The DX series is entirely different to conventional temperature measuring devices. Designed specifically for the highest possible accuracy, it is said to be the only infrared instrument which can be certified with NIST-traceable accuracy on real surfaces of unknown emissivity, while remaining completely free of the contact errors and heatsinking errors of contact devices.
The true emissivity of a surface can never be accurately determined by conventional infrared devices. IR devices with a pre-set emissivity setting can only display an approximate temperature over their entire temperature range. The sensing area of the DX scanner is equipped with a reflective surface to correct for emissivity variations.
The accuracy specifications given by most manufacturers are only for a 'blackbody' calibration and do not hold outside laboratory conditions. Blackbody calibrations totally ignore emissivity shifts, ambient change effects on the target, and other phenomenon. Exergen's DX series is unaffected by these distortions.
Even if an IR 'gun' is set to the correct emissivity to read a surface accurately at a particular temperature, it does not mean that the gun will read the same target correctly at other temperatures. Emissivity of virtually all surfaces changes with temperature and common assumption for conventional IR thermometry is that emissivity is constant with changes in target surface temperature. Real materials do not have this characteristic. The average value for nonmetals for which the change in emissivity with respect to surface temperature has been reported is approximately -2% per 100°F target temperature change (-3% per 100°C).
A setting of emissivity = 0,9 on an IR gun from one manufacturer will not necessarily match that of a different gun from another manufacturer. No industry-wide standards exist for the precise use of emissivity in measurement. Therefore, quality assurance programmes should not rely upon any instrument that allows users to alter the instrument settings and to let it display whatever the user wishes.
Even if emissivity were constant at all temperatures, there would still be errors induced by changing ambient temperatures. For example, with emissivity = 0,9, ambient reflections account for 10% of the signal that the IR gun will see. If the ambient temperature changes, the IR gun will display a different target temperature, even if the target remains at the same temperature.
Thermocouples, RTDs, thermistors and other contact devices only measure their own temperature. They do not measure surface temperature. Published 'accuracy' specifications are for the probes only, not the surfaces they must measure. Users must guarantee that the probes are brought to the same temperature as the surface. Can you guarantee that your probes are brought to the same temperature as the targets to be measured?
For moving surfaces, a contact probe is prone to frictional heating. The size of the error is dependent on the roughness of the surface, the speed, the coating on the probe and so on. It is impossible to control all the variables.
For most nonmetals, heatsinking errors can be quite large. The metal leads required on contact probes conduct heat faster than the target material can replace it, resulting in unknown and fairly sizeable errors. In general, the less thermally conductive the target material, the larger the heatsinking error with a contact probe.
Contact temperature probes are slow. The temperature of a target can change more quickly than most probes can measure, resulting in errors in realtime measurement.
Absolute Automation
(011) 453 6662
© Technews Publishing (Pty) Ltd | All Rights Reserved
printer friendly version