can be achieved by pushing the two wires 11 and 12 through two open ended bores in the insulator 13, the insulator having in one end a transverse slot which communicates with the two bores. The two wires are welded together at the one end of the insulator to form the junction 10 and the wires are then pulled back to withdraw the junction into the transverse slot. The one end of the insulator is then closed by filling the transverse slot with ceramic paste.

The insulator 13 is butted onto a two-bore ceramic tube 14 of greater diameter than the insulator 13.

The insulator 13 and tube 14 are mounted in a tubular ceramic housing 15. A plug 16 of ceramic paste inserted between the housing 15 and the insulator 13 and/or tube 14 forms the base of a melting point cell 17 into which the insulator 13 extends sufficiently far to eliminate heating or cooling of the junction 10 by heat flow along the wires 11 and 12 and the tube 14.

The cell 17 is closed by a ceramic cap 18 and is almost filled with melting point material 19. A small gap 20 is left to allow for expansion. The material 19 is a pure metal or alloy of which the melting point is known, e.g. any appropriate metal listed in tables 34 and 35 Pages 107 and 110 of "Manual on the use of Thermocouples in Temperature Measurement", and is introduced into the cell in a molten state. The size of the gap 20 is dictated by the meniscus formed between the material 19 and the cap 18. In order to obtain an effective seal between the cap 18 and the housing 15 ceramic paste is interposed therebetween.

The material 19 is selected to have a melting point close to but below the normal operating temperature ortemperature range of the thermocouple in order to obtain the most relevant calibration.

The housing 15 is mounted in a ceramic protection tube 22 having an integral closed end 23 so that the interior of the protection tube 22 is not exposed to the gases in the temperature sensing zone.

A thermocouple as described above is in effect self-calibrating. When used for instance to measure the temperature of a furnace, each time the furnace is heated to its operating temperature the temperature of the measuring junction 10 will pass through the melting point of the material 19.The recorded EMF wiill exhibit a plateau (or discontinuity in a graph of voltage against time) at the melting point while the heat absorbed by the material 19 is used to melt it. Any semi-skilled person can easily be trained to observe this plateau. If the plateau shifts from its expected position due to the junction 10 or material 19 becoming contaminated the thermocouple can be replaced.

Preliminary trials have shown calibration in this way to be significantly better than known methods despite the fact that the melting point and not the freezing point of the material 19 is being used. This is because the thermocouple is calibrated in situ.

Moreover, the calibration requires no furnace downtime and capital does not need to be tied up in the laboratory.

The overall diameter of the thermocouple described above may be about 8mm, that is to say similar two that of a conventional thermocouple.

Although, it is believed to be better to rely on the melting point of the material in the cell 17 to calibrate the thermocouple, it may be possible to use the freezing point of the material at least in some applications.

Moreover, it may be possible to make the protection tube of platinum instead of a ceramic material, particularly for thermocouples for use at relatively low temperatures. However, it is important that the melting point material should not be contaminated with gas, as otherwise its accuracy will be impaired.

The invention is also applicable to resistance thermometers and thermistors in which case modified versions of the insulator 13 will contain the resistance element of the resistance thermometer and the thermistor, respectively.

Claims (9)

1. Atemperaturesensing device comprising a temperature sensor for producing an electrical signal representative of a sensed temperature and contained within a ceramic insulator, a ceramic housing containing the ceramic insulator and a miniature melting or freezing point cell into which the sensor, contained within the insulator, extends, the cell being closed at one end by a plug of ceramic paste interposed between the housing and the insulator and at the other end by a ceramic cap, and containing a mass of material of which the melting or freezing point is known and a protection tube having an integral closed end, the housing being mounted in the protection tube with the ceramic cap of the housing adjacent to the integral closed end of the protection tube, whereby the device can be calibrated without removing it from a temperature sensing position.

2. The device of claim 1, wherein the material is a pure metal or an alloy of defined melting point.

3. The device of claim 1 or claim 2, wherein the protection tube is of ceramic material.

4. The device of anyone of the preceding claims, wherein ceramic paste is interposed between the cap and the housing to obtain an effective seal.

5. The device of anyone of the preceding claims, wherein the insulator is in two parts butted together, the part nearer to the cap and which extends into the mass of material being of smaller cross-sectional dimensions than the other part.

6. The device of any one of the preceding claims, wherein the sensor is the measurement junction of a thermocouple.

7. The device of any one of claims 1-5, wherein the sensor is the resistance element of a resistance thermometer.

8. The device of any one of claims 1-5, wherein the temperature sensor is a thermistor.

9. A temperature sensing device substantially as hereinbefore described with reference to and as shown in the accompanying drawing.