Temperature Profiling Optimizes Batch Oven Processes
Using dataloggers to track temperature can increase profitability and product quality.
Whether you are preheating, drying or curing a part, oven temperature data is integral to operational audits for batch ovens and other types of process oven applications. Looking at product performance for each run, you will have the hard data that proves whether your batch oven’s temperature is uniform or if hot and cold spots inside the oven reduce consistency.
An oven’s interior temperatures gradually fluctuate and become erratic as the three-phase compressors age. Using an oven temperature datalogger, however, you can obtain a complete temperature profile for each oven. This provides proof that your products have passed through the process at the right temperature for the right amount of time. You also can print quality assurance (QA) reports for compliance, traceability and complete quality control. For example, you can use the temperature profile during single-batch thermal processing to minimize curing times and prevent undercured parts.
Placed in the oven along with your products during the treatment process, oven temperature dataloggers are designed specifically for temperature profiling in process monitoring and surface-coating applications. To this end, they are well suited for placement in walk-in, bench or other batch ovens. Many oven temperature profiling systems can record temperature data via external sensors that trail out from the logger’s enclosure. After a few runs, you should have the temperature profile data necessary to start fine-tuning your process efficiency with the aim of increasing quality, profitability and repeatability.
Which Temperature Sensor Should Be Used?
Four types of temperature sensors are used with dataloggers. It can seem like a hassle to find the type that is best for your batch oven process. However, you can quickly narrow down the choices by considering the pros and cons of each type:
- Thermocouples are the most common type of temperature sensor due to the wide operating range. They offer a typical accuracy of 1 to 2°F. Consider thermocouples when you want a low cost device that is easy to use.
- Semiconductor devices are equally low cost and provide an output voltage or current proportional to temperature. They have a limited operating range and accuracy comparable to thermocouples. Consider using semiconductors when budget is a factor and you have a specific temperature range in mind.
- RTDs and thermistors provide a change in resistance that is related to temperature. While they offer higher accuracy than thermocouples, these sensors are more expensive and have a more limited operating range. Consider RTD/thermistor sensors when you need high precision measurements for a narrow temperature window.
How Do Sensors Create a Temperature Profile?
Thermocouples are the most typically used sensors. A thermocouple is nothing more than two dissimilar metal wires of specific alloys fused together at a single point. A thermocouple produces an output voltage (typically at the millivolt level) related to the temperature in your oven. The datalogger measures the voltage and then applies a calibration equation to convert the voltage to temperature. 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 logger.
A thermocouple temperature measurement actually involves multiple voltage measurements of the thermocouple and reference junction as well as scaling the voltage to temperature and compensating for the reference temperature. If you need higher accuracy, wires with reduced errors are available.
Because of the low voltages involved with thermocouple measurements, their loud noise is often an issue, particularly in industrial environments. Two popular methods of reducing measurement noise are to use of a shielded thermocouple wire and to use a temperature transmitter. The transmitter provides a 4 to 20 mA output that can then be measured with the datalogger.
Semiconductor sensors incorporate a solid-state device such as a diode or voltage reference that has a well-established voltage versus temperature characteristic. Semiconductor sensors also incorporate signal-processing electronics to generate a voltage or current output proportional to temperature. These devices only require a simple voltage or current measurement and linear calculation to scale the signal to temperature.
An RTD sensor is a device whose resistance increases linearly with the temperature within the oven. The most common RTD consists of a fine platinum wire wound around a cylinder. To measure temperature, the datalogger will source a known current through the RTD and measure the resulting voltage, from which it can calculate the resistance. Finally, using the slope of the resistance versus temperature curve and the 32°F (0°C) resistance, the datalogger can calculate the surrounding temperature. RTDs are typically more stable and accurate than thermocouples, but they have a more limited operating range.
Thermistors are similar to RTDs — their resistance changes with temperature — but their resistance change is highly nonlinear. Because of this, thermistors can offer accurate temperature measurements down to an accuracy of 0.01°C, but they operate over a limited temperature range (typically -40 to 302°F [-40 to 150°C]). Like RTDs, thermistors are designed to have a specific resistance at 0°C, and each family of thermistors has a specific resistance versus temperature characteristic that the oven datalogger must be able to accommodate.
If your application requires you to view data in real time, fast-response probes are available in clip-on, magnetic and combination types to capture both surface- and air-temperature measurements. Examples of dataloggers that can read the signals from these sensors include portable, power-over-Ethernet and wireless models. You then can use software to create a highly accurate oven temperature profile.
Need to Alarm Your Oven Doors?
If frequent oven door opening is a problem for your process, an intelligent temperature datalogger can provide you with an automated way to detect and alarm how often your oven doors are opened and closed. Using a logger with digital outputs for connection to an alarm device, you can set the logger to go into alarm mode every time your oven door is opened.
Additionally, if your logger software supports a feature called alarm hysteresis, you can set more specific alarms. For example, you can specify that the logger only triggers an alarm when the door is opened two or three times in a row over a certain time period.
Getting the Inside View of Your Process
Typically, a temperature datalogger’s analog input channels can connect with a range of thermocouple probes (for example Type T and Type K), capturing data at a high sample rate. Other logger models are designed to connect to other sensor types. Universal loggers have universal inputs for flexible connection.
As the battery-operated datalogger passes through your process along with your products, it regularly records the surrounding temperature via the connected external temperature sensors. The oven datalogger stores and time stamps the temperature data onto its internal or external memory. The logger can store thousands of readings.
After the run is finished, you can use the logger software to view the stored temperature data and see how uniform the process actually is. You might also select areas for long-term improvement. For setup and data download, many loggers support USB interface for communication with a PC.
How Do I View and Print the Finished Oven Profile?
Nowadays, many oven loggers include temperature profile software for setup and configuration. While some are simple interfaces for the logger, many powerful software packages are designed exclusively for the paint and finishing industry. These programs enable storage of preprogrammed cure data, display of calculated cure percentages and more. Typically, you also can print graphical reports of each run via USB port without having to use a PC.
Using temperature profiling software, you can document batch-to-batch thermal processing and validate that your product has been produced in accordance with specific QA and product procedures. It is relatively simple to analyze a certain oven’s temperature data to identify warmer and cooler spots, helping to develop a stabilized, repeatable process over time.