Within the process industry, two basic types of measuring resistors are used for electrical temperature measurement: wire-wound and film measuring resistors. Their different behaviour is reflected in the provisions of the European IEC 60751 (DIN EN 60751) standard, while the American ASTM E1137/E1137M treats the two types of resistors equally. This leads to confusion among users.
IEC 60751 leaves no room for interpretation. It lists the tolerance classes with their temperature ranges separately according to the nature of the measuring resistor. The basic values listed in the standard, the formulae for the permissible tolerance values and the test requirements apply to both types.
The detailed information has been present in the standard since 2008. The original EN 43760 standard from 1954 and its successors, from IEC 751 up to DIN EN 60751 from 1996, were developed on the basis of experience with wire-wound measuring resistors. Up to the revision of the standard, temperature ranges and class accuracies depended exclusively on the behaviour of wound platinum wires, which – embedded in ceramic or glass – change their resistance value as a function of the temperature.
The most important information, laid down as formulae and tables, was very clear. This included the basic values of -200 to 850°C, achieved using polynomials, and also two tolerance classes (A and B) and the associated temperature ranges -200 to 650°C (class A) and -200 to 850°C (class B).
Over the course of time, knowledge about the physical effects that occur in the platinum of the sensor at higher temperatures over longer periods of time has expanded. This has led to the necessity to adapt the standard to account for the drift behaviour of the measuring resistors so that it corresponds to the reality. A further reason for the revision was the development of film measuring resistors (also known as thin-film and flat measuring resistors or even chips for short) and their rapid market penetration.
This sensor type was not explicitly mentioned in the previous editions of IEC 60751: 2008 (DIN EN 60751: 2009). Users were therefore unsure whether the standard also applied to these new, cost-effective and exceptionally vibration-resistant measuring resistors. In particular, the specification of the temperature ranges suggested that film measuring resistors could be used at the same temperature limits and with the same accuracies as wire-wound measuring resistors. In reality, however, film measuring resistors behave differently to wire-wound measuring resistors, due to stretching and shrinkage effects as well as due to the diffusion of foreign atoms into the thin platinum layer. The result is a characteristic curve that leaves the standard range of permissible measuring deviations at low and high temperatures earlier than is the case with wire-wound measuring resistors. This behaviour physically arises as a result of the design and the materials used and must not be understood as a quality deficiency.
Readers wanting more information about updates to IEC 60751 and how the effects translate to the corresponding ASTM guidelines can download the full paper at http://instrumentation.co.za/+J1889
Telco sensors in conveyor belt monitoring
Sensors & Transducers
Gail Norton Instrumentation Agencies has developed a low-maintenance conveyor belt monitoring unit built on robust Telco infrared sensors, detecting belt tear and misalignment reliably in harsh coal mining environments.
Read more...Inductive ring sensors monitor material flow in feeding technology ifm - South Africa
Sensors & Transducers
ifm’s inductive ring sensors detect metallic parts such as screws and nuts as they pass through tubes at speeds up to 35 m/s, enabling precise counting, flow control and blockage detection, with sensitivity and output functions configurable via IO-Link.
Read more...Reliable 1D and 2D code reading made simple SICK Automation Southern Africa
Sensors & Transducers
SICK’s new sensingCam SEC200 image-based code reader brings supermarket-checkout simplicity to logistics and industrial identification, reading 1D and 2D codes reliably with quick browser-based setup, rugged IP65 construction and CRA-compliant cybersecurity.
Read more...Market opportunity for sensors in humanoid robots
Sensors & Transducers
A new IDTechEx report forecasts the market for sensors in humanoid robots will exceed $6,59 billion by 2037, driven by perception sensors, force/torque sensors, encoders and inertial measurement units across a growing range of commercial robot platforms.
Read more...The wireless route to safer, more efficient mining operations Emerson Automation Solutions
Editor's Choice Sensors & Transducers
Modern wireless level measurement technologies are helping mining operations improve process control, increase safety and reduce installation costs. Emerson’s Rosemount 3408 wireless non-contacting radar level transmitter uses 80 GHz frequency-modulated continuous wave radar with HART communication to reach remote assets where cabling is impractical.
Read more...Telco sensors for harsh environments
Sensors & Transducers
Telco sensors are designed to operate where moisture, particulate contamination and surface fouling would disable conventional sensors, with a range including an adjustable five-metre infrared proximity sensor for harsh industrial environments.
Read more...The data-driven path to sustainable manufacturing ifm - South Africa
Editor's Choice Sensors & Transducers
Smart automation is closing the gap between operational efficiency and environmental responsibility, and the data to do it is already inside the plant. With ifm’s sensing and digitalisation technologies, manufacturers can turn machine intelligence into measurable sustainability gains.
Read more...Expanding visibility in mining operations Emerson Automation Solutions
Editor's Choice Sensors & Transducers
Advances in wireless communications, condition monitoring and sensing technologies are helping mining operators extend measurement into previously under-monitored areas, improving equipment reliability, process performance and water stewardship.
Read more...Fiber-optic amplifiers for precise object detection ifm - South Africa
Sensors & Transducers
The OCF fibre-optic amplifier detects even the smallest or most transparent objects in confined spaces, making proven fibre-optic technology easier and more user-friendly than before.
While every effort has been made to ensure the accuracy of the information contained herein, the publisher and its agents cannot be held responsible for any errors contained, or any loss incurred as a result. Articles published do not necessarily reflect the views of the publishers. The editor reserves the right to alter or cut copy. Articles submitted are deemed to have been cleared for publication. Advertisements and company contact details are published as provided by the advertiser. Technews Publishing (Pty) Ltd cannot be held responsible for the accuracy or veracity of supplied material.
printer friendly version