Temperature measurement can be contact and remote. The most common are thermocouples, resistor sensors, and thermometers that need to be in contact with an object, because they measure their own temperature. They do it slowly, but are inexpensive.
Non-contact sensors measure the infrared radiation of an object, give a quick result, and are usually used to determine the temperature of moving and non-stationary bodies that are in a vacuum and inaccessible due to the aggressiveness of the environment, shape peculiarities or safety hazards. The price of such devices is relatively high, although in some cases it is comparable to contact devices.
Monochrome Thermometry
The monochrome method for determining the total energy brightness uses a given wavelength. Implementations range from hand-held probes with simple remote sensing to sophisticated portable devices that allow you to simultaneously observe an object and its temperature with recording readings or printing them out. Stationary sensors are presented as simple small detectors with a remote location of electronics, as well as high-strength devices with remote PID control. Fiber optics, laser aiming, water cooling, a display and scanner are optional options for monitoring technological processes and control systems.
The configuration, spectral filtering, operating temperature range, optics, response time and brightness of the object are important elements that affect performance and should be carefully considered during the selection process.
The sensor can be either a simple two-wire or complex wear-resistant, highly sensitive device.
The choice of the spectral response and the range of operating temperatures is associated with specific measurement tasks. Short wavelengths are for high temperatures and long ones are for low. If objects are transparent, for example, plastic and glass, then narrow-wave filtration is necessary. The CH absorption band of the polyethylene film is 3.43 μm. The selection of the spectrum in this range simplifies the calculation of the emissivity. In the same way, glass-like materials become opaque at a wavelength of 4.6 microns, which allows you to accurately determine the surface temperature of the glass. The radiation range of 1-4 microns makes it possible to measure through the inspection openings of the vacuum and pressure chambers. An alternative is the use of fiber optic cable.
The optics and response time in most cases are insignificant, since a 3 cm field of view at a distance of 50 cm and a response time of less than 1 s is sufficient. For a small or rapidly moving discontinuous object, the need arises for a small (3 mm in diameter) or even smaller (0.75 mm) measurement spot. Long-range aiming (3-300 m) requires optical adjustment, since the standard field of view of the device becomes too large. In some cases, the method of two-wave radiometry is used for this. Optical fiber allows you to distance the electronics from aggressive environments, eliminate the influence of interference and solve the problem of access.
Laser thermometer basically has an adjustable response time in the range of 0.2-5.0 s. A fast response can increase the noise level of a signal, while a slow response affects sensitivity. Induction heating requires an instant reaction, and a slower response for a conveyor.
Monochrome IR thermometry is simple and is used in cases where temperature control is extremely important to create high-quality products.
Two-wave thermometry
For more complex tasks where absolute measurement accuracy is critical and where the product is physically or chemically exposed, two- and multi-wave radiometry are used. The concept appeared in the early 1950s, and recent changes in the design and hardware have increased its productivity and lowered cost.
The method consists in measuring the spectral energy density at two different wavelengths. The temperature of an object can be read directly from the device if the emissivity is the same for each wavelength. The readings will be true even if the field of view is partially obscured by relatively cold materials such as dust, wire screens, and gray translucent windows. The theory of the method is simple. If the energy brightness of both wavelengths is the same (for a gray body), then the emissivity is reduced and the ratio becomes proportional to temperature.
A two-wave laser thermometer is used in industry and research as a simple, unique sensor that can reduce measurement error.
In addition, multiwave thermometers have been created for materials that are not gray bodies, the absorption coefficient of which varies with the wavelength. In these cases, a detailed analysis of the surface characteristics of the material with respect to the relationship of this coefficient, wavelength, temperature and chemical composition of the surface is necessary. Given these data, it is possible to create algorithms for calculating the dependence of spectral radiation at different wavelengths on temperature.
Evaluation Rules
To evaluate the accuracy of measurements, the user must know the following:
- IR sensors do not distinguish colors by nature.
- If the surface is shiny, then the device will establish not only the emitted, but also the reflected energy.
- If the object is transparent, infrared filtration is required (for example, glass is opaque at 5 microns).
- In nine out of ten cases, an absolutely accurate measurement is not required. Repeated readings and the absence of bias will provide the necessary accuracy. When the energy brightness changes and data processing is difficult, you should focus on two- and multi-wave radiometry.
