Infrared Thermometer

An infrared thermometer is used to measure the surface temperature of an object without touching the object's surface. In practice, the object for which the surface temperature is to be measured can be anything from a rubber tire to a furnace heating element. This versatility makes an infrared thermometer an incredibly useful tool for inspection professionals in many diverse industries. However, to ensure accurate measuring results, the specifications of an infrared thermometer must align with the requirements of the application.

How do you determine what infrared thermometer is best suited to your application? The following text is designed to help you make an informed purchasing decision.

Key Questions to Ask Yourself When Choosing an Infrared Temperature Measuring System

- What type of device does your application require (e.g., portable or stationary infrared thermometer)?
- What emissivity value is recommended for measuring the temperature of your material surface?
- What distance-to-target or distance-to-spot (D/S) ratio does your application require?
- What degree of accuracy of the infrared thermometer is necessary for the successful completion of your task?
- What memory size and data transfer interface is needed, if any?
- What level of after-sales service and support is desired?
- What is your budget for an infrared thermometer?

Following is a more detailed analysis of criteria to keep in mind when selecting an infrared (IR) thermometer.

Portable or Stationary
The answer to this question largely depends on the intended application. If you want to use the IR thermometer to sporadically collect readings for quality assurance, a portable infrared thermometer should be used. If you want to measure continuously and use the readings for process control, it will be better to use a stationary infrared thermometer. A stationary IR thermometer should be equipped with an interface to transfer readings to a process control system or controller. The transfer can be analog or digital.

Emissivity
An infrared thermometer can have its disadvantages. One such disadvantage is the need to know the required emissivity. The emissivity of a material is the relative ability of its surface to emit or absorb energy by radiation. Emissivity not only depends on the kind of material, but also on the anticipated surface temperature and the wavelength (µm) of the infrared thermometer. Another disadvantage is that the emissivities of metals vary widely, making accurate measurement difficult. For example, at 25 °C (77 °F) highly oxidized copper has an emissivity of 0.78, but at 527 °C (980.6 °F) the same highly oxidized copper has an emissivity of 0.91, while polished copper has an emissivity of 0.012 at 327 °C (620.6 °F). A table of approximate emissivity values for common materials can be viewed here. Warning: The accuracy of the values shown in the table cannot be guaranteed, because emissivity is dependent on several variables (surface texture, color, temperature at time of measurement, etc.).

Whereas most glass, ceramic, plastic, wood and organic materials have very high emissivities (about 0.95) within the middle infrared range (MIR) and far infrared range (FIR), blank metals have much lower emissivities within the MIR and near infrared range (NIR) -- e.g., polished gold within the MIR has an emissivity of approximately 0.02. However, when metal is anodized (like aluminum) or highly oxidized, it will have a higher emissivity of about 0.9 within the MIR. When it comes to painted metals, the higher emissivity of the paint will be significant to the tinfrared thermometer. The intensity and the emission maximum depend on the temperature.

Many infrared thermometer devices have a feature to adjust the emissivity. Often the emissivity is adjusted using a rotary knob with a range of 0 ... 1. Some infrared thermometer devices have an additional measuring input for a contact temperature sensor (or thermocouple). If you want the IR thermometer calibrated for an unknown material to determine the emissivity, the temperature can be measured with this additional sensor. The setting for the emissivity in the infrared thermometer will be adjusted until the non-contact measurement produces the same measurement result as the measurement with the contact sensor.

Distance-to-Target Ratio
Every infrared temperature meter has a lens with a certain distance-to-target or distance-to-spot (D/S) ratio. Specifications like 2:1, 10:1 or 20:1 are very common for low-priced IR thermometer devices. When looking at high-quality infrared thermometer devices, the specifications can be up to 75:1. These values also can be expressed as x:y. This means that the measuring spot has a diameter of y when the distance to a surface is x. For example, if an IR thermometer has a distance-to-target ratio of 20:1, you can stand 20 centimeters or 20 inches away from your target and measure the temperature of a one-centimeter or one-inch circle. This is similar to the cone of light emitted by a flashlight. If you approach a wall very closely with a flashlight, the cone of light will be smaller than when you are farther away from the wall. More advanced infrared thermometers can reduce or increase the size of this measuring spot to meet the size requirements of the application.

