It is a given that manufacturing ovens need to be properly maintained and have an even temperature distribution to produce quality parts. However, many ovens actually have hot and cold zones that go undetected until part quality begins to suffer: they’re like the guy who shaves every morning but always misses the same spot. To troubleshoot their ovens and to fine-tune their heat treatment process before problems occur, operators can establish an oven temperature profile, also known as a thermal profile. By collecting, viewing and analyzing this set of time-stamped temperature data, operators have the true picture of what is going on inside their process. This enables repeatability and reduces variance, leading to substantially increased product quality.
Here are seven ways you can generate highly accurate temperature profiles for your oven process:
1. Capture Thermal Profiling Data with Thermocouple Sensors
Temperature measurements are among the most common datalogging applications. While no thermocouple type predominates for temperature measurement, Type J thermocouples are commonly chosen for oven temperature profiling purposes due to their low cost, high durability and wide measuring range. Meanwhile, Type K is a more general-purpose thermocouple offering many different probes. You also can opt to use more expensive types for specific applications and higher accuracies (Types B, R, S/C, D and G). Having your sensors calibrated will verify their readings.
Place the temperature sensors onto a test board/sensor grid for insertion into your static or conveyor oven. You can either install them in known problem setpoints for effective troubleshooting or opt to space them out in a distributed grid (attached by epoxy for stability) for a more comprehensive view of your heat treatment process. Aim for a mix of both occupied and unoccupied spots in your oven. Keep your sensors clear of areas affected by air currents, or you can opt to use fast response, high accuracy probes that can measure air and surface temperatures.
2. Oven Dataloggers Record and Store Temperature Data
Dataloggers are an ideal way to create oven temperature profiles by recording, storing and downloading thermocouple data. Because thermocouple sensors generate a small voltage proportional to the surrounding temperature, a datalogger measures this voltage and then applies a calibration equation to convert the voltage to temperature units (°C or °F). The datalogger also incorporates a cold-junction reference to compensate for any offset voltage that occurs at the connections between the thermocouple wires and the datalogger.
Oven dataloggers run through your process, along with the products, over multiple trial runs to assess and improve process capability. These devices have high temperature operating ranges, enabling them to travel through the oven while enclosed within a suitable thermal barrier. Some logger models connect to a single type of thermocouple, often Type J or K, while others have flexible inputs accommodating both types. Additionally, many oven loggers can automatically start and stop recording based upon a preset time or temperature.
Different models exist for different applications, but typically they are multi-input, battery-operated devices with adjustable sampling rates. Note that a fast sampling rate is important if you need to troubleshoot your ovens frequently or extensively.
If you plan on using the logger for multiple runs or for a long duration, it helps to choose a model with a large memory. After the test run, you can download the temperature readings to a PC for analysis using the logger software. Downloading is most commonly done via USB/flash drive.
3. Shield Thermocouple Wires for Accurate Data
Thermocouple noise is signal interference encountered as a result of the very low voltage involved in thermocouple measurements. This loud sensor noise is commonly an issue in industrial environments. The severity depends on the specific application and your type of oven (electric ovens will definitely be a source of noise).
The most common source of noise is that induced by AC line power from nearby cables or equipment, especially from long cable runs. This noise is periodic at the line frequency, which is 60 Hz in the United States or 50 Hz in other parts of the world. Many dataloggers are able to reject most of this noise by performing an integrated measurement of the input for exactly one line-cycle period (16.7 ms or 20 ms).
To reduce thermocouple noise and ensure that the datalogger records accurate temperature data, use thermocouple wire with an extra layer of electrical shielding around the conductors. This is a simple way to compensate for the negative effects of electrical interference from nearby equipment. For optimal performance, tie this shield to a good electrical or earth ground at only one end.
4. Use a Temperature Transmitter to Reduce Noise
Another way to reduce signal noise is to use a small temperature transmitter that connects to a standard thermocouple input and provides a 4 to 20 mA output that the datalogger can measure. Temperature transmitters are useful when you have very long cable runs between the oven measurement setpoints and the datalogger. You will find that the 4 to 20 mA current signal is much less susceptible to noise than the millivolt-level thermocouple output.
5. Source the Right Thermal Barrier for Your Profiler
An effective thermal barrier — a datalogger enclosure — is necessary to protect the oven logger as it passes through static or conveyor ovens along with the product. A variety of stainless steel thermal barriers give years of solid protection from high temperature ranges at various runtimes. Be sure to make a note of the thermal barrier’s operating temperature. This will tell you the maximum time you can leave the barrier inside the oven without damage — even the best barrier can only protect the datalogger for so long! Also, be sure to adjust the operating temperature based onyour particular oven’s temperature.
For extreme oven temperatures or lengthy oven runs, you can insert a phase-change heat sink inside the barrier to add another layer of protection for the logger. Phase-change material (PCM) shields the datalogger from excess heat to further insulate it within the thermal barrier.
6. Customize your Process with Temperature Profiling Analysis Software
Specialized datalogger software helps users organize and analyze their temperature data in the form of a detailed temperature profile. Analysis and implementation helps to prevent overheating and premature cooling as well as reduce defect output rates. By viewing the thermal graph data, users easily can identify the source of known problems by examining anomalous data and further optimize their existing process.
Using process window index (PWI) algorithms, thermal profiling software shows how well the run meets process specifications or if it goes outside specified values. For example, with the datalogging software, you can view slope and peak temperatures and view the effects of conveyor speed to stabilize your food product or component temperatures. After verifying a successful test run, simply repeat the process in the production phase.
Users also can purchase customized software for specific applications such as for monitoring paint ovens and producing cure percentage calculations.
7. Generate Datalogging Software Reports as Proof
With some datalogger software applications, users can directly output the data to a printer in the form of a report, showing adherence to a number of industrial standards. This is useful to demonstrate best practices to customers and show that your products are up to manufacturer and component specification.
Data collection in the form of a temperature profile is a cost-effective way to benefit your business. By troubleshooting specific oven issues and optimizing your heat treatment process, temperature profiles allow users to make repairs and other improvements that lead to improved product quality and lower defect output. You will also benefit by having hardcopy proof of your best manufacturing practices to give to customers.