Selecting the right infrared temperature sensor

Transcription

1 48 June 1998 InTech Selecting the right infrared temperature sensor By Karen Ackland What is the temperature range of your process? What size is the target? How close to the target can the instrument be installed? Choosing an infrared temperature sensor can be a straightforward procedure.,,,, Does the target fill the field of view? What is the target material? How fast is the process moving? Will you measure discrete objects or a continuous process? What is the ambient temperature? Are the ambient conditions contaminated (dust, smoke, steam)? Do you want to connect to existing control equipment? Do you need to keep records for audits and/or quality programs? Infrared temperature sensors have been successfully used for years in process industries for ongoing temperature monitoring and control. Although the technology is proven, choosing among units with different specifications is sometimes confusing, leaving the process engineer to rely on more traditional temperature measurement methods (e.g., those involving contact) or on vendor recommendations. Recent innovations in infrared temperature sensor design have provided process engineers with enhanced functionality, and more questions about how to integrate and use infrared temperature sensors in their process. Infrared technology explained An infrared temperature sensor collects radiation from a target in the field of view defined by the instrument s optics and location. The infrared energy is isolated and measured using photosensitive detectors. The detectors convert

2 InTech June the infrared energy to an electrical signal, which is then converted into a temperature value based on the instrument s internal algorithms and the target s emissivity (a term referring to the emitting qualities of the target s surface). Infrared or noncontact temperature sensors are very successful in measuring hot, moving, or difficult-toreach objects, or where contact temperature sensors would damage the target. A block diagram of an infrared temperature sensor is shown in Figure 1. Understanding the process application helps determine which type of infrared temperature sensor to use. What is the temperature range of the target? How big is the measurement spot? How far away is that spot from the sensor? These are the first of several questions to ask to help find the right temperature sensor for your application. Environmental and operating conditions determine other sensor specifications (e.g., ambient temperature, display and output, and protective accessories). Finally, ease-of-use, maintenance, and calibration considerations may uncover hidden costs that will further influence the choice of an infrared temperature sensor. Determine temperature range Infrared instruments are available for lowtemperature applications (from below freezing) to high-temperature applications (over 5,000 F). In general, the narrower the temperature range, the better the resolution of the output signal for monitoring and controlling process temperatures. If monitoring start-up or cool-down temperatures is critical, it is necessary to choose a temperature sensor with a wider measurement range. Object Atmosphere This is critical in heat-treating applications, for example, where temperature must be held within a specific temperature range for a period of time to affect a material s metallurgical properties. Establish target size In infrared temperature measurement, the area to be measured (i.e., the target) should fill the instrument s field of view. Suppliers of infrared temperature sensors typically recommend that the measurement target exceed the field of view by 50%. If the target is smaller than the field of view, background objects (e.g., furnace wall) will influence the temperature reading. Conversely, if the target is larger than the instrument s field of view, the instrument will not capture a temperature variation outside the measurement area. An illustration of field of view is shown in Figure 2. To collect all the emitted radiation, single wavelength infrared temperature sensors (i.e., point sensors) need a clear line of sight between the instrument and the target. Sighting optics allow the user to visually sight through the instrument on the target. Some instruments have a built-in laser that pinpoints the target, which is especially helpful in dark areas. Two-color or ratio Infrared temperature sensor Optics Amplifier Electronics Detector Figure 1. The infrared temperature sensor collects energy emitted from the object based on its optics and location. Detectors measure the energy and convert it into an electrical signal. Figure 2. For accurate temperature measurement, the target should be larger than the instrument s field of view, or spot size. If the spot size is larger than the target, energy emitted from the background or surrounding objects will also be measured. Best Good Incorrect Sensor Target greater than spot size Target equal to spot size Target smaller than spot size Background