Accuracy
Each infrared thermometer will have its own accuracy specifications that are dependent upon the temperature range being measured. Typically a higher accuracy measuring instrument will come with a higher price tag, so it is important to consider practicality as well as precision.

In addition to emissivity, influencing factors such as the surface thickness, geometry (even, concave, convex), finish (polished, rough, oxidized, sand-blasted) and transmissivity (thin plastic films) can affect results, as can the measurement angle and spectral range. Thus, when taking comparative temperature measurements with an infrared thermometer, it is vital to establish a control to minimize the effects of as many variables as possible to capture the most accurate readings.

Memory and Data Transfer
Some infrared thermometer products save temperature readings to an internal memory or SD card memory. The size of the memory can be presented as a number of GB or as a maximum number of readings stored. This memory usually allows for the temperature measurement data to be downloaded to a PC using a USB port, RS-232 interface or SD card reader. In most cases, an IR thermometer with temperature data recording capabilities is equipped with a USB connection. In some cases, special PC software is required to transfer the measurement data.

Service and Support
This is another important point to keep in mind. Find out how long the supplier has been in business. The longer the company's tenure, the more likely it is that you will be able to order spare parts a few years down the road after buying your infrared thermometer. PCE Instruments has been in business since 1999. (For more details, please visit the About Us / Corporate History section of our website.) Also find out what kind of technical support will be available to you for your infrared thermometer. Call PCE Instruments, talk to the technical support team prior to purchasing and see for yourself the level of service provided.

Budget
Often more is involved in the budget than just the cost of the infrared thermometer. Calibration costs are common expenses that can occur when using an infrared thermometer. For instance, if you need to meet the requirements of an ISO quality standard, a regular calibration interval needs to be adhered to. ISO calibration costs can occur at the time of purchase and reoccur on an annual or even semi-annual basis, depending on your accuracy needs and usage of the infrared thermometer / IR thermometer. It is also possible that recalibration becomes necessary later on, due to the drift IR thermometers and their sensors can experience over time. Consumables like (rechargeable) batteries should be included in your calculations as well.

Brief Background on the Infrared Thermometer

An infrared thermometer is often called a laser thermometer, non-contact thermometer, IR temperature gun or pyrometer. An infrared thermometer uses infrared radiation, also known as thermal radiation, to indirectly measure surface temperature without contact. William Chandler Roberts-Austen was credited with the invention of the radiation thermometer. Any object with a temperature above 0 degrees Kelvin (K) emits radiation. An infrared thermometer solely measures the temperature of a visible surface. Therefore, an infrared thermometer cannot take measurements through glass. If the object being measured is colder than the IR thermometer, the radiation flow is negative. To calculate the temperature, the infrared thermometer sends thermal radiation to the measured object to correct the negative flow.

Advantages of an Infrared Theromometer

An infrared thermometer offers many advantages. Measurement results can be obtained very quickly, from 10 µ to 1 second depending on the device. An IR thermometer also elminates errors due to poor thermal contact. There is little wear or tear on an infrared thermometer, even on portable handheld units. High voltage, electromagnetic fields and corrosive materials do not affect the measurement process. Measurements taken with an infrared thermometer do not cause damage to sensitive objects such as plastic films or paper products. Another important advantage is that, because measurements can be taken without contact, an infrared thermometer is a much safer tool for use in high-temperature situations.

Measurement of Wavelength (NIR, MIR, FIR)

The ideal conditions to measure wavelength using an infrared thermometer depend on the material and its temperature. For the best readings at room temperature, wavelengths in the middle infrared range (MIR) are recommended. Thermal sensors inside the infrared thermometer, such as pyroelectric detectors, are used for this purpose.

Temperatures from approximately 350 °C (662 °F) can be determined in the near-infrared range (NIR) with infrared photodiodes, using an infrared thermometer. In this way, for instance, a germanium photodiode can have a lower receiving wavelength of approximately 1.9 µm. Temperatures from approximately 700 °C (1292 °F) in the visible spectral range can be measured with photodiodes. The IR thermometers receiving wavelength range for high temperatures are often determined by photoreceptors. For instance, the smallest receiving wavelength of silicon photodiodes is approx. 1.1 µm. An object with a temperature of 3000 K is at its maximum radiation, so with this you can measure temperatures from approx. 700 °C (1292 °F).

