GB2114362A - Measurement transducer arrangements - Google Patents

Abstract

Two or more measurement sensors are enclosed in protective tubes 3,4 which extend parallel side-by-side from a common measurement head 1. The sensors are thermocouples or oxygen probes. Comparison of temperature measurements from the sensors in each tube can be used to indicate the presence of a leak in one tube.

Description

SPECIFICATION Measurement transducer arrangements This invention is related to the improvement of the exactness and reproducibility of the temperature measurement and -control. Two or more protection tubes are suggested, which are fixed in one thermocouple head side by side and not one tube in the othertube as before.

Usually a protection tube is fixed in another protection tube and both together into a thermocouple head, which contains the electric terminals of the thermocouple wires, These protection tubes are made of different materials to damp the thermal stress of the internal protection tube (or of the coils of the resistivity thermometer). The other goal of this coaxial construction is to get an advantageous atmosphare for the thermocouple wires and to get a neutral protection tube for the process.

To simplify this description only the thermocouples are mentioned below, but the same description is valid also for resistivity thermometers with protection tube.

Also well known is the construction in which one protection tube of a thermocouple contains two thermopairs in a four bore capillary tube or as a double compacted thermocouple (with ceramic powder isolation).

This construction was developped to separate a temperature controller from a recorder.

A new development is to fix a protection tube into the head with two times diameter. The goal of this construction is to fulfil only the half part of the protection tube by the built in thermopair. By this way control thermocouple can be inserted into the same protection tube, and so the temperature can be controlled during the run of the furnace.

Disadvantages of these constructions are following: 1. Two thermopairs in one protection tube are very common, but broken protection tube makes it possible that all the thermocouple wires get the same disadvantageous gas (or fluid) and therefore their thermal emu will aged similarly. The controlling of a thermoco uple with the aid of the other thermocouple is therefore not possible in spite of the theoretical independance of the two thermopairs.

This construction can be hard controlled during the run.

2. The controlling thermocouple with the double diameter protection tube will cooled down if a control thermocouple is inserted. This cooling down causes an overheating of the furnace as long as the control thermocouple reaches the same temperature as before the controlling. If the charge is sensitive this controlling is not possible in spite of the existing p.lace for the inserted thermocouple.

The thermocouples for the recording instruments and for the security controllers can be checked theoretically by this method, but these thermocouples shall be cooled down in the practice by some 100 K. During this controlling process a disturbing of the temperature curve is caused on the recording chart. This cooling down of the temperature curve can be hardly explained later. This controlling process consumes a lot of time.

Another disadvantage of this thick protection tube (for example with a diameter of 24 mm) is its very large dimension. It has a large thermal lag because of its large mass and because of the large gap between the wall of the protection tube and the thermopair.

The idea of the invention is to fix two or more protection tubes side by side, but mainly parallel into one thermocouple heaad. This group of protection tubes enables the temperature measuring quicker and more reliable.

This invention is shown in Figure 1: 1. is the thermocouple head with the electric terminals of the thermocouple wires which can contain an adapter tube (2), the protection tubes are (3) and (4) which are fixed into the thermocouple head advantageously gas-tight.

To simplify the describing the word "thermocouple head" is used here as the construction element of the thermocouple which can be seen from the built in thermocouple: in Figure 1 the "thermocouple head" contains two parts - 1 and 2 together.

The advantages of this new thermocouple are following: 1. The protection tubes side by side are thinner than 10 mm and therefore they have time constante of 40 % than usual. This little time constante enables very smooth temperature curve. In a concrete case the two-point controlling oscillated between 879,3 "C and 880,0 "C (the set-point was 880 "C). The amplitude was + 0,35 K which could not be achieved before.

2. The protection tubes in the same thermocouple head have the same temperature with a tolerance oft 0,1 K in industrial furnaces measured.

Therefore the thermocouples show the same temperature.

3. In an empty protection tube the otherthermocouples of the group can be controlled each time without the cooling down of them. Waiting of a longer soaking time for this controlling is not necessary anymore.

4. If one of the protection tubes in the group breaks and therefore for example the recording is disturbed, the empty protection tube can be used for the protection of the recording thermopair. The process can be led up to the end. The empty protection tube is not only a control tube but also a reserved protection tube.

5. An old feeding-through in an existing furnace can be used now to insert two or more thermocouples instead of the only one thermocouple before.

The completing of the temperature measuring is possible for example by security controlling.

Much less protection gas streams out in the case of a broken protection tube in the group compared to the old one. The new protection tubes are thinner than the old ones and they are more fulfilled.

7. The thin protection tubes of the invention are more elastic and not sensitive against thermal stress. They can work in such furnaces which are vibrated by motors and pumps. These thin protection tubes are cheaper and they can be made from very pure and expensive alumina of 99 or 99,7 %.

These very pure protection tubes can be used up to 1500 "C (up to 2300 "C in the case of berylia) without banding.

In a concrete case a very little drift of 0,7 K was found after 17 months at 1050 "C. This drift was exactly the same for both thermopairs in the group.

8. If a protection tube breaks because of a wrong moving of a charge or because of other factors, the thermocouple wires in this broken protection tube will be poisoned and therefore they indicate an other temperature than the thermocouple wires in the not broken tube. As long as two or more temperature indications at this multitube thermocouple are identical the temperature measuring or -control is reliable.

The thermocouple head - cavities (1) and (2) - can unify not only protection tubes with the same diameters, but also protection tubes with different diameters. It is possible, that a thin protection tube covers a thermopair, a thick one, however, an oxygen probe. By this way the multitube thermocouple enables the improvement of a furnace with protective atmosphere. The old feeding-through can be used within minutes to fix a new high quality thermocouple group and an oxygen probe at once.

Optimally the thermopairs are made in the thin protection tube and in the oxygen probe from the same roll of thermocouple wires. The thermocouple indication from the oxygen probe can be used to control the temperature indication from the thin protection tubes by the same way as shown in point 8.

Two oxygen probes fixed in one head make possible to control not only the temperature, but also the measuring of the quality of the protection atmosphere.

The electrical terminals of the thermocouples and of the oxygen probe(s) have the important advantage to be in the same head and therefore in the same cavity getting the same temperature.

Claims (4)

1. Thermocouple or resistivity thermometer consisting head with electrical terminals and protection tubes and thermopairs - or perhaps without thermopairs - having inventionally two or more protection tubes side by side, mainly parallel each with other fixed in one head with electrical terminals,

2. thermocouple or resistivity thermometer definited in claim 1 having inventionally an empty protection tube for enabling control measurings,

3. thermocouple or resistivity thermometer definited in claim 1 having inventionally high purity alumina protection tubes,

4. thermocouple or resistivity thermometer definited in claim 1 having inventionally in one or in more protection tube(s) and/or oxygen probe(s).