Non-Contact Temperature Monitoring in Poultry Processing
By John Stewart

Infrared cameras and point sensors, like these from Exergen and Mikron, are commonly used in non-contact temperature monitoring.

Non-contact temperature measurement has many applications in poultry processing. For example, such devices can be employed to measure the surface temperature of birds leaving the scalder for a quick determination of overscalding. Similar applications exist for birds in a chilling process. Another important application is to monitor product entering and leaving a cooking process to determine that it was cooked properly.

Point sensors and cameras that can measure temperature are relatively inexpensive and available from many different vendors. Properly applied, the sensors can provide accurate temperature measurements to 1 or 2 ¾C and identify trends in temperature variation down to 0.1 ¾C. While the operation of these devices is simple, deploying them in a production environment can be tricky. This article addresses some of the issues with deployment.

All surfaces emit radiation when they are heated above absolute zero (-273 ¾C). As the surface heats, it emits more radiation and emits at shorter wavelengths. For processing temperatures between 0 and 100 ¾C, objects emit infrared radiation, and devices that measure temperatures in this range typically are sensitive to infrared radiation between 6 and 12 mm. Devices that limit the responsive band to wavelengths between 8 and 12 mm are less affected by water vapor.

Not all surfaces emit radiation as well as others. Emissivity, e, is a measure of radiation emission efficiency. An object with an e of 1 is a perfect emitter called a black body. Most objects have an e of less than 1. For example, typical food objects have an e between 0.6 and 0.95 for wavelengths between 8 and 12 mm. One challenge of measuring temperature with non-contact devices is knowing the emissivity of the sample, which can change with temperature and other process variables.

Another pitfall to avoid with non-contact sensors is reflections from other hot surfaces. The total radiation leaving a surface is a combination of self-emitted radiation and reflected radiation from other surfaces. The reflectance of a surface is related to emissivity through the following equation:
r = 1 – e, so objects with high emissivity are not very reflective. Even though food objects are not very reflective, placing an inspection station near bright infrared sources like the mouth of an oven can corrupt measurements.

Measuring the temperature of individual products on a moving production line can be difficult because sensors often have a slow temporal response and typically have a large measurement area. Typical point sensors need one-tenth of a second to measure temperature. For shackle lines moving at 180 birds per second with 6-inch centers on the shackles, one-tenth of a second equates to 1.6 inches. In addition to the size needed to accommodate product movement, the point sensors typically have a large measurement area. Many devices ship with a 1:1 spot size. In a 1:1 configuration, the diameter of the measurement area will be the same as the distance to the object being measured. In many processing applications, it is necessary to place the device several inches or more from the product to ensure it remains a non-contact device.

Optics can be added to the devices to change them to a 10:1 configuration or even higher ratios. However, even with optics, the minimum spot size is typically about an inch. When purchasing devices with optics, choose plastic lenses that are safe and capable of handling the sanitary environment on the processing line. Another important option to consider is an air purge system for a point sensor. An air purge jacket can help protect the sensor from water encroachment and it can help keep the lens clean. In a production environment, a dirty lens can cause large errors in measurement. The air purge system can also help to keep a stream of clean air between the sensor and the target object. Some non-contact sensors are quite sensitive to water vapor and a clean air stream between the sensor and the product can help reduce errors due to transmission through clouds of steam rising from the product.

For applications that require temperature measurement of small features or many samples of product spanning the width of a belt, an infrared camera may be a better choice. Micro-bolometer cameras typically use arrays of Vanadium oxide sensors, which are built with a micro-machining process. Current commercial offerings include 320x240 or smaller arrays and can measure a temperature in about a hundredth of a second. Ethernet or IEEE 1394 interfaces and software development kits make it relatively easy to interface the cameras into a real-time production environment. Unlike more expensive cooled cameras, micro-bolometer cameras use little power and do not require expensive maintenance. All cameras, however, require an external enclosure to protect them from processing environmental conditions.

As true with any measurement device, an important part of adding non-contact temperature measurement sensors is adding a calibration procedure for the device. Calibration sources are commercially available, and sensors should be checked against calibration sources often to ensure they are operating properly as part of a routine calibration schedule.

Vendors in this field are an excellent source of application information. Mikron, Exergen, Omega, IRCon, Raytek, and Flir Systems all sell non-contact temperature measurement devices designed for industrial inspection.

John Stewart is a research engineer with Georgia Tech’s Agricultural Technology Research Program. His areas of expertise are computer vision software and simulation.