Temperatures in the food industry are commonly measured using bimetal thermometers, gas-filled sensors, thermocouples or resistance temperature detectors (RTDs). When installed in the walls or roof of a cooling system, these devices generally provide accurate temperature measurements for the environment surrounding the food. However, small food temperature changes that occur due to variations in line speed, food thickness, moisture content or other parameters are not detected through proximity with these devices. Because some processes move quickly, considerable scrap can be generated before the improper temperature is detected. Worse yet, temperature-related problems might not show up until the food is in the retail store or in the hands of the consumer. For these sensors to indicate the correct temperature, they have to be inserted directly into the food, and this option often is neither practical nor desirable.

As food quality requirements continue to increase, many processing plants have begun looking for ways to increase the accuracy of their food temperature measurements using other noncontact devices. Infrared (IR) thermometers provide a possible solution. Typically associated with measuring high temperatures in processes such as induction heating, steel production and radiant heating of multiple materials, infrared technology also can be used to measure various products as they are being cooled. Increasingly, infrared temperature-measurement devices are finding use in food processing applications.

Technology Basics

Enlarge this picture Sensors can be obtained with a stainless steel housing and a hard plastic, shatter-resistant lens to meet food industry specifications.

The human eye can detect electromagnetic radiation at wavelengths between 0.3 to 0.7 μm. Known as visible light, the radiation emitted at these wavelengths comprises only a small part of the electromagnetic spectrum. Directly below this level is the infrared region. The term "infrared" literally means "below red," with red being the color of the longest wavelengths of visible light.

All objects emit infrared energy in the range of 0.65 to 14 μm, and the amount of radiation emitted increases with temperature. Infrared thermometers use this energy to "see" an object's temperature. Most food temperatures are measured at wavelengths of 8 to 14 μm. At these wavelengths, infrared instruments can measure temperatures as low as -58°F (-50°C) with an accuracy of ±0.5 percent of the temperature indication, or ±3.6°F (±2°C), whichever is greater.

One property that must be determined for all targets and infrared thermometers is emissivity, which is based on the black body radiation law. A black body has an emissivity of 1.00, or 100 percent. It is a perfect emitter -- no other object can emit more infrared energy at any temperature or wavelength. To correct for the inevitable loss in signal when measuring the temperature of other objects, every infrared thermometer has an emittance control. For example, at 8 to 14 μm wavelengths, the emissivity of almost all foods is 0.94 regardless of color, texture or moisture content. Setting the instrument's emittance at 0.94 allows the instrument to add a 6 percent gain to the signal received from food targets, which ensures that the instrument will indicate the right temperature.

To meet food industry specifications, infrared sensors can be obtained with a stainless steel housing and a hard plastic, shatter-resistant lens. Infrared sensors for the food industry are coated with an FDA-approved epoxy to prevent any exposed metal surfaces.

Infrared instruments typically use a 24 VDC power supply and provide a linear 4 to 20 mA output. Some units also have digital outputs of RS485 or RS232. With these simple outputs, they can be connected to a PLC or PC for closed-loop control, alarms or simple temperature indication.

Infrared Considerations

Enlarge this picture Infrared instruments measure temperatures with an accuracy of ±0.5 percent of the temperature indication or ±3.6°F (2°C), whichever is greater.

Infrared thermometers require a clear line of sight from the sensor to the target. The sensor generally is mounted approximately 24 to 36" from the target to ensure accurate temperature readings.

Infrared thermometers can operate in ambient temperatures as low as 32°F (0°C). However, if the ambient air is cooler than the sensor, condensation can form on the lens and prevent the sensor from seeing the target objects. To prevent such fogging, the lens can be purged with a dry gas such as nitrogen or dry air at an airflow rate of 6 cfm. Installing the sensor at the exit end of the cooling system -- rather than within the system -- also can help prevent condensation problems. For applications in which the temperature must be measured within a cooling unit, an opening of about 1.5" in diameter is required to hold an infrared sensor.

It is important to note that applications with a significant temperature differential between the food being processed and the surrounding environment can experience inaccurate temperature readings. For example, if the food is maintained at a temperature of -30°F (-34°C) and the surrounding room temperature is 35°F (2°C), infrared energy from walls or equipment will reflect off the product and cause the thermometer to provide a reading as much as 20°F (11°C) higher than the actual food temperature. This problem can be avoided by using sensors that can be programmed with the background temperature. These sensors use internal software to compensate for the difference and provide the correct food temperature.

Another consideration with using infrared sensors in food processing applications is that the sensors must be covered during wash downs. Otherwise, the hot caustic solution used for cleaning can damage the lens. If the cleaning solution is above 131°F (55°C), the sensor might have to be removed completely to prevent overheating.

A Cool Measurement Tool

Because the infrared sensor does not have to come in contact with the food to provide an accurate temperature measurement, it does not interfere with the process or cause possible contamination. The air surrounding the food is transparent, so the thermometer "sees through" the air and measures the actual product temperature.

Infrared thermometers are also much faster then contact sensors. Response times as fast as 100 msec are common. In comparison, thermocouples often require 1 to 2 sec for full-scale indication. The faster response time provided by infrared technology provides much more accurate temperature control even in rapidly moving processes.

Infrared temperature measurement also helps food and beverage producers save energy and reduce waste by not overcooling their products -- especially when freezing is required. Refrigeration systems consume the majority of the power required in the frozen food industry, and temperature inaccuracies are a major contributor to inefficiency. Of course, overfreezing also can destroy the product or make it unacceptable to the consumer because of color or texture.

Infrared sensors provide an accurate noncontact method for measuring the actual food temperature. With this information, operators are better able to control the line speed and the amount of food processed to optimize productivity and quality.