Platinum measuring resistor

Platinum measuring resistors are temperature sensors that use the dependence of the electrical resistance on the temperature of platinum as a measuring effect . They are designed for installation in industrial resistance thermometers or in an integrated circuit. They are widely used and are standardized in EN 60751 . Due to their low limit deviations , they are usually interchangeable without changing the parameters.

The standardization covers the range from −200 ° C to 850 ° C, the actual application range of a platinum measuring resistor is usually more limited and specified in the data sheet.

Characteristic curve of the Pt100

Face value

Platinum measuring resistors are named after their material and their nominal resistance at a temperature of 0 ° C. Is very common ${\displaystyle R_{0}}$

• Pt100 ( = 100 Ω ). ${\displaystyle R_{0}}$

In addition, higher nominal resistances have found importance

• Pt500 ( = 500 Ω) and ${\displaystyle R_{0}}$
• Pt1000 ( = 1 kΩ). ${\displaystyle R_{0}}$

The range of possible nominal values ​​extends up to 10000 Ω.

Standardized specifications

Resistance characteristic

	DIN EN 60751
Area	measuring technology
title	Industrial platinum resistance thermometers and platinum temperature sensors (IEC 60751: 2008)
Brief description:	Industrial temperature sensors
Latest edition	2009-05
ISO

The dependence of the resistance of a platinum temperature sensor with the nominal resistance on the Celsius temperature is specified in DIN EN 60751 as a formula , as specified under resistance thermometer. The standard also contains a tabular definition for the Pt100, called the basic value series (see also table ). ${\displaystyle R_{t}=R(t)}$ ${\displaystyle R_{0}}$ ${\displaystyle t}$

The realization of this definition is not exact, but possible within the limit deviations that are specified by accuracy classes.

The mean temperature coefficient over the range 0 ... 100 ° C is given as the characterizing value . He surrenders to

${\displaystyle \alpha _{0}={\frac {\Delta R/R_{0}}{\Delta t}}={\frac {R_{100}-R_{0}}{R_{0}\cdot 100^{\;\circ }\mathrm {C} }}=3{,}851\cdot 10^{-3\;\circ }\mathrm {C} ^{-1}.}$

Accuracy classes

For the manufacturing tolerances of platinum measuring resistors, four accuracy classes have been defined for wire-wound resistors and film resistors . The associated limit deviations largely agree with those of platinum resistance thermometers.

The limit deviations are smaller compared to those with standardized thermocouples, which is a significant advantage.

construction

Wire-wound and thin-film measuring resistor

The platinum

The platinum contains a targeted admixture of other materials, through which it changes its electrical values ​​at high temperatures far less than pure material and has a higher long-term stability.

The temperature-sensitive area

Platinum measuring resistors can be divided into two subgroups. In their metrological quality, e.g. B. in their limit deviations, both techniques are comparable. In the layering technique, smaller physical dimensions are possible, so that the temperature of the material to be measured is accepted more quickly and measured more punctually. This minimizes the disadvantages that they have when compared to thermocouples .

Wire measuring resistors

The temperature-sensitive element is formed by a platinum wire. The adjustment is carried out of the nominal resistance by shortening the platinum wire. The wire is melted in many turns in a glass rod or embedded in a ceramic compound and housed in a glass or ceramic tube as a housing to protect against corrosion. Coiled sensors are mainly manufactured as Pt100 and are preferably used for higher temperatures.

Film measuring resistors

The platinum is applied in a meandering shape to a ceramic substrate using thin-film technology . After the bonding of the connecting wires and the adjustment of the nominal resistance by laser trimming the platinum layer is provided with a coating to protect them from chemical attack. The thin-film sensor produced in this way can also be installed in a glass or ceramic tube and hermetically sealed in order to increase its mechanical and chemical resistance. The advantage of thin-film sensors, in addition to the variety of shapes, is their efficient production and calibration process, the use of smaller quantities of platinum and the feasibility of higher-resistance designs. They are preferred when the temperatures do not have to be high and long-term stability does not have to be particularly good.

Designs

Depending on the application, the platinum is in wire or layer form on a glass or ceramic carrier with a coating of the same type. For use with low chemical and mechanical stress, for example for temperature measurement within devices, no further protection against environmental influences is necessary. The electrical connection can be made, for example, by soldering into a circuit board or by surface mounting ( SMD ).

