Platinum Resistance Thermometer2.3 Standardisation and Tolerances Our product range includes a series of platinum resistance thermometers - starting with tolerance class B (DIN EN

Platinum Resistance Thermometer

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Gräff GmbH, Bonner Str. 54, D-53842 Troisdorf, Tel.: 02241/4907-0, Fax: 02241/4907-77I n t e r n e t : h t t p : / / w w w. g r a e f f - g m b h . c o m E - M a i l : i n f o @ g r a e f f - g m b h . c o m

Operation and Maintenance Manual

English



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Operation and Maintenance Manual for

Resistance Thermometer Table of Context 1 General 3 2 Functionality 4

2.1 Measuring Principle 4 2.2 Linearization 4

2.3 Standardisation and Tolerances 4 2.4 Self-Heating 6 2.5 Disturbances 6 2.6 Heat Dissipation Error 6 2.7 Thermoelectric Voltages 7

2.8 Insulation Resistance 7 2.9 Response Time 7 2.10 Selecting an Appropriate Device 8 2.11 Temperature Probes with Connection Head 8 3 Handling 8

3.1 Warnings 9 3.2 Connection 10 4 Maintenance 11 5 Technical Data 12 6 Drawings 13 Last update: January 2010 This manual is subject to change without notice!


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1 General Please read this manual carefully and completely before installing the sensor! Any damage caused by non-compliance with the operation and maintenance manual voids all warranty claims. We admit no liability for consequential damage. Warranty: For the sensor we grant a warranty of 24 months from the date of purchase.

The warranty includes free rectification of defects verifiably caused by the use of faulty materials

or poor workmanship. The defective device shall be returned to the manufacturer immediately

after the defect has become known together with original sales receipt and fault description.

The right to further claims shall be reserved.

The liability for defects does not cover natural wear and tear and transport damages as well as

damages due to non-compliance with the installation instructions, local installation regulations

or improper installation.

The manufacturer shall not be liable for any damages not arising on the delivered item itself,

especially not for indirect damages, consequential damages or property damages.

We shall reserve the right to repair, rectify or replace the defective goods or refund the

purchase price.

Unauthorised removal of our marking (serial number) negates the warranty. This product is manufactured by Gräff GmbH Temperature Measurement and Control Technology Bonner Strasse 54 D-53842 Troisdorf


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2 Functionality Platinum resistance thermometers provide high-accuracy measurement over a wide temperature range (from -200°C to +850°C). In their processing pure Platinum has purposefully been contaminated to preserve the quality of the pure metal and at the same time provide increased resistance to chemical contamination during the measuring process. Unlike thermocouples, resistance thermometers do not need special cable to connect the sensor. 2.1 Measuring Principle Resistance thermometers use the characteristic of an electrical conductor to change its resistance with temperature The principle of operation of these probes is to measure the resistance of the platinum element. The most common type (PT100) has a resistance of 100 ohms at 0°C and 138.4 ohms at 100°C. Corresponding ly, PT25 and PT1000 sensors have a resistance of 25 ohms and 1000 ohms at 0°C. Platinu m resistance thermometers are cold conductors, whose resistance increases with rising temperature. 2.2 Linearization The relationship between temperature and resistance is almost linear over a small temperature range: e.g. if we assume that the measuring range between 0°C and 100°C is linear, we will receive a deviation of 0,4 °C at 50 °C. For precise measurement, it is necessary to linear the resistance in order to receive an accurate measurement result. The most commonly used definition of the said relationship between temperature and resistance is the International Temperature Standard 90 ( ITS-90 ). 2.3 Standardisation and Tolerances Our product range includes a series of platinum resistance thermometers - starting with tolerance class B (DIN EN 60751): these sensors have an accuracy of +/- 0.3°C at 0°C. For more precise measurements tolerance class A sensors (+/- 0.15°C at 0°C) or 1/10 DIN class B sensors (+/- 0.03°C at 0°C) are used. Note: These tolerances only refer to the temperature sensor. Possible errors in the entire measuring system must be added. Equations to calculate the measuring resistance plo tted against temperature. A second-order polynomial applies for the temperature range between 0 and 850°C

R(T) = R0 * ( 1 + A * T + B * T2 ) A third-order polynomial applies for the temperature range between -200 and 0°C

