Temperature should be most accurately measured by contact methods. However, contact probes have many drawbacks: they need to be immersed into the temperature they should be measuring; they need good contact with a surface; they can be intrusive; they are not very good for touching moving, distant, high voltage or very hot items; they are bad at preventing cross-contamination; they only measure a single point; and they are slow, wear out, and suffer from electromagnetic interference.
All temperature sensors are improving with time, many are getting cheaper. Non-contact infra-red (IR) thermometry is getting much, much better and much cheaper - and thermal imaging automatically detects what matters.
Why use non-contact IR?
Non-contact infra-red (IR) thermometers are passive devices that measure naturally emitted radiation from a surface and tell the user the temperature. They provide fast temperature readings without physically touching the object. You simply aim, pull the trigger, and read the temperature on the LCD display with data transmission and logging.
Compact, and easy-to-use, IR thermometers can safely measure hot, hazardous, or hard-to-reach surfaces without contaminating or damaging the object. Also, IR thermometers can provide several readings per second, as compared to contact methods where each measurement can take several minutes.
Thermal imaging systems such as the fixed 2D Ircon Maxline imager (Fig.1) and linescan process imagers such as the Thermalert MP150 (Fig.2) for moving items or scanned areas give a complete visualisation of surface temperatures.
Increasingly refined software allows users to define 'areas of interest' such as potentially overheating machinery and items on conveyor belts. Predictions suggest that while more and more temperature measurement is moving towards non-contact IR, the percentage of esoteric applications of process imaging will grow faster still.
Handheld thermal cameras cost £20000-£35000 five years ago. The Raytek (now Fluke) Ti series was introduced in 2002 at £7000, and while the 160 by 120 pixel resolution Ti10/25 starts at under £3000 now, the end of 2009 will see a 240 by 320 pixel Ti32 at near £6000.
There are several factors that determine accurate measurement, the most important being: emissivity; field of view; distance to spot size; and location of a hot spot (Fig.3).
Emissivity. All objects both reflect and radiate energy. Only the emitted energy indicates the temperature of the object. To determine the surface temperature it is necessary to know how reflective - or more importantly how emissive (non reflective) - a surface is. Reflective materials can be measured, but only if their emissivity calibration factors are known.
Emissivity may be nearly 100 per cent (for example E=0.97) on what looks to be a dark/matt finish, and as low as E= 0.1 (90 per cent reflective) on some shiny metals. However, the human eye may not suggest same emissivity factor as IR sensors at different wavelengths. If in doubt use look-up tables as a guide or better still ask an IR sensor manufacturer.
Some IR thermometers allow you to change the emissivity in the unit while other units have a fixed, pre-set emissivity of 0.95 - the emissivity value for many organic materials and painted (any colour but aluminium paint), or heavily oxidised surfaces.
To determine the surface temperature of a shiny object, you can find the emissivity by pointing the sensor at a temporarily covered part of the surface to be measured with masking tape or matt black paint (assuming it does not insulate the surface by being too thick). Then view the shiny surface and adjust the emissivity to give the same value. A trend is the use of IR sensors connected by software - free from some manufacturers - where the true temperature value can be typed in and the emissivity calculated automatically.
Distance to spot ratio. The optical system of an IR thermometer collects the IR energy from a circular measurement spot and focuses it on the detector. Optical resolution is defined by the ratio of the distance from instrument to the object compared to the diameter of the spot being measured (D:S ratio). The larger the ratio number the better the instrument's resolution, and the smaller the spot size that can be measured.
Built-in laser sighting included in some instruments helps to aim at the measured spot. More recently inbuilt video cameras provide a remote visual display of the target area being measured. Raytek multidrop software allows the image to be automatically captured and stored as data embedded with the measurements - optionally this can be done if an alarm threshold is exceeded.
Another recent innovation in IR optics is the addition of a Close Focus feature, which provides accurate measurement of very small target areas without including unwanted background temperatures. The new Ircon Modline 5 has excellent close focus abilities - with another new feature of a dirty lens alarm to trigger should the IR sensor lens get dust or similar on it to the extent that reading may become inaccurate.
Protocols and wireless
Sensors are getting cheaper, yet technical staff costs are increasing. A trend in all industrial instrumentation (not just IR) is toward reducing cable connection and wiring complexity.
Existing customer plant systems may use established and rightly popular 4 to 20mA analogue signals but, along with mV and thermocouple simulation outputs, IR manufacturers can supply Modbus, Hart, Profibus, multi drop RS232/485, Ethernet, and simple USB connections for set-up.
Companies such as Radir do not stop at the measurement of temperature (and inferred parameter such as moisture and thickness). There are also special systems with examples being cement kiln scanning, food conveyor control, glass toughening, and more where the data is presented to operators, engineers and financial departments.
PC costs have dropped so much that what would have been a complex assembly of hardware with digital displays can now be networked data presented to match precisely the information wanted. With thermal imaging, the parameter being measured is multiplied by several thousand but reliable software programming means that the end measurement is simple and effective. Why use 10 sensors to locate a temperature hotspot when a thermal process imager can find the hot spot and notify staff of its location with more precision in just one instrument - and less overall cost?
Yet behind these simple tasks, a hidden software system is measuring 80000 pixels of data 100 times per second, 24 hours a day, and it can record conveyor speed, power, humidity, pressures and much more at the same time.
Pete Clarke is Radir's UK/Ireland technical support specialist with for Raytek, including Ircon, Milton Keynes, UK. www.radir.com