In industrial use, however, the protection of the measuring resistor and its ease of installation are crucial. Both are achieved by installing the sensor in corrosion- and heat-resistant material; there are also standardized housings as measuring inserts . This is often also separated from the medium to be measured by a protective tube; For pictures see under resistance thermometer .

By installing the measuring resistor in a flexible tube made of corrosion-resistant steel, a so-called jacket resistance thermometer is created. There are other versions for measuring gas temperatures, surface temperatures or for piercing the object to be measured. The electrical connection of these measuring resistors is established using permanently installed cables or plug connectors.

Films are also used as a carrier. The platinum is applied using wire, thin-film or thick-film technology.

Measuring circuit

The electrical connection from the resistance thermometer to the evaluating measuring device is available in a two-wire, three-wire or four-wire circuit . With three-wire and four-wire measurements, the measurement deviation is eliminated by the resistance of its connecting wires. These measuring circuits are described in the article Resistance thermometer .

For further processing of the electrical signal , in the simplest case the voltage drop across the measuring resistor through which a constant measuring current flows is measured. So the voltage is  proportional to the resistance  . When choosing the measurement current, a balance must be made between the useful signal that can be achieved, which increases with the measurement current, and the measurement deviation that also increases due to the sensor's self-heating. Difference- forming bridge methods allow the measurement of a voltage  proportional to the change in resistance compared to a fixed value. ${\displaystyle U}$ ${\displaystyle R_{t}}$ ${\displaystyle U}$  ${\displaystyle \Delta R_{t}}$

Measuring resistors with a large nominal value offer advantages in their measuring circuit compared to conventional Pt100. For example, a Pt1000 versus a Pt100 should be mentioned:

1. To achieve the same electrical voltage in a measuring circuit, the current intensity can be reduced 1 to 10, an important argument for battery-operated devices.
2. Furthermore (with 1 to 10 reduced current strength) the self-heating decreases 1 to 10, since the current goes into the self-heating as a square and the resistance linearly.
3. With the same absolute influence of the line resistances contained in the measuring circuit, their relative influence is 1 to 10 smaller, so that the more cost-effective two-wire circuit is more sufficient.

Pure platinum measuring resistors

In addition to the measuring resistors standardized worldwide according to IEC 60751, those made of pure platinum are used in North America and in the Far East (with a falling trend). There are corresponding standards for this, e.g. B. Japanese JIS C 1604. The mean temperature coefficient of such spectrally pure platinum is ${\displaystyle \alpha _{0}=3{,}92\cdot 10^{-3\ \circ }\mathrm {C} ^{-1}}$ 

Also normal resistance thermometer (Standard Resistance thermometer, engl. Standard Resistance Temperature Devices , such as the short SRTD) Physico-Technische Bundesanstalt , to avoid using spectrally pure platinum (99.999% Pt) in the form of freely suspended wire winding to thermal stresses. The temperature-dependent resistance curve of this pure platinum is used by the international temperature scale ITS-90 to interpolate the temperature scale between the fixed points.

Web links

Wikibooks: Linearization of Resistive Sensors  - Example of a linearization and formula inversion using a spreadsheet

• Measuring principle and circuits for Pt100
• Formulas and online calculator (by Andreas Hofmeier)

Individual evidence

1. ↑ ^{a b} roessel-messtechnik.de (PDF) p. 11
2. ↑ jumo.de
3. ↑ Resistance thermometer. temperature-messtechnik.de
4. ↑ heraeus-sensor-technology.de
5. ↑ Frank Bernhard (Ed.): Technische Temperaturmessage , Springer 2004, p. 603
6. ↑ Description. jumo.de
7. ^ Frank Bernhard: p. 635
8. ^ Frank Bernhard: p. 618 ff.
9. ^ Frank Bernhard: p. 609
10. ↑ Klaus Irrgang (Ed.): Temperaturmesspraxis with resistance thermometers and thermocouples. Vulkan, 2004, p. 100
11. ↑ Georg Bauer, Konrad Ruthardt: Handbuch der Analytischen Chemie , Volume Elements of the Eighth Subgroup II: Platinmetalle , Springer-Verlag 2013, 254 pages, page 244
12. ↑ Stefan Messlinger: For temperature measurement with platinum resistance thermometers and Prema 5017 DMM . epub.uni-bayreuth.de (PDF) accessed on May 12, 2019