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Resistance thermometer theory

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Platinum resistance thermometers (Pt100/Pt1000)

Building blocks of the Pt100 sensor

Properties and sources of error

Wiring of Pt100 sensors

 

Platinum resistance thermometers (Pt100/Pt1000)

Resistance thermometers work on the principle that the resistance of a metal varies with temperature. When accurate laboratory measurements are required, only standard platinum resistance thermometers (SPRTs) are used.

There are two reasons for this:

  1. Platinum is not only a noble metal but also the most electrically stable metal known to man.
  2. The platinum resistance thermometer, PRT, is the reference on which the international definition of temperature is based. So long as the international temperature scale, ITS-90, is valid, the PRT is the most accurate means of measuring temperature.

Industrial platinum resistance thermometers are known as IPRTs, the most widely used of which is the Pt100, which has a resistance of 100 ohms at 0 °C.

 

SPRTs—for calibration

Standard platinum resistance thermometers (SPRTs) are used for calibration under ITS-90. These are made of extremely pure platinum and therefore respond more faithfully to the laws of physics (thermodynamics) andare more predictable than any other type of temperature sensor.

The temperature scale is created using a number of fixed points - phase-transition points for different materials. The PRT serves as an inter-polation instrument for the intervals between the fixed points on a scale from approx. -259 °C (14 K) to 962 °C.

The SPRT has the following properties:

  • Extremely pure platinum wire (alfa-value at least 0.003926).
  • Freely suspended bifilar-wound wires (0.07-mm diameter for Pt25)
  • The wires are enclosed in ceramic in high-purity aluminium or quartz glass.
  • The wires are heat-treated to reduce strain and oxidation.
  • The leads are welded platinum to platinum
  • The probe is sealed in a controlled atmosphere.
  • The entire process from drawing of the wire through to final assembly should take place under the cleanest possible conditions.

This results in an extremely accurate detector, although it is unfortunately too delicate for use outside the calibration laboratory.

SPRT-200

The reference detector for calibration is the SPRT, which is a
highly accurate and delicate platinum resistance thermometer.
Hysteresis is minimized by free suspension of the wires. The detector can practically be constructed in different ways. Normally the length of the device is 50 - 70 mm.

 

IPRTs—for industrial use

The industrial platinum resistance thermometer (IPRT) is of more robust design than the SPRT and capable of withstanding greater physical stresses. As far as possible, it corresponds to the laboratory standard.

Nonetheless, industrial platinum probes always constitute a compromise in order to optimize certain required properties. Common requirements include:

  • Low hysteresis
  • High stability
  • High tolerance to vibration
  • Hermetically sealed enclosure
  • Wide temperature range
  • Automated production
  • Minimum production cost
  • Small size

When accuracy is crucial Pentronic usually uses detectors made by TDI-Isotech in Britain. The construction is as close to that of the SPRT as it is possible to get at present.

  • 80% of the length of the wire is freely suspended to minimize hysteresis
  • Bifilar-wound wires
  • Alfa-value = 0.3851 ohms/°C (Pt100) as per IEC 60751
  • Ceramic sheath of pure aluminium oxide
  • Clean environment for manufacture
  • All detectors assembled by hand
  • Pure-platinum leads
  • Heat treated

 

The mixing of different metals along the leads from the resistor to the sensor terminals can give rise to Seebeck voltages that cause reading errors.

IPRT-200

Pentronic mainly uses the Pt100 detector,
the design of which is very close to that of detectors used in the laboratory.
80% of the length of the bifilar-wound wire is freely suspended to make it suitable for industrial applications.
The body must not be less than diameter 1,5 x 15 mm to include the best properties
.

Småelement -w 200

Photo showing IPRTs. Above two thin film Pt100 detectors and below one wire wound device which measures diameter 2,8 x 25 mm.

Bobinlindad -Pt 100-w 200

Bobbin wound Pt100 detector in principle.
The platinum wire is fixed to the supporting materials by glass or aluminium oxide.
When the wire expands due to heat cycling hysteresis will be the result. See below
.

 

Film detectors (Pt100/Pt1000)

Film detectors first had their market in whiteware applications. Film detectors are made in automated processes and fit the measurement needs of household equipments very well.

Now these detectors are used in industrial applications where low cost takes priority over accuracy and high temperature measurements.

Be aware of the fact that film detectors have limited temperature ranges compared to the wire wound resistors. For example class A -30 to +300 °C for assembled sensors.

Tunnfilm

Film detectors have a platinum pattern fixed onto its surface that gives 100 or 1000 ohms resistance at 0 °C. They are known as Pt100 and Pt1000 respectively.

 

Building blocks of the Pt100 sensor

An industrial Pt100 sensor usually comprises three main components: a sensing element or detector, a protective tube and a connection. These can be combined in numerous ways to achieve the desired properties.

