Used to monitor untouchable or fast-moving processes, infrared thermometers measure the infrared radiation from your process. In this second column of a two-part series, I'll explain how to convert detector voltage to a temperature display and take a look at emissivity compensation and other performance variables.
Converting Detector Voltage To a Temperature DisplayThis relationship depends on the radiation detector material, optics used and selected part of the infrared spectrum. A typical curve shows that the raw output voltage vs. temperature is severely nonlinear. Usually, a signal conversion circuit is incorporated in the infrared thermometer, and the output is presented as a linear 4 to 20 mA, 0 to 10 V signal, or scaled for an output of 1 mV per degree. Alternatively, indicators and controllers are available that compensate the raw signal and display the temperature in °F/°C.
Some infrared thermometers - often called infrared thermocouples, which they are not - have conversion circuits that put out a millivolt signal imitating a Type J or K thermocouple over a limited range. The upside of this approach is that many users already have thermocouple extension cable in the plant wiring and have thermocouple-input controllers that will work from these signals. The downside is that you have to accept some emissivity errors and operate in a small temperature band to achieve acceptable accuracy. Also, you have to live with the less robust (10 to 50 mV) signals from a source of several thousand ohms, which some controllers cannot tolerate.
Emissivity Compensation. Usually, you will sense less than the theoretical radiant energy from your target - i.e., emissivity is less than one. Many infrared thermometers and controllers have an emissivity adjustment. If you know the target's emissivity, turn the setting to that value. This boosts the signal just enough to bring it up to a value corresponding to emissivity equals one. In emissivities tables, for a particular material, usually you will find approximate values or a range of values. Rather than depend on such figures for your material, it is better to measure the target temperature when it is stationary (using, say, a thermocouple and indicator), then adjust the emissivity setting to bring the infrared thermometer reading into agreement.
Gold Cup Emissivity Enhancer. This is a metal block with a 3" hemispherical cavity machined into one face and gold plated. In the center is a small hole for a fiber optic cable. You place the cavity facing the target, up to 0.8" away. Multiple internal reflections convert the target to a blackbody as seen by the fiber optic, which now transmits the radiation to an infrared thermometer.
This is an ideal solution for targets with low and temperature-dependent emissivities such as hot aluminum billets. Shiny metal targets suffer not only emissivity but also reflection errors. For example, a boiling chrome-plated tea kettle at 212°F (100°C) can reflect the room wall's temperature (70°F) or some neighboring hot body into the infrared thermometer lens, giving gross errors.
For transparent materials, a wide infrared band detector could sense the temperatures of objects behind the target or reflected from it and give deceptive readings. In such cases, a filter is applied that selects a narrow band in the infrared spectrum where the material is opaque.
Dual Wavelength Thermometers. This design put two sensors in one enclosure. Each sensor has its own filter selecting a different part of the infrared spectrum radiating from a single target. The circuit derives a signal that is the ratio of the two outputs and which represents the target temperature. This design is used when the target does not fill the field of view or is partially and intermittently obscured by smoke, dust or vapor.
Response Time of Infrared Thermometers. Response times range from 5 to 500 ms after signal conversion. Compared to a thermocouple or RTD in a control system, it can remove a sizable time lag from the loop, giving tighter control and stability.
Peak Picker. When you are monitoring intermittent targets such as parts on a conveyor, the speed of the infrared thermometer allows fast capture and hold of the temperature until the next sighting. Decay rates can be set up to match the process speed.
Averaging. A smoothing circuit (averager) can calm dancing digits on the display and present less confusion to a controller. Time constant adjustments typically range from 5 ms to 5 sec.
These columns barely scratch the surface. Always talk to your supplier - you won't believe the troubles he's seen.