R(T) = R0 * (1 + A * T + B * T2 + C * ( T - 100°C ) * T 3 ) In compliance with standard DIN EN 60751 the follow ing coefficients apply: A = 3.9083 * 10-3 °C -1 B = -5.775 * 10-7 °C -2 C = -4.183 * 10-12 °C -4 Calculation example:

The measuring resistance has a temperature of 200°C and the R0 has a value of 100 ohms (PT100). R(T) = 100Ohm * ( 1 + 0.0039083°C -1 * 200°C + (-0.0000005775°C -2) * (200°C) 2 ) R(T) = 175.856 Ohm


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Calculating the temperature from the measuring resi stance (temperature range between 0 and 850°C)

T = ( - A / 2 / B ) - ( A / 2 / B )2 - (( R0 - R(T) ) / B / R0 ) Tolerance classes Tolerance class Temperature range Formula

class AA -200 to 850°C +/- (0.10K + 0.0017 * T) class A -200 to 850°C +/- (0.15K + 0.002 * T)

class B -200 to 850°C +/- (0.30K + 0.005 * T) class C -200 to 850°C +/- (0.60K + 0,0100*T)

1/5 DIN B (Gräff) -150 to 350°C +/- (0.06K + 0.001 * T) 1/10 DIN B (Gräff) -150 to 350°C +/- (0.03K + 0.000 5 * T)

T = Measuring temperature at which the tolerance shall be determined.


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2.4 Self-heating The measuring current flowing through the measuring resistor generates some self-heating: e.g. a measuring current of 1 mA flowing through a 100 ohm resistor will generate 100 µW of heat. If the sensor cannot provide sufficient heat dissipation, a systematically higher temperature will be reported. The amount of self-heating depends on various factors, e.g. how much of the generated dissipation loss can be discharged by the measured medium. Due to the relation between of electric power and P = R x I2, the effect also depends on the basic value of the temperature sensor: At the same measuring current the heating is ten times greater on a PT1000 temperature sensor than on a PT100. Additionally, the measurement error is defined by design characteristics as well as the conductive heating and heat capacity. This effect is also heavily influenced by the flow rates of the measured medium. These effects can be reduced by either using a larger sensor or by ensuring that the sensor has good thermal contact with the medium to be measured. 2.5 Disturbances For a Pt100 sensor, a 1°C temperature change will c ause a 0.384 ohm change in resistance. This means that even a small error in measurement (e.g. poor contact with the connection line or disregarding the offset of a two-wire circuit) can have a great impact on the temperature measuring result. Due to a small voltage and current range during the measurement, it is important to separate the wires of the temperature resistor from electric cables, motors, control systems or other electric systems that can generate interference voltage. Using shielded cables with one end of the shielding being grounded can help to minimise disturbances. When the using long cables, please ensure that the measuring system is able to consider the impedance. If a measuring current of 1 mA is used, a signal of 100 mV (PT100 at 0°C) will be received. Since the change in resistance is very small at 1°C , a small error in voltage measurement (e.g. the use of two different materials can result in a thermo voltage) has a heavy impact on the temperature measurement, for example, an error of 100 µV in voltage measurement generates a deviation of 0.4°C in the temperature reading. Si milarly, a 1 µA error results in a deviation of 0.4°C in temperature reading. 2.6 Heat Dissipation Error A thermometer is rarely used in the ambient temperature range. If the measuring temperature is above or below the ambient temperature, a temperature gradient between the measuring point and the surroundings will occur at the thermometer. The result is a false temperature reading. The heat flows from the warmer to the cooler point across the protective tube or through the internal components of the thermometer. Furthermore, the sensor is connected to the cable, forming a direct metallic connection between the sensor and the surroundings. This connection is a thermal bridge which also causes an error. Good electrical conductors always have a low thermal resistance. Therefore, the requirement for a low resistance of the cables is counteracted by the fact that they cause a heavy heat dissipation error. Furthermore, the design of the thermometer influences the heat dissipation error. It is important that the sensor has a good thermal connection to the protective tube and is at the same time thermally decoupled from the connecting cables. Make sure that the total length of the thermometer is not too short since otherwise to much heat will be dissipated. The depth of immersion (length of the part of the thermometer that is exposed to the measured medium) depends on the type of medium and the rate at which transports heat. A fast flowing liquid, for example, will transfers more heat than still air, and therefore will be able to compensate the heat dissipation of the thermometer. Generally, measurements in liquids only require about 50 percent of the total length compared with that used with gases.