Structural elements
A non-contact laser thermometer works according to the principle: IR energy at the input to and a signal at the output. The basic circuit of the device consists of collecting optics, lenses, spectral filters, and a detector as an external interface. Dynamic processing is carried out in different ways, but it can be reduced to amplification, thermal stabilization, linearization, and signal transformation. Ordinary window glass is used for short-wavelength radiation, quartz for medium frequencies, and germanium or zinc sulfide for the range of 8-14 microns, optical fiber - at wavelengths of 0.5-5.0 microns.
line of sight
Laser remote thermometer is characterized by a field of view (PZ) - the size of the spot temperature control at a given distance. The change in the diameter of the field of view is directly proportional to the change in the distance between the thermometer and the measurement object. Its value depends on the manufacturer and affects the price of the device. There are models with a PZ less than 1 mm for point measurements and with long-range optics (7 cm at a distance of 9 m). The working distance does not affect the accuracy of the readings if the object fills the entire measurement spot. In this case, the maximum signal loss should not exceed 1%.
Aiming
Conventional IR thermometers measure without additional devices. This is acceptable for working with large objects, for example, paper web, where spot accuracy is not required. For small or distant objects, a laser beam is used. Several laser aiming options have been created.
- Beam offset from the optical axis. The simplest model is used in low-resolution devices for large objects, since the deviation is too large near.
- Coaxial beam. It does not deviate from the optical axis. The center of the measuring spot is accurately indicated at any distance.
- Double laser. The diameter of the spot is marked with two points, which eliminates the need to guess or calculate the diameter and does not lead to errors.
- Circular pointer with offset. Shows the field of view, its size and external border.
- 3-point coaxial pointer. The beam is divided into three bright points located on the same line. The midpoint indicates the center of the spot, and the outer points indicate its diameter.
Aiming provides effective assistance in directing the thermometer exactly at the measurement object.
Filters
Thermometers use short-wave filters for high-temperature measurements (> 500 ° C) and long-wave filters for low temperatures (-40 ° C). Silicon detectors, for example, are resistant to heat, and a short wavelength reduces the measurement error. Other selective filters are used for plastic film (3.43 microns and 7.9 microns), glass (5.1 microns) and flame (3.8 microns).
Sensors
Most sensors are either photoelectric, generating voltage when exposed to infrared radiation, or photoconductive, that is, changing their resistance under the action of the source energy. They are fast, highly sensitive, have acceptable temperature drift, which can be overcome, for example, by a thermistor temperature compensation circuit, an automatic zero-circuit, amplitude limitation and isothermal protection.
In the IR thermometer circuit, the detector output signal of the order of 100-1000 μV is subjected to a thousand-fold amplification, is regulated, linearized, and, as a result, it is a linear current or voltage signal. Its optimal value is 4-20 mA, which minimizes external noise. This signal can be sent to the RS-232 port or to the PID controller, remote display or recording device. Other uses for the signal:
- enable / disable the alarm;
- peak value retention;
- adjustable response time;
- in the sampling and storage scheme.
Performance
An infrared laser thermometer has an average response time of about 300 ms, although using silicon detectors can achieve a value of 10 ms. In many instruments, the response time is varied in order to dampen the incoming noise and adjust their sensitivity. Minimum response time is not always necessary. For example, with induction heating, the time should be in the range of 10-50 ms.
Characteristics of Laser Thermometers
Etekcity Lasergrip 630 - infrared 2-laser thermometer, price $ 35.99. Specifications:
- temperature range -50 ... +580 ° C;
- accuracy +/- 2%;
- the ratio of distance to spot size 16: 1;
- emissivity 0.1 - 1.0;
- response time <500 ms;
- resolution 1 ° C.
Laser thermometer (photo) also informs about the highest, lowest and average temperature. The measuring spot is offset 2 cm below the aiming point. Laser guidance is most accurate at the intersection of the rays (36 cm).
Amprobe IR-710 - infrared laser thermometer, price $ 49.95. Specifications:
- temperature range -50 ... +538 ° C;
- minimum spot size 20 mm;
- accuracy +/- 2%;
- the ratio of distance to spot size 12: 1;
- emissivity 0.95;
- 500 ms response time
- resolution 1 ° C.
This laser thermometer (photo), in addition to the current temperature, also indicates its minimum and maximum values.