3 50 June 1998 InTech Figure 3. The smallest spot this instrument can measure is 0.25 inch at a distance of 8 inches. It would still be possible to accurately measure from a distance of 24 inches, but the minimum spot size would increase to 2.0 inches. Target spot size at focal point IR sensor Spot diameter (in) Spot diameter (mm) Diameter of target spot size 150 instruments, where temperature is determined from the ratio of the radiated energies in two separate wavelength bands, are a good choice when targets are very small or moving in and out of the field of view. Energy received from two-color instruments may be attenuated up to 95% and still provide accurate temperature measurement. Two-piece fiber-optic units, where the cable can snake around the obstructions, may be a good choice if a direct line of sight between the instrument and the target is otherwise impossible. Determine optical resolution Optical resolution is specified by the D:S ratio, which is determined by comparing the distance from the object to the sensor (D) with the size (i.e., diameter) of the spot being measured (S). For example, a 1-inch spot on a target being measured at a distance of 10 inches has a D:S ratio of 10:1. Infrared sensors on the market today have D:S ratios ranging from 2:1 (low optical resolution) to more than 300:1 (high optical resolution). The higher the optical resolution, the more expensive the instrument optics tend to be. The choice of D:S ratio really depends on the size of the object to be measured and the distance the sensor is from the target. For example, high resolution is needed for hightemperature applications (e.g., heat treating) where the sensor must be mounted far away from the target but must still measure a small spot. Optical charts help determine the target spot size at a specific distance for fixed-focus Distance: sensor to object (in) in mm Distance: sensor to object (mm) D:S = 32 Distance to spot Spot diameter Distance from sensor to object instruments. An optical chart for one sensor is shown in Figure 3. Infrared temperature sensors are available with both fixed- and variable-focus lenses. The instrument s focal point is the smallest spot it can measure. On a fixed-focus instrument, there is a single focal point at a set distance. While it is possible to accurately measure temperature at a distance closer to or farther from the focal point, the spot size will be larger than at the focal point. Variable-focus instruments have a minimum focal point that can be adjusted to correspond to the distance from the target. Target material impacts measurement The target material s emissivity and surface characteristics determine the spectral response or wavelength needed in a sensor. Highly reflective metals with different alloy compositions tend to have low or changing emissivities. Thus, the optimum wavelength for measuring hightemperature metal is the near infrared, around 0.8 to 1 micron. Because some materials are transparent at certain wavelengths, choose a wavelength at which the material is opaque. For example, 5 microns is a good choice for surface measurement of glass. Plastic films have transmission coefficients that vary according to the wavelength and thickness of the materials. Choosing 3.43 microns for polyethylene or polypropylene or 7.9 for polyester allows measurement of thin films (less than 10 mils). The typical spectral response for low-temperature applications is 8 to 14 microns. If there is any doubt, the manufacturer can test a sample of the material to determine the optimum spectral band to use. If processes are run with different target materials, select an instrument with adjustable emissivity. Fixed-emissivity instruments are sufficient for some materials, especially in low-temperature applications. Fast response time Infrared temperature sensors reach 95% of the final temperature reading a common definition of response time much faster than contact temperature sensors (e.g., thermocouples). This is particularly important when measuring moving or quickly heated objects. New infrared sensors on the market have response times selectable down to 1 millisecond. However, a fast response time is not desirable for all applications, especially in those where a fast sensor may exceed the capability of existing control instruments. In addition, when there is significant thermal lag in heating a process, speed in the instrument may be unimportant.

4 TEMPERATURE In Tech June 1998 Signal-processing needs vary Discrete processes (e.g., parts manufacturing), as opposed to continuous processing, require instruments with signal processing (e.g., peak or valley hold and averaging). Peak hold may be used, for example, to measure the temperature of glass bottles on a conveyor belt with temperature output fed into a controller. Without peak hold, the temperature sensor would read the lower temperature between the bottles and respond by increasing the process temperature. With peak hold, the instrument response time is set slightly longer than the time interval between bottles so there will always be at least one bottle represented in the temperature measurement. A sensitive control system can be fine-tuned by averaging the temperature output. Ease of use is important Infrared temperature systems should be easy and intuitive for plant operators to use. Today, user interfaces may be located directly on the sensor, on a remote monitor panel, or through a software program. Sensors with a built-in display and user interface are easy to install and set up. A separate, more accessible monitor is appropriate for ongoing temperature monitoring when sensors are installed in hard-to-reach locations. A typical instrument with display is shown in Figure 4. The simplest monitors provide a remote display of the current temperature. Additional features include adjustable set points that generate an alarm or process correction. Digital displays, which are replacing traditional analog displays, provide averaging and trend plotting and help minimize operator error. LED displays are easier to read in low light, but may be difficult to see in bright light. Graphical displays that plot temperature data over time are also available. Infrared smart sensors house microprocessors and support bidirectional, serial communications between a sensor on the plant floor and a PC. Software available with smart temperature sensors, often running on the familiar Windows platform, makes it easy to remotely monitor temperature data and modify sensor parameters from the safety of the control room, as shown in Figure 5. Environmental considerations Sensors are specified for performance within certain ambient temperature ranges. Dust, gases, or vapor can cause inaccuracies in measurement and/or damage sensor lenses. Noise, electromagnetic fields, or vibration are other conditions that should also be considered before installation begins. A protective housing, air purging, and/or water cooling can protect the sensor and ensure accurate measurements. These accessories are available from most manufacturers. In choosing accessories, consider the cost of bringing services (e.g., power, air, and water) to the unit. When possible, choose accessories that require standard services to minimize installation costs. The manufacturer will specify cable lengths, and all cables must be rated for the required ambient environment. Two-color instruments are a good choice when smoke, dust, or other particulates degrade the measurement signal. Fiber-optic sensors, where the optical head is separated from the sensor electronics with a fiber-optic cable, provide a solution around electromagnetic fields or other harsh environments. In applications involving hazardous materials (e.g., vacuum chambers), the sensor is mounted to look through a window into the enclosure. Window materials must be able to transmit the wavelengths used by the sensor. When specifying window materials, it is important to determine if the operator also needs to be able to see through the window. For example, in low-temperature applications, the window may make the target invisible to the eye, since it is often made of an opaque material such as germanium or amorphous material transmitting infrared radiation. If the operator needs to see through the window, zinc selenide or barium fluoride windows are recommended. Figure 4. Typical instrument with display. 51