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2.7 Thermoelectric Voltages The effect of thermoelectric voltages can also be seen when the temperature is measured by resistance thermometers. In this case it is a rather undesirable effect. Thermoelectric voltages can develop at the joint between two different materials. Under normal conditions, it can be assumed that both joints are at the same temperature and therefore the resulting thermoelectric voltages neutralize each other. The magnitude of the resulting error highly depends on the characteristics of the evaluation electronics. A simple method for diagnosing such an error is to perform two measurements with inverted measuring current. If the larger difference between the two measurements, the higher generated thermoelectric voltage. 2.8 Insulation Resistance Because of the limited resistance between the cable and the insulation material in which the sensor is embedded, another measurement error due to poor insulation resistance can occur, which reduces the indicated temperature. This problem results from the fact that the insulation resistance is placed in parallel to the actual sensor. The result is a lower total resistance. Because the insulation resistance varies with temperature, the error can vary with the measuring conditions. Especially in ceramic insulation materials the resistance will decrease with increasing temperature. Due to the relatively low maximum temperature of approx. 600°C this effect is unlikely to be important for platinum temperature sensors. More important is any moisture penetration because of a poor isolation resistance, which may lead to substantial measurement errors. Therefore, the sensors are generally hermetically sealed. The measuring insert itself is also sealed in order to prevent moisture penetration into the probe tube. It is safe to replace measuring inserts since they are sealed units. If resistance thermometers without inserts have to be repaired, a reliable sealing is absolutely necessary. 2.9 Response Time Due to its internal thermal resistance, the probe will never respond instantaneously, but always with a delay. The result is a temporary measurement error caused by an erratic change in the temperature of the measured medium and the lagging of the measured quantity. How quickly the thermometer responds primarily depends on the ratio of the thermal resistance to the thermal storage capacity of the thermometer. This means, the larger the thermal resistance, the slower the temperature probe warms up. As a consequence, the measurement error exists for a sustained period of time. In order to achieve a fast response sensor should be kept as small as possible and thin heat conductive materials should be used, provided that the mechanical stress would allow this. Air gaps between the measuring insert and the heat protection tube form a very large thermal resistance. This can be solved by embedding the sensor in thermal compound or metal oxide (which heat transfer medium is used depends on the operating temperature). Because of their lower thermal mass, thermocouples principally have shorter response times than resistance thermometers. This mainly applies to mineral-insulated thermocouples. Since most thermocouples are also integrated in a protective tube, this effect will not be significant. Generally, the response time increases with increasing protective tube diameter. The 90%-time T09 indicates the period during which the measured value reaches 90% of its final value. This time can be used to compare the responses of different probes.


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2.10 Selecting a Suitable Device It is important to carefully choose sensor type and cable to ensure that they meet your requirements. Special attention should be focussed on the temperature range and the effects of the medium to be measured (corrosive or conductive). Ordinary solder joints between the connection cables and the measuring resistor cannot be applied used if the temperature exceeds 170°C. Therefore nearly every joint is form ed by means of a laser (except for special requirements). This method prevents thermoelectric voltages that may distort the measuring result. 2.11 Temperature Probes with Terminal Head These probes have a modular design consisting of measuring insert, protective tube, terminal head with the terminal base inside, and possibly flanges or screw fittings. Measuring inserts are ready-made units comprising temperature sensor and terminal base. They are inserted into the protective tube, which is often made of stainless steel. The base plate of the insert tube is flush with the base plate of the protective tube to ensure good heat transfer. The bolts retaining the measuring insert are positioned on springs so that even with a different expansion between insert tube length and protective tube length the measuring insert is flush to the base plate. As a consequence, the measuring insert can easily be replaced. It is also possible to produce measuring inserts with integrated two-wire measuring transducer. If no measuring insert is used, the temperature sensor will be located directly inside the protective tube, embedded in aluminium oxide or a thermal compound. The advantage of this design is that a better heat transfer to the sensor is ensured. A disadvantage is that the sensor cannot be replaced later. If an immersion sleeve is used, a thermometer can be removed without the need to depressurise or drain the system. The immersion sleeve is a type of protective tube firmly attached to the measuring point in which the thermometer is inserted and secured. Since the immersion sleeves are in direct contact with the measured medium, the requirements concerning the chemical and mechanical consistency are the same as the requirements for the protective tube. Terminal heads are made of cast iron, aluminium or synthetic material. 3 Handling 1. Unpack the thermometer and inspect it for damage from shipping.

