While traditional contact temperature sensors like thermocouples and RTDs will always have a role to play in industrial heating and drying processes, increasingly firms are recognising the advantages of measuring in-process materials directly.
A good example is a drying application for food products, where thermocouples may be used to measure air temperature in a conveying dryer, but infrared thermometers are used to measure actual product temperature as it moves through the drying process.
The above example illustrates what is probably the key application advantage of infrared thermometers: they are non-contact and, therefore, they can measure moving materials. Other key applications for infrared thermometers include:
- Processes that require prompt or frequent temperature measurement.
- Processes that undergo rapid thermal changes.
- Processes that are physically inaccessible to contact thermometers.
- Products that will be damaged or contaminated, if contacted.
- Products that have varying surface temperature distribution.
Another key advantage of IR thermometers compared with contact thermometers is response time. The thermal mass of a contact thermometer, the process of conducting heat from the object into the sensor, and the associated thermal resistance at the point of contact dramatically limit response time. By contrast, IR thermometers have no such limitations. By their very nature, these thermometers respond almost instantaneously to temperature changes, permitting measurement of fast-moving objects or objects that change temperature rapidly.
IR thermometers focus and record thermal radiation, the energy any object emits due to its temperature. For most materials, the emission of thermal radiation is a surface phenomenon. An object’s surface characteristics (resulting from its physical and chemical properties) strongly influence its ability to radiate thermal energy. This ability is referred to as emissivity. Emissivity values range between 0 and 1. A perfect radiator, or blackbody, has an emissivity equal to 1
(by contrast, a mirror, or perfect reflector, has a value of zero).
The primary issue to understand in applying IR temperature sensors is that real objects do not behave like a perfect radiator (tables are available to help determine an object’s emissivity).
For example, an object with an emissivity of 0.7 emits only 70 per cent of the energy of a blackbody, so unless emissivity is accounted for, the indicated infrared reading will be lower than the object's actual temperature.
Besides emissivity, there are other important factors influencing the choice of IR sensors:
- The object or process temperature.
- The size of the object – will it fill the field-of-view of the instrument?
- Are there physical obstructions, or can the object be viewed via direct line-of-sight?
- Is there likely to be smoke, dust, or other particulates in the measurement area?
- What are the control output or PLC interface requirements?
The above considerations normally apply to single point thermometers, but for many applications, single point measurement isn’t sufficient. Many web-based processes require a complete temperature profile of the material as it moves through the line. By providing more complete temperature information compared to that of a point instrument, linescanners can significantly improve product consistency and quality.
A linescanner works by collecting IR radiation at several hundred points (typically over a 90º field-of-view) along a line.
As the process moves forward, the instrument optically scans at a rate of 48 lines-per-second and calculates the temperature of each measured point. The software assembles the various temperature points into a 2-dimensional thermal image or picture.
The growing acceptance of this technology is no doubt attributable to the wide range of models and potential users. Today, Raytek makes scanners for low (<300°), medium (<800°) and high temperature (<1200°), as well as special models for both plastic and glass applications. End-users can be found in textile, food, gypsum, paper, cement, steel to plastic and glass firms.
Raytek has pioneered a number of application specific programs. Starting with the extrusion process, Raytek offers the ES100, an automated inspection system for detecting and measuring defects in sheet extrusion or cast film processes. This system allows operators to monitor die bolt heaters via the melt temperature profile for early detection of problems or plugged dies.
A related system for extrusion coating and laminating, the EC100 system provides automatic inspection for detecting and classifying defects. Wavy or torn edges can be quickly detected. Complete thermal profiles, coupled with high and low alarm capability, ensure proper bonding between the plastic and the substrate.
Finally, the TF100 allows thermoformers to visualise the temperature distribution of the plastic sheet prior to forming. The system can be used either with inline or rotary thermoformers. TF100 users report significant reductions in scrap rates, reduced change-over times and improved defect detection.
For tempered or safety glass manufacturers, the GS100 system allows manufacturers to measure the temperature distribution of the heated glass in annealing, tempering, and bending operations.
Users include automotive safety glass manufacturers and tempering operations for architectural glass.
A key feature of the GS100 is the ability to create and display software zones overlaid on the glass thermal image. These zones typically correspond to the heating element configuration within a reheat oven. In addition, closed loop control is possible to each zone via analog output modules.
The building products industry is another major user of Raytek systems. The TIP450 wallboard system provides detailed dryer balance analysis and board thermal mapping. In addition to the MP50 scanner, the TIP450 consists of a console cabinet with touch screen display, remote I/O assemblies and a fixed IR point sensor.
Already in use at over 85 gypsum board plants worldwide, users are reporting significant quality improvements, increased throughput, energy savings and reduced scrap rates. In many cases, they are reporting paybacks in as little as six months.
The advantage of direct material measurement using infrared scanners in web-based applications has always been compelling. However, as new application-specific software is introduced, the advantages become all the more compelling.
Marv Maddox is with Raytek Corporation, a subsidiary of Fluke Corporation, Santa Cruz,