Detectors

Pentronic stocks a wide range of Pt100 wire wounded detectors, (also called resistors) the models varying in size and tolerance class. The diameter of the resistor element ranges between 0.9 and 2.8 mm, and the length between 6 and 50 mm.

There are two principal sizes of industrial sensor for applications in which priority is given to accuracy and stability: 2.8-mm dia. x 25 mm and 1.5-mm dia. x 15 mm. If you need to specify a different length, we will ensure that the diameter is the size best suited to the protective tube being used.

 

Tolerances as per IEC 60751:2008

Industrial Pt100 detectors are divided into four tolerance
classes under IEC 60751:2008: AA, A, B and C. (Earlier there were only two classes A and B).

The new standard makes a difference between wire wound resistors and film resistors. Experience shows that film resistors can't handle as wide temperature ranges as wire wound resistors under the different tolerance classes.

The sensitivity of Pt1000 is ten times greater than the one of Pt100 in ohms/°C. Tolerances are 10 times greater as well in ohms. In degrees C both sensor's tolerances are equal. 

 

Tol -Pt 100-A-B-X-w 200

Pt100 tolerances less than class AA
are only valid close to 0 °C where the selection of already manufactured
detectors takes place. Depending on which platinum wire quality (A or B) is used the slope of the selected detector tolerances outside the icepoint will not be better than the one of the material. See graphs A and B.
Thus the graph X ("1/10 DIN") does not exist.
Only relevant calibration can settle closer tolerances outside the zero point
.

 

Closer tolerances

Most manufacturers employ closer tolerances, but
these apply to just one temperature: 0 °C. This means,
for instance, that a tolerance of ± 0.03 °C applying to
“1/10 DIN” (Class B/10) is valid only at 0 °C. For other temperatures, the tolerances will follow the respective slope of curve for Class A or B, depending on which materials have been used. See diagramme to the right.

Thus, the Class-B value cannot be divided by ten: it is impossible to achieve this tolerance curve using the IEC platinum alloy.

Pentronic uses wire-wound Pt100 detectors normally made of Class-A material. Unless we have been given specific instructions to the contrary, during final inspection of the temperature sensor we check the tolerance against the Class-B specification, so that other types of error are accommodated as well. Details of the measured values are included with the delivery. Results from the final inspection testing are available under Test certificates in this web site.

Film resistors are gradually used more often in industrial applications. The test limits are suited to the customer's requirements.

Closer tolerances at other temperatures can only be achieved through calibration that determines the specific properties of the individual sensor.

This is because the quality of platinum specified in IEC 60751 is reduced by the material being an alloy with added palladium - done for compliance with the traditional DIN standard. The alloy gives rise to departures from the ideal curve, necessitating a safety margin corresponding to the slope of curve A or, at worst, of curve B.

Some Japanese and American standards specify a platinum alloy of higher purity, which gives closer tolerances over a wider temperature range. However, these detectors generate a different output signal, which is not compatible with European instruments. For competetive reasons American and Japanese instrument manufacturers usually produce their indicators including the IEC 60751 scale.

Temperature-resistance relationship as per IEC 60751:2008

IEC 60751:2008 describes resistance as a function of the temperature of a Pt100 which relationship was established in 1983. See Tables and polynomials.The amendments made in 1995 have little significance to measurements made in practice. Pentronic will be pleased to explain the effect of these on calibration.

IEC 60751:2008 is not at all changed compared to the 1955 version concerning resistance as a function of temperature.

Tolerances for assembled Pt100 sensors as per IEC 60751:2008

IEC60751-2008-w 590-en

Protective tubes

Detectors usually need to be enclosed in a protective tube before being used. Tubes are of two types, depending on the temperature range. Pentronic uses seamless tubes of stainless, acidproof steel unless otherwise stated. Up to 250–300°C. A steel tube with PTFE or polyimid-insulated leads is generally used. The temperature
limit is determined by the insulation.

Detectors and tube diameters are chosen to obviate the formation of problematic air gaps. As regards largebore tubes, we usually prevent the formation of air gaps by packing the tube with metallic material.

This results in the detector being centrally located in the tube, giving it greater protection against vibration. The heat-transfer properties are also greatly enhanced, which means shorter response times and smaller reading errors caused by heat losses through the protective tube.

Other manufacturers pack the Pt100 sensing element in powder, which has inferior heat-transfer properties and can be dispersed by vibration, allowing air gaps to form below.

Up to 600°C. Pentronic uses a sheathing material (mineral-insulated cable, MIC) in which the leads are protected by densely packed magnesium oxide into which the sensor element is inserted. Here, again, good heat-transfer properties and antivibration protection are assured. MIC has the following advantages:

  • High-temperature tolerance
  • Provides excellent moisture protection and can be immersed, eg, in water.
  • Mechanically strong and pliant.