2. Check the thermometer for proper operation by means of an ohm-meter at an ambient room temperature of approx. 20°C the resistance to be me asured by a resistance thermometer Pt100 must be approx. 109 ohms and it must increase when heated.

3. Check the dimensions: Diameter, measuring surface, cable length, screw thread and attaching parts must comply with your order.

4. Use a suitably sized wrench to install the temperature probe together with a suitable seal into the corresponding sensor hole so that the probe tip has heat-conducting contact with the measured medium.

5. Connect the measuring line according to the circuit diagram.

6. Make sure that the maximum operating temperature is not exceeded.

7. Special maintenance is not required.


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3.1 Please observe the following warnings!

• Torsion of the connection cables shall be avoided. • The cable must not be exposed to mechanical tensile loading. • The temperature range of the extension / probe tip / cable must be observed. • If the device is not moisture-proof, the line / measuring sleeve must not be exposed to

moisture. • For all standard types vibrations and oscillations shall be avoided. This does not apply

to products specifically intended for this purpose (see quality characteristics). • Use only suitable tools for installation. • Any modification of the sensor structure and the use for other purposes are prohibited. • Mechanical loading can destroy the temperature sensor. If a risk-free operation can no longer be assumed for men and machine, the sensor shall be put out of operation and assured against unintended operation. It can be assumed that a risk-free operation is no longer possible if:

- the sensor or the connection cable show visible damages, - the sensor does no longer operate in accordance with its specifications, - after the sensor has been exposed to severe stresses during transport.


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3.2 Connection Resistance Thermometers in Four-wire Circuits The four-wire technology provides ideal connection possibilities for resistance thermometers. The measurement result is not affected by impedance or temperature-related fluctuations. Cable compensation is not required. The sensor receives the measuring current through the cable. The voltage-drop is tapped off by the measuring lines. If the input resistance of the downstream electronics is much higher than the output impedance, it can be neglected. Thus, the determined voltage-drop is independent of the cable characteristics.

Resistance Thermometers in Three-wire Circuits The three-wire technology also works with two measuring circuits, one of which is used as a reference. Because of the three-wire circuit the amount of the impedance as well as its temperature dependence can be compensated. However, it is required that all three cores have identical properties and are subject to the same temperature conditions. Since in most cases this is true, the three-wire technology is the one most frequently used these days. Cable compensation is not required.

Resistance Thermometers in Two-wire Circuits Evaluation electronics and temperature probe are connected by a two-core cable. Like any other electrical conductor this cable has a resistance which is in series with the temperature sensor. These two resistances are added, and the result is a systematically higher temperature reading. To prevent this error the resistance of the cable (called offset ) must be subtracted from the measurement result. This offset value can be input in the most evaluation systems where the impedance is taken into account.

PT100

red (black)

red (black)

white (yellow)

white (yellow)

PT100

red white

PT100

red (black)

red (black)

white (yellow)


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Circuit Diagrams for Resistance Thermometers with T erminal Head:

Connector Assignment For the connector assignment please refer to our quality characteristics. Bibliography: Matthias Nau; Elektrische Temperaturmessung (Electrical Temperature

Measurement); Fulda 2003

rot/ red

rot/ red

weiß/ white

8

Pt100

rot/ red

3

8

weiß/ white

weiß/ white

Pt100

5

6

8

Pt100

rot/ red

35

rot/ red

6

Pt100

rot/ red

weiß/ white rot/ red

3 5

weiß/ white

Pt100

rot/ redrot/ red

34

5

rot/ red

weiß/ white

67

Pt100

Pt100

8

3 5

rot/ red

6

3

weiß/ white

rot/ red

weiß/ white

Pt100 Pt100

weiß/ white

rot/ red4 5

rot/ red

67

Pt100

weiß/ white

2-wire circuit

3-wire circuit

4-wire circuit

4 Maintenance The sensor is maintenance-free.

Documents

Platinum Resistance Thermometer2.3 Standardisation and Tolerances Our product range includes a series of platinum resistance thermometers - starting with tolerance class B (DIN EN