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The instruments we construct in order to measure temperature are called "THERMOMETERS" and most often they consist of a measuring system.

In all thermometers physical changes are used for measuring the temperature.

The most common temperature measuring principles are based on the following four main groups:

In liquids, gases, or solid substances.

- Resistance (nickel, platinum, thermistors)
- Thermo voltage (thermo element)
- Charging voltage (semi-conductor)
- Frequencies (quartz thermometers)

- Colour-changing paint
- Wax balls
- Label indicator

The term "Thermometer", e.g. concerning types based on electrical properties, cf. point 1.2.2., normally covers a finished manufactured unit consisting of an element with insulated feed lines ending in terminals and covered by a protecting tube.

IN EVERYDAY USAGE A THERMOMETER IS CALLED A SENSOR.

The following is a description of thermometer principles based on electrical properties.

The main emphasis will be placed on RESISTANCE THERMOMETERS and THERMO ELEMENTS.

Principle :

The principle is based on the fact that electric resistance changes in relation to temperature.

The resistance thermometer (element) works by sending electric current through the element so that it is possible to read the voltage variation, indicating the temperature variation.

The element itself does not create any electro-motive force. It is a PASSIVE ELEMENT.

The material most used in element production is platinum. The reason is essentially that platinum is a rather stable material, and elements of platinum are reproducible in mass production. And not least, the interdependence between resistance and temperature is almost linear.

Nickel can be and has been used for elements, but is becoming less popular.

Pt-100 (Platinum-100 element) :

The Pt-100 element is defined by the resistance in the element being 100 ohms at 0°C. It is constructed by adapting diameter and length of the platinum wire.

Other Pt-values are widely produced. Most common next to Pt-100 are:

Pt-500, 1000 :

These are defined by the resistance values at 0°C being 500 ohms and 1000 ohms respectively.

The advantage of using an element having a higher resistance value is that the resistance values in the feed line will have a relatively smaller influence on the total measuring result.

Previously the Ni-100 element was widely used for measuring temperatures in industry. But because of mechanial problems in the production and because Ni-100 is less linear than Pt-100 the use of the element is rapidly declining.

A Ni-100 element shows the same non-linearity over an area of 180°C, as does a Pt-100 element over an area of 600°C.

External glass tube
Platinum winding Fig. 3	Internal glass rod	Feed lines

Glass elements can be used in the temperature range -200°C to 400°C. The price of this type of elements is at the high level.

The ceramic element consists of a platinum wire which is spiralized and mounted in a two-hole ceramic tube. The ceramic material is often made of aluminium oxide.

The construction method makes the element more sensitive to vibrations than the glass element.

Platinum winding
		Feed lines
Ceramic jacket Fig. 4

Ceramic elements are used in the temperature range -200°C to +850°C, but it is not recommendable to exceed +600. The price is considerably lower than that of the glass elements.

Thin film - (Platinum) :

The photo technique is used in circuit production. And this technique has been applied to the production of thin film elements.

A thin platinum film is steamed on to a ceramic plate. This thin film allows very small dimensions.

Ceramic plate
		Feed lines
Platinum film Fig. 5

This small dimension also allows a quick response time. The stability is not quite as good as for wire resistances. Thin film resistances are applicable in the temperature range -50°C to +400°C. Most often they are used for max. 200°C. Mass production makes it the cheapest form of platinum element.

Principle

If two contact points are held between two different metals A and B at different temperatures, an electromotive force/potential will usually occur and drive a current through the circuit. Cf. fig. 6 a.

The electromotive force/potential per degree of temperature difference is called the thermoforce/-potential or the Seebeck coefficient. This depends exclusively on the two metals selected and is a splendid way of measuring differential temperatures if a µ-voltmeter is inserted into the circuit. Cf. fig. 6 b.

To measure the absolute temperature, one end, the reference point, must be kept at 0°C e.g. with ice water, while the other end, the measuring point, is measured. Often the reference point is called "the cold end" and the measuring point "the hot end". This is a little misleading as negative temperatures can also be measured with the measuring point. IEC 584 includes interdependent table values of voltage and temperature when the reference point is kept at 0°C.

In fig. 6 b the m V-meter has terminals of a third metal, C. Thus the points BC and CB will occur at the instrument. As they have the same temperature, they will have the same size and be opposite of each other, which is why they are excluded from the measurement.

In the industry the most common instruments which are compensated for the cold point, i.e. the instrument measures the temperature at the connecting terminals and adds an internal voltage corresponding to the junctions AC and CB and the voltage that should come from the reference point.

As the instrument compensates for the leak AC and CB when the temperature is measured at the terminals of the instrument, it is important that compensation cables are of the same material as the thermoelement. If not, the cold point will move to the terminal of the thermocouple, where the instrument cannot compensate. The indication error in that case will be ti-tk. (Cf. fig. 7).

In order to make different makes compatible, the International Electronical Commission has prepared standards for these sensors - IEC 584 (DIN 43710).