Ans -frialedare -w 200

(1). Non-terminated wires

Ansl -skarvhylsa -w 200

(2). Extension fitting (sleeve connector) with lead and optional
armour

Ansl -kontakt -w 200

(3). Fitted connector.

Output signal connections

Sensors are equipped with one of the following types of connection:

  • Nonterminated wires (1)
  • Extension fitting (sleeve connector) with lead and optional armour (2)
  • Fitted connector (3)
  • Probe with provision for retrofitting of terminal block or transmitter (4)
  • Probe-mounted terminal block (5)
  • Probe-mounted transmitter (6)

 

Temperature ranges

Pt100 sensors can usually be used to measure temperatures up to approximately 250 °C. For higher temperatures, protective tubes, eg, with mineral-oxide insulation, are required.

There is little point in measuring temperatures in excess of about 600 C using a Pt100. This is because the resistance of the platinum wire is shunted unpredictably by the surrounding material. Special detectors are available for measuring temperatures up to approximately 700 °C. In this temperature range, type-N and type-K thermocouples constitute a good alternative.

Ansl -fria -ledare -w 200

(4). Terminal block.

Ansl -plint -w 200

(5). Mounted transmitter.

Ansl -transmimtter -w 200

(6). For retrofitting terminal block or transmitter.

 

Properties and sources of error

Hysteresis

All IPRTs exhibit hysteresis, ie, give different readings depending on whether the temperature is rising or falling. In 1982, D.J. Curtis at Rosemount investigated different designs.

He found that the best design was an expensive special sensor element, closely followed by the wire-wound Pt100 (the model used by Pentronic). Film-type thermometer detectors and bobbin-wound elements exhibited errors 5–10 times higher.

The following error values are percentages of the measuring range:

  • Wire-wound Pt100 0.008%
  • Bobbin-wound Pt100 0.08%
  • Film-type Pt100 0.04–0.08%

Pt 100-hysteres -w 200

Theoretical example of hysteresis in Pt100 detector elements on cycling between low and high temperatures. The deviation (dT) depends on different expansion coefficients between platinum and supporting material which are more or less locked to each other physically.

Film and bobbin wound detectors follow graph A while the 80% free wire detector will be affected by a factor down to 0,1 less (graph B). SPRTs with very free wires are subject to hysteresis in the order of < 1 mK.

 

Stability

Platinum detectors are highly stable over time but flaws introduced in the design and during manufacture can adversely affect the properties.

Detectors need to be heat treated to homogenize the crystal structure and remove any oxides that may have formed.
Wires secured to the base are stressed during heating: the freer the wires, the smaller the drift of the sensor on temperature changes.

The resistance value changes, eg, if a kink is introduced during manufacture, if the sensor is subjected to impact or vibration, or if it cycles between high and low temperatures.

A typical drift value for a Pt100 detector is 0.05°C per annum.
High-quality detectors exhibit maximum drift of 0.01°C. If the temperature range is confined to 25–150°C, drift is as low as 0.005°C a year.

 

Response time

Because it takes longer for the components of a Pt100 detector
to warm up, Pt100 detectors generally have a longer response time than thermocouples. All thermometers, of course, are only measuring their own temperature, which means that the response time is governed by the time it takes for the surrounding medium to heat up the probe and sensor.

For the response time to be short, the sensor must have good thermal-conductivity properties and a low mass. Pentronic’s design is unique in that there is continuous metallic contact between the surrounding medium and the sensor. This gives a faster response, a smaller measuring error due to heat dissipation through the protective tube, and greater tolerance to vibration.

Measuring-junction location

A Pt100 measures across the entire length of the wire. The temperature reading is therefore the average for the wire length.

It is important to remember this if you are measuring
surface temperatures and the like with a spring-loaded probe.

2_7_w 200_sv

A Pt100 measures across the entire length of the wire. However, some very small film detectors can make measuring locations almost as short as thermocouple ones possible.

Faults introduced during manufacture

Pt100 detectors are delicate instruments and need to be treated with the utmost care during manufacture. Impurities can create defects that cannot be detected during final inspection but manifest themselves only after some time in use.

If the tube to the sensor insert is moist or has traces of oil on it, it will adversely affect the insulation of the sensor. At room temperature, this will probably not matter but, at higher temperatures, the contaminants may be vaporized and able to penetrate to the platinum wires.

Shortcomings in the manufacture can also lead to the platinum wires being contaminated by other metals, eg, iron.

Detectors from the “1/10 DIN” range increase the influence of the lead connections. To make allowance for these and other variations, Pentronic usually permits a deviation of 0.05 °C during the final inspection of sensors at 0 °C, which corresponds to “1/6 DIN”.

