Temperature Measurement Flexibility Helps Quality and Validation
Understanding different sensor types helps create better temperature profiles.
Oven temperature data is critical to the entire process, including preheating, curing and drying. Using different types of temperature sensors, you can find out if your oven interiors are uniform in temperature or if there are uneven zones impacting consistency.
With universal inputs, all the sensors are recording data on the same device using the same software to keep it simple and to store the data to internal memory. It is all with a goal of increasing profitability, product coverage and optimizing the oven heat treatment process.
Why Use Different Sensor Types?
Available in several designs, temperature sensors use different methods to record data at varying degrees of accuracy. This means that all temperature data is not equal. Each type has advantages and disadvantages for particular applications.
Flexibility. Thermocouples are the most typically used sensors available, due in part to their wide operating range (typical accuracy of 1 to 2°F). Consider thermocouples when you want a low cost device that offers ease of use. A thermocouple uses two metal alloy wires fused together to produce an output voltage related to the temperature in the oven. A datalogger measures this voltage and applies an equation to convert the voltage to a temperature reading. Because of the low voltages involved with thermocouple measurements, signal noise is often an issue, particularly in industrial environments. If you need higher accuracy, wires with reduced errors also are available.
Cost. Semiconductor devices are another low cost choice. They incorporate a solid-state device to generate a voltage or current output proportional to temperature. Much like thermocouples, they have a limited operating range and accuracy. Consider using semiconductors when budget is a factor and you have a specific temperature range in mind.
Stability. Resistance temperature detectors (RTDs) are sensors which have a resistance that increases linearly with the temperature within your 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 0°C resistance, the datalogger can calculate the surrounding temperature. RTDs typically are more stable and accurate than thermocouples, but they have a more limited operating range.
Similar to RTDs, thermistors provide a change in resistance that is related to temperature. However, thermistors’ resistance change is highly nonlinear. This means that they offer higher accuracy than thermocouples (down to an accuracy of 0.01°C) but only over a limited temperature range (typically -40 to 302°F [-40 to 150°C]). Each family of thermistors has a specific resistance versus temperature characteristic that the oven datalogger must be able to accommodate.
Consider RTD/thermistor sensors when price is not a factor, and you need high-precision measurements for a narrow temperature window.
Temperature Measurement for Process Optimization
While many logger models are designed to connect to specific sensor types (for example, Type K thermocouples), universal loggers are able to record data from many different types. This includes thermistors, RTDs and air temperature sensors.
For thermocouple applications, some loggers also utilize a cold junction reference to compensate for any offset voltage that occurs at the connections between the thermocouple wires and the logger. Additionally, an analog-to-digital converter (ADC) helps to ensure precision measurements.
Using a temperature datalogger equipped with universal analog inputs, you can log data from several different types of temperature sensors. This is especially useful if you do not know which sensor types you need, or if you want to experiment with different temperature sensor types. For example, maybe you want to run thermocouples to the logger, but your readings keep getting signal interference. You also can connect RTDs.
Adding Sensor Power. In the setup section of many logger software applications, you can select a sensor power timer for the temperature input channel you want. For example, you can choose to have the connected temperature sensors take a minute or more to power up, helping to ensure that they take a stable reading. Many software packages also will show you sensor power wiring diagrams for the logger’s blocks. When all this is completed, you can send this setup to the datalogger so it is saved for future use.
Setting up a Temperature Channel. To set up a temperature channel, follow these steps: run the logger software, go to the setup section or tab, select the first temperature channel, select your sensor type (for instance, thermocouple Type K), and input the temperature range you want to monitor. You can also add scaling, choose the temperature scale (°F/°C)and other options. Diagrams show where to connect into the block of the logger. To simplify multichannel applications, you also can enter names for different sensors for easy reference.
Monitoring Air Temperature inside the Oven
You may think that the interior of your oven is a relatively stable environment, but inside there are air currents and flows that often cause product defects due the unpredictable temperature. To help ensure a quality product and process repeatability, you can use specialized sensors to monitor air temperature and airflow in oven interiors simultaneously with temperature monitoring.
The universal inputs on a datalogger can connect with air temperature and airflow sensors to give you the necessary multivalue data. Along with standard temperature monitoring, this allows you to create an accurate temperature profile. If you need to view data in real time, fast-response probes are available in clip-on, magnetic and combination types to capture both air temperature and surface measurements.
Typically, a temperature recorder’s analog input channels can accept different types of thermocouple probes (for example, Type T and Type K), capturing data at a high sample rate (for example, eight times each second). This creates the high level of detail needed to generate a full temperature profile for surface coating and through process applications.
Naturally, temperature recorders need thermal protection as they collect data alongside your parts or products during a run or sit within walk-in ovens. When housed within a stainless steel thermal barrier for heat absorption, the oven logger is safe to record for the duration of the specific time period listed for the enclosure. In this way, you can derive temperature profiles for different applications including curing, baking and more.
As the datalogger passes through the oven, it samples the interior temperature via the connected external temperature sensors trailing out of slots in the enclosure. The oven datalogger then stores and timestamps the temperature data onto internal memory for data download and analysis via USB interface with PC.
Document Product Quality and Validate Your Process
With the data collected over several runs, you should be able to optimize your process efficiency — all with the goal of increasing repeatability, product quality and profitability. This can take the form of minimizing curing times, ensuring more even part coverage and other benefits.
Equipped with this temperature data, you can use datalogger analysis software to display stored data and get the inside view on the heat treatment process. For example