TechnologiesElectronics

Laser thermometer: the principle of action. Laser Remote Thermometer (photo)

Temperature measurement can be contact and remote. The most common thermocouples, resistor sensors and thermometers that need to contact the object, because they measure their own temperature. They do it slowly, but they are inexpensive.

Non-contact sensors measure the IR radiation of the object, give a quick result, and are usually used to determine the temperature of moving and unsteady bodies in a vacuum and inaccessible due to the aggressiveness of the environment, the features of the form or security threat. The price of such devices is relatively high, although in some cases it is comparable with contact devices.

Monochrome Thermometry

The monochrome method for determining the total energy brightness uses a given wavelength. The implementations range from hand-held probes with simple remote measurement to complex portable devices that allow you to simultaneously observe the object and its temperature with the recording of the readings in the memory of the instrument or their printing. Stationary sensors are presented as simple small detectors with remote electronics location, and high-strength devices with remote PID-control. Fiber optics, laser sighting, water cooling, the presence of a display and a scanner are optional options for monitoring technological processes and control systems.

Configuration, spectral filtering, operating temperature range, optics, response time and object brightness are important elements affecting performance and should be carefully considered during the selection process.

The sensor can be either a simple two-wire or a complex wear-resistant high-sensitivity device.

The selection of the spectral response and the operating temperature range is related to the specific measurement tasks. Short wavelengths are designed for high temperatures and long ones for low temperatures. If the objects are transparent, for example, plastic and glass, then narrow-wave filtration is necessary. The absorption band of CH polyethylene film is 3.43 microns. The selection of the spectrum in this range simplifies the calculation of the radiation coefficient. In the same way, glass-like materials become opaque at a wavelength of 4.6 μm, which makes it possible to accurately determine the temperature of the glass surface. The radiation range of 1-4 μm makes it possible to measure through the inspection openings of vacuum and pressure chambers. An alternative is to use a fiber optic cable.

Optics and response time are in most cases unimportant, since a field of view of 3 cm at a distance of 50 cm and a response time of less than 1 s is sufficient. For a small or rapidly moving intermittent object, there is a need for a small (3 mm in diameter) or even smaller (0.75 mm) measurement spot. Long-range aiming (3-300 m) requires optical control, since the standard field of view of the instrument becomes too large. In some cases, a two-wave radiometry method is used for this. Fiber optics allows you to distance electronics from corrosive environments, eliminate the effects of interference and solve the access problem.

The 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 the signal, while a slow one affects the sensitivity. When induction heating requires an instantaneous reaction, and for the conveyor - a slower response.

Monochrome IR thermometry is simple and is used in cases where temperature control is extremely important for creating high-quality products.

Two-wave thermometry

For more complex problems where the absolute accuracy of the measurements is of decisive importance and where the product is exposed to physical or chemical effects, two- and multi-wave radiothermometry is used. The concept appeared in the early 1950s, and the latest changes in the design and hardware have increased its productivity and lowered the cost.

The method consists in measuring the spectral energy density at two different wavelengths. The temperature of the object can be read directly from the instrument if the emissivity is the same for each wavelength. The readings will be correct, even if the field of view is partially blocked 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 radiation coefficient is reduced and the ratio becomes proportional to the temperature.

A two-wavelength laser thermometer is used in industry and scientific research as a simple, unique sensor capable of reducing the measurement error.

In addition, multiwave thermometers have been developed 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 is required with respect to the relationship of this coefficient, wavelength, temperature and chemical composition of the surface. In the presence of these data, it is possible to create algorithms for calculating the dependence of spectral radiation at different wavelengths on temperature.

Evaluation rules

To assess the accuracy of measurements, the user should know the following:

  • IR-sensors by their nature colors do not distinguish.
  • If the surface is shiny, then the device will set not only the emitted, but also the reflected energy.
  • If the object is transparent, IR-filtering is necessary (for example, glass is opaque at 5 μm).
  • In nine out of ten cases, an absolutely accurate measurement is not required. The repeated reading of the readings and the absence of bias will ensure the necessary accuracy. When the energy brightness changes and the processing of data is difficult, we should dwell on two- and multi-wave radiometry.