Types according to IEC-584-1 :

The thermoelement consists of two non-insulated wires welded together, often called a thermocouple.

For most purposes it is necessary to protect the thermoelement from the surroundings. This can be done by insulating the wires and by using a protecting tube. Depending on the response time desired, three different forms are available. The most common has an insulated measuring point.

NTC (negative temperature coefficient) resistances can be used as temperature sensors. These thermistors are strongly non-linear and cover a very limited temperature range. The non-linearity is compensated for in the instrument, which has to be trimmed exactly for that particular thermistor. This can cause problems in connection with replacement of the sensors. Today precision thermistors having a tolerance of the resistance value down to ±0,5% or ±0,1°C are produced.

A semi-conductor junction works in a narrow temperature range, almost linearly to the temperature, but has different basic value and steepness.

The semi-conductor is very popular due to its price, good linearity, and reproducible temperature dependence. However, in a professional context it is of limited use, as adjustment of sensor and instrument is necessary, which is not possible with the service equipment normally available. However, there are certain types with current, voltage, or resistance junction that can be used and which do not require a high degree of accuracy.

Quartz thermometers :

In some places it may be impossible to place conventional sensors. E.g. rolling plastics lanes from a plastics extruder. In such cases, infrared measurement can be used. The heat radiation is measured with an IR detector and transferred to a display or a 4-20 mA measuring signal.

This type of measurement can also be used in places where conventional sensors would be burnt.

For any temperature measurement, it is of utmost importance to choose the right type and design of thermometer.

The following is a brief description of the most important parameters to be observed.

Temperature range - limits and running temperature depending on type

Tolerances :

IEC 751 (previously DIN 43760) defines two tolerance classes, A and B, which producers of Pt-100 observe.
The tolerance classes define the deviations permitted at 0 degrees centigrade and the constant at which the tolerance is allowed to develop.

Table of correlated temperatures and permitted deviations (tolerances) in degrees C and ohms.

For classes A & B - according to IEC 751

IIEC 584-2 states 3 tolerance classes based upon wire thicknesses of 0.25 mm to 3 mm. The tolerance only applies to thermocouples fresh from factory.

Tolerances for the IEC 584-2 standard :

Table 4

Tolerance curve for IEC 584-2 Class 2

It will always be very difficult to specify the long-term stability of a given thermometer at a given use. It depends on its structure as well as its use. The maximum temperature, vibrations and temperature fluctuations that the sensor is exposed to are of great importance.

The IEC 751 standard indicates how to test long-term stability and the limits the thermometer has to be within in order to keep the standard. The test consists in bringing the thermometer to its extreme limits and keeping it there for 250 hours. Having been exposed to this load, the values of the thermometer are not to have changed by more than 0,15°C for class A and 0,3°C for class B.

In the same way the standard indicates the criteria for testing the "STABILITY TO TEMPERATURE FLUCTUATIONS" of the thermometer. The test is performed by changing the temperature 10 times between the extreme limits of the thermometer. After this test, the thermometer is to keep within the same limits as described under testing of long-term stability.

The long-term stability varies from one type to another depending on the quality of the protection and the scope of the changes in temperature. Generally, the ageing is diminshed by good protection and by using the thermosensor for the temperature it was designed for without exceeding the upper temperature limit.

Lack of stability is explained by the thermovoltage having the character of a hysteresis. The voltage follows one curve at heating and another at cooling. The difference can be up to 5°C, depending on the age of the thermoelement, time and temperature.

Like any other form of electric resistance, heat is generated in the sensor element when a current is driven through it.

In order to make the signal from the sensor element big, it is desired to make the current big. This leads to an increase in the input P = R x I² , generating a rise in temperature in the element. The difference between 1 mA and 10 mA measuring current can lead to spontaneous heating of up to 0,8°C.

In order to achieve the best measuring result it is important that the sensor is placed and mounted in the optimum way.

Where the response time does not have to be very quick, it may be suitable to mount a pocket, which makes it possible to replace the sensor without emptying the system or container.

Alternatively, in the low- and medium pressure range, it is possible to use sensors with removable insert in order to obtain the same advantage.

IEC 751 defines the necessary immersion depth as the one giving a maximum error of 0.1°C when the sensor point is in steam at 100°C and the handle at 0°C.

The immersion depth depends of the design of the sensor, the length of the sensor element, and the thickness and material of the jacket. Normally the smallest immersion depth recommended is the length of the element + 5 times the protecting tube diameter, in liquids, and in gases, the length of the element + 15 times the protecting tube diameter.

In connection with measurement of temperatures in pipes having very large cross sections, it cannot be taken for granted that the temperature is the same in all of the cross section. In such cases, it will be necessary to mount several sensors in a specific way, in order to measure the average temperature.

Standard VDI/VDE 2640, sheet 4, includes a description of the method.