Infrared Theromometer Practical Applications

As discussed, the main advantage that an infrared thermometer possesses, in comparison to a regular contact thermometer, is that it provides non-contact measurement using emissivity. This means that within just a few seconds it is possible to determine the temperature in parts of machines, engines or systems that cannot be accessed easily without shutting down. In fact, because an infrared thermometer measures the emission coming from the surface of the material or object (as all objects emit energy and electromagnetic waves depending on their temperature), and since the energy depends on the temperature of the object to be measured, the temperature of the object's surface can be measured without immediate proximity and direct contact. Due to the installed software and laser sight, the infrared thermometer determines the coefficient of the emission of the object to be measured and provides a precise reading of the surface temperature in the exact target area.

Thus, an infrared thermometer / IR thermometer is an ideal solution for monitoring heating, ventilation and air conditioning (HVAC) systems, parts and components. An infrared thermometer can be used for regular monitoring of HVAC systems as well as for troubleshooting and searching for the cause of an unidentified malfunction. The condition of an HVAC system plays an important role in any company, factory, warehouse, school or home. An infrared thermometer can be used to examine air ducts, pipes and walls. HVAC system maintenance is a challenging task, since not all the elements of the HVAC system are open to immediately access and inspect. That is why in HVAC system testing, adjusting and balancing (TAB), application of direct contact measuring devices is not always appropriate or easy. Often difficulties result not only from obstacles creating limited access to key measuring points within the HVAC system, but also from potential risks and dangers for the HVAC technician, for example, when the temperature of an HVAC system component becomes too hot for measurement to be carried out safely by contact.

An infrared thermometer enables HVAC inspection professionals to carry out necessary HVAC system tests and examinations on site, without using ladders or too much other auxiliary equipment, making inspection of the condition of the ventilation ducts, air conditioning vents, heating elements cooling pipes and the like simpler.

Most infrared thermometer devices are small in size, which makes transportation and handling absolutely problem-free. Plus, the accuracy and measurement results from most IR thermometer products are impressive. An infrared thermometer can help to save a lot of time, effort and cost arising from unplanned shutdowns of equipment, machinery and systems. Non-contact measurements with the help of an infrared thermometer are carried out in real time and, when using devices with preset limits, it is easy to detect when operating conditions exceed permissible limits (as a rule, the infrared thermometer emits a sound or flashes a colored light as an audible or visual alarm warning signal).

Another area where infrared thermometer devices are implemented is in the field of medicine. For example, patients suspected of having highly contagious health conditions can be quarantined and have their body temperatures measured with a high degree of accuracy using special infrared thermometer technology.

The measurement of temperatures of glass or on different types of glass brings some difficulties, because of the reflection and transmission of the glass. To limit the problems which may arise during the application of the infrared thermometer, there are several ways to carry out the temperature measurement.

The first way to measure the temperature with a pyrometer is to use an infrared thermometer, which is in the wavelength range of 1.0 microns, 2.3 microns or 3.9 microns. However, these wavelengths make it possible to measure not directly the surface temperature, but just below the glass surface. Wavelengths of the IR thermometers with 5 μm and 7.9 μm ensure that also the surface is measured. A disadvantage of these wavelength ranges is that the temperature of the glass must be very high. It is therefore not suitable for the temperature measurements in the lower measuring range.

A second option for measuring the temperature of glass is to use a thermometer with a wavelength of 8 ... 14 microns. In this case, however, there is a difficulty that the reflection of the surface must be compensated by the thermometer. Often, this can be done only by very expensive pyrometers. It is also important to remember that the degree of emissivity should be set to 0.85 and that the infrared thermometer should have a fast response time, because glass can change the temperature quickly due to the poor thermal properties.

In order to be able to measure the surface temperature still quickly and easily with an infrared thermometer with a wavelength of 8 ... 14 μm, so-called emission grade stickers can be applied. These stickers have a degree of emissivity of 0.95. They are simply glued to the glass. After a while, the sticker gets the temperature of the glass and the surface temperature can be measured with any standard infrared laser thermometer. Practically, the temperature of the glass is measured then. This is a fast, easy and inexpensive method to measure the glass temperature in a contactless way.