5 TEMPERATURE 52 June 1998 In Tech sors are part of the control instrumentation. Most manufacturers offer calibration services for their customers. Smart temperature sensors can be calibrated on-site using calibration software and a blackbody calibration source. Figure 5. PC software provides remote temperature monitoring, sensor configuration, and data analysis for smart infrared sensors. Behind the byline Karen Ackland is marketing manager for Raytek Corporation in Santa Cruz, Calif. She has an M.B.A. from UCLA, and has more than 15 years of experience marketing technology products, including instrumentation, distributed systems, and robotics. Maintainability important The cost of an infrared sensor is usually minor compared to the risk of process downtime. A sensor is a long-term investment that, for the most part, can be expected to provide reliable use for 10 years or more. With this expectation, product reliability and vendor responsiveness become important evaluation criteria. If a unit needs repair, what kind of turnaround can you expect from the vendor? What is the average cost of repair compared to the cost of a new unit? Are spare or loaner units available? Does the vendor provide on-site operator training? While these issues are harder to quantify, they potentially represent expenses beyond the cost of the unit. The new smart sensors offer functionality that extends the life of the sensor. Because smart sensors contain processing capabilities at the sensing node, if something goes wrong (e.g., a high ambient condition or failed component in the sensor), fail-safe conditions are automatically used to protect the sensor. Updates to the sensor firmware can be downloaded from a PC without removing the sensor or returning it to the factory for an upgrade. Vendors recommend that infrared temperature sensors be recalibrated to a known temperature source at least once a year. This is required in ISO 9001 plants where infrared temperature sen- New uses for temperature data Infrared temperature sensors are available with voltage, current, and digital outputs. In many cases, the choice of output will depend on the existing control equipment. The most common output is the 4-20 ma current loop, which can easily be integrated into the control environment. When an infrared temperature sensor is replacing a thermocouple, choose an instrument with the appropriate thermocouple output. Many smart infrared systems support simultaneous analog and digital output using RS-232 or RS-485 serial communications. The analog output is usually integrated into an existing control environment, while the digital output is used for ongoing analysis and quality control. Digital output supports sophisticated analysis where temperature can be combined with other process variables. For example, on a paper line a process engineer may be concerned with the temperature, moisture, and weight of the product. In this case, the temperature data is used as a variable in a model to optimize process efficiency. Digital output capabilities of smart infrared units have recently caught the attention of quality managers who can now capture temperature data for each product or run. This data can be archived, graphed, or printed to document that the job was performed according to specifications. If documentation of process temperatures is required for IS or other quality programs, digital output should be considered. Choosing an instrument is straightforward Infrared temperature measurement is based on field-proven technology. With a basic understanding of infrared theory and the main selection criteria, choosing an infrared instrument is a straightforward procedure. In addition to the key specifications, ease-of-use, installation, and maintenance requirements help determine if the instrument is a good match for the process. In addition, new process requirements (e.g., ISO 9001 documentation or statistical process control) can benefit from the latest smart sensors. A wide variety of infrared temperature sensors on the market today provide ongoing, accurate temperature measurement, regardless of whether the instrument is selected to replace a thermocouple or to integrate into a multivariable process. IT