The IEC 751 standard includes no requirements as to the response time for a thermometer, but indicates methods to measure it.

The response time is defined as the time needed by a thermometer exposed to a sudden change in temperature, in order to fully demonstrate the new value.

Ambient temperature:

In order to avoid a too high temperature in the connection head damaging the cables and any transmitter, it will often be necessary to extend the protecting tube with a piece of tube - the "EXTENSION LENGTH". This will reduce the heat transmission considerably.

If the tube or the container is insulated, it can also be an advantage to extend the protecting tube in order to make the connection head accessible for mounting.

Electric connection:

The IEC 751 standard suggests the following 3 methods of connection and colour codes:

1) Symbol and colour codes :

In resistance thermometers, it is necessary to be aware of the resistance value in the feed lines.

In a copper wire the resistance is:

L = Length in meters
A = Cross section area in mm ²
0.0175 = Specific resistance of copper

2) 2-conductor system :

The measuring current "I" is driven through L1, the sensor element and back through L2. The voltage "U" is measured inside the instrument. In this way the resistance in L1 the sensor element and L2 is included as a total value in the result of the voltage measurement.

Evidently this can lead to grave errors.

Example :

If a 2-conductor 1 mm² copper wire of 5 m is chosen, the conductivity resistance will be:

If a Pt-100 element is used, 0.385 ohm corresponds to a change in temperature of 1°C.

This deviation will affect the total exactness of the measuring system and generate an error. Usually it is necessary to use a better solution, 3- or 4-conductor.

3) 3-conductor system :

The measuring current "I" is driven through L1, the sensor element and L3.

The voltage U1 is measured over L1 and the sensor element. The voltage U2 is measured over L3. (L2 is currentless).

We have the current I and can calculate the voltage over the element by saying:

U = U1 - U2

which will express the temperature.

The approach in the 3-conductor system is based on the presupposition that L1 and L3 have the same length and cross section area. In most cases this solution will be sufficient.

But if a system without the mentioned approaches is desired, the 4-conductor system must be used.

4) 4-conductor system :

The measuring current "I" is driven through L1, sensor element and L4. In L2 and L3 the current is 0 or very close to 0 as the resistance in the voltmeter is very high.

The voltage is measured by means of L2 and L3 directly over the element. In this way we have the very most exact measuring method.

If the weak electric signal from resistance or thermoelement is to be changed into a power signal, a transmitter can be used. The transmitter is designed to change the current depending on the temperature. The most common power signal is a 2-conductor 4-20 mA signal.

The advantages of mounting the transmitter in the connection head of the sensor are:

- A 2-conductor wire will be sufficient for resistance sensors and eliminate the need for compensation cable for thermoelement.
- The signal is less sensitive to electrical noise.
- The signal is compatible with most modern instruments, PLC, PC etc.
- Several instruments, such as digital instrument and printer, can be connected in series.

In connection with mounting of sensors with built-in transmitter in the connection head, attention should be paid to the following:

- The ambient temperature should not be too high since all electronic devices are sensitive to this. It can reduce the exactness considerably or even ruin the measurement.
- Vibrations.
- Electrical noise.

According to the IEC 751 standard, producers of Pt-elements are to check class A elements at 0°C, as well as at another temperature, e.g. 100°C. Class B elements are only to be checked at 0°C.

Often it is necessary to know the exact value in several points. Such metering is called CALIBRATION.

Within calibration, two concepts are of importance: absolute calibration and comparative calibration. In everyday usage the term calibration covers the latter form.

Absolute calibration is based on the fixed points defined in ITS 90 (fig. 2). For each fixed point there is a specially produced measuring cell.

If e.g. a measuring cell with the metal Gallium is placed in a bath where the temperature can be controlled exactly around the melting point of gallium, the temperature can be regulated so that the gallium starts melting. The process of heating involves heat, which will keep the temperature at exactly 29.7646°C (an absolute temperature) as long as there is metal melting in the cell.

The thermometer in the cell is read and now we know the deviation of our thermometer above or below the absolute temperature at this fixed point.

We carry out the same procedure at the other fixed points in the temperature range we are inerested in. Between the values measured interpolation is carried out. We now know the deviations of the thermometer in relation to the absolute temperature. When measuring with this thermometer we are now able to calculate the absolute values of the measurement.

Such a thermometer is called a reference thermometer. All measuring equipment used during the process has to be traceable.

In this process the reference, calibrated as described above, is placed in a bath closely together with the thermometer which is to be calibrated.

The bath is placed at the temperature levels at which the thermometer is to be calibrated. Both thermometers are read at the points chosen and the values are written down. The points between them are interpolated. Via the resistance values of the reference thermometer we are able to calculate and tabulate values (curve) for the deviation of the thermometer from the absolute temperature. Afterwards a certificate can be issued.

Senmatic has a modern measuring laboratory with the necessary baths and reference thermometer - equipment which is fully up to that of the National Laboratory.