Platinum oxides

The surface of platinum becomes oxidized. Since the sensor wires are thin (approximately 0.02 mm), the coating of oxide will have a measurable effect on the resistance.

In normal environments, the process is a slow one and the effects can usually be minimized through regular calibration.

 

Wiring of Pt100 sensors

Two-wire connection

Two-wire connections make life easy for the technician but at the expense of a much greater likelihood of measuring errors arising when long extension leads are used.

The problem results from the resistance of the lead being added straight into the measured value. At 20°C, the resistance of a 10-m, 2 x 0.25 mm2 copper lead is 1.4 ohms. In terms of temperature, this translates into a measuring error of 3.6 °C.

It is possible to eliminate the error through calibration, but every time the ambient temperature around the lead changes, the resistance will also change, producing a new error. A Pt1000 film detector would reduce the magnitude of the error to a tenth. If this is acceptable still there would then be a greater risk of self heating.

Three-wire connection

A three-wire connection will eliminate most of the effect of the lead resistance on the measured value, although this is conditional on all three wires having the same resistance. This is almost impossible to achieve in practice. In fact only two wires have to have equal resistances but as two of them are colour coded red you cannot easily know which is the critical one. This is the reason we say all three wires should hav equal resistances.

  • Wires often come from different drawing mills and can therefore have a resistance that differs by 5–10%.
  • One or more strands may be missing in one of the leads. Pentronic has encountered leads in which two of the seven strands were missing - a difference of 28% in the lead resistance.
  • Connections, switches and the like can introduce different resistances.

In extreme cases, there may be a combination of these factors at play. For example: a ten-metre length ofthree-wire lead that has a resistance difference across the wires of 10% will give a reading error of 0.18°C. However, a three-wire lead will solve the problem of the effect of the temperature range on the wires.

Four-wire connection

All instruments designed for optimum measuring accuracy have four-wire connections. The current and the signal are separated into two circuits, which renders the unbalance in the resistances of the wires insignificant.

This is true provided that the difference is not of an excessive magnitude - up to 100 ohms is often acceptable with modern instruments. The four-wire system was previously confined to
laboratory use but is now used in process equipment, eg, indicators and controllers.

 

NOTE!
The notations in the diagrammes DO NOT refer to Pentronic's markings on screw terminals and similar devices.

IEC60751-färgkod

Colour coding for extension lead to two-, three- and four-wire single Pt100 as per IEC 60751. One side of the Pt100 detector is connected to red wires while the other side is connected to white ones.

Färgkod -dubbel -Pt 100-w 200

Because the former IEC standard offered no recommendations of colour code for dual Pt100s, Pentronic employs its own standard (above) whenever possible. The extra circuits are connected to blue and yellow wires. The IEC:2008 recommends black or grey colour where we chose blue.

4-plint -w 200


Terminal-block markings for connection of Pt100 sensors. Deviating colors might occur.

Colour coding

The IEC recommends that different colour codes be used for Pt100 leads, in order to ease connection work. Pentronic uses the recommended codes whenever possible. See the picture and comments in the right-hand column.

Depending on market demand probably cable manufacturers will follow the IEC colour recommendations sooner or later.

We can also supply cable of the same colour code for additional runs. See illustrations in the righthand column.

 

What the Pt100 indicator really measures

Resistances included in the measured value for different
connections. Calibration of 2-wire and 3-wire connections
assumes that the lead resistances and the differences in the
resistances in the current winding are known.

Configuration Measuring parameter

Remarks

2-wire Sensor + (R1 + R2) All lead resistance included
3-wire Sensor + (R1 - R2) Difference in lead resistance included
4-wire Sensor Unaffected by lead resistance


Synopsis of error sources

Typical error sources and ranges for the Pt100 detector.
The table does not include any allowance for environmental
effects, such as contamination by metal migration, platinum
oxides and the like.

Source of error Error contribution interval °C
Sensor construction/installation 0,1 - 3
Pt100 sensor Tolerance 0,03 - 0,3 (at 0 °C)
Lead configuration 2 wire 0,1 - 5
  3 wire 0,01 - 0,5
  4 wire 0,00
Instrumentation 0,02 - 3

 

Comparison - thermocouple vs Pt100

The following table describes briefly the main differences between thermocouples and Pt100s in general terms:

Property Thermocouple (TC) Pt100
Measuring range Large: -200 < T < 1000 + °C Limited: -200 < T < 600 °C
Stability Not good, especially in high temperature Excellent. Annual drift < 0.01 °C
Measuring location From hot junction to reference location Across entire Pt-wire length
Ageing Significant in high temperature Insignificant, see Stability.
Response time Very short possible < 1 s Not as short as corresponding TC
Excitation power None Significant < 1 mW
Physical strength Very good Limited
Pricing Slightly lower than corresponding Pt100s Slightly higher than corresponding TC