Elements of construction

The non-contact laser thermometer works according to the principle: the infrared energy at the input and the output signal. 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 conversion. Conventional window glass is used for shortwave radiation, quartz for medium frequencies, and germanium or zinc sulphide for the range of 8-14 microns, optical fiber for wavelengths of 0.5-5.0 microns.

line of sight

The laser remote thermometer is characterized by the field of view (PP) - the size of the temperature control spot 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 object of measurement. Its value depends on the manufacturer and affects the price of the device. There are models with a PZ of less than 1 mm for point measurements and 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. The maximum signal loss should not exceed 1%.

Aiming

Conventional IR-thermometers produce measurements without additional devices. This is permissible for working with large objects, for example, a paper cloth, where no point accuracy is required. For small or remote objects, a laser beam is used. Several laser targeting options have been created.

  1. A beam with a shift from the optical axis. The simplest model is used in low-resolution devices for large objects, since the deviation is too large.
  2. Coaxial beam. Does not deviate from the optical axis. The center of the spot is accurately indicated at any distance.
  3. Double laser. The diameter of the spot is marked with two dots, which eliminates the need to guess or calculate the diameter and does not lead to errors.
  4. Circular pointer with offset. Shows the field of view, its size and the outer border.
  5. 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, while the outer dot indicates its diameter.

Aiming provides effective assistance when the thermometer is directed precisely to the measurement object.

Filters

Thermometers use short-wave filters for high-temperature measurements (> 500 ° C) and long-wavelength 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 μm and 7.9 μm), glass (5.1 μm) and flame (3.8 μm).

Sensors

Most sensors are either photovoltaic, generating voltage when exposed to infrared radiation, or photoconductive, that is, changing their resistance under the action of the energy source. They are fast, highly sensitive, have an acceptable temperature drift, which can be overcome, for example, by a thermistor circuit of temperature compensation, automatic zero-circuit, amplitude limitation and isothermal protection.

In the IR thermometer circuit, the detector output signal of the order of 100-1000 μV undergoes a thousandfold amplification, is regulated, linearized, and, as a result, represents a linear signal of current or voltage. Its optimum value is 4-20 mA, which minimizes external interference. This signal can be sent to the RS-232 port or to a PID controller, a remote display or a recording device. Other uses of the signal:

  • On / off alarm;
  • Holding the peak value;
  • Adjustable response time;
  • In the sampling and storage scheme.

Speed

The infrared laser thermometer on average has a response time of about 300 ms, although using silicon detectors can achieve values of 10 ms. In many instruments, the response time varies in order to damp the incoming noise and to regulate their sensitivity. It is not always necessary to have a minimum response time. For example, with induction heating, the time should be in the range 10-50 ms.

Characteristics of laser thermometers

Etekcity Lasergrip 630 is an infrared 2-laser thermometer, the price is $ 35.99. Characteristics:

  • Temperature range -50 ... +580 ° C;
  • Accuracy +/- 2%;
  • Ratio of distance to spot size 16: 1;
  • Emissivity 0,1 - 1,0;
  • Response time <500 ms;
  • Resolution 1 ° C.

The laser thermometer (photo) also informs about the largest, smallest and average temperature. The measuring spot is displaced 2 cm below the aiming point. Laser guidance is most accurate at the intersection of the rays (36 cm).

Amprobe IR-710 is an infrared laser thermometer, priced at $ 49.95. Characteristics:

  • Temperature range -50 ... +538 ° C;
  • The minimum spot size is 20 mm;
  • Accuracy +/- 2%;
  • The ratio of the distance to the spot size is 12: 1;
  • Emissivity: 0.95;
  • Response time 500 ms;
  • Resolution 1 ° C.

This laser thermometer (photo), besides the current temperature, also indicates its minimum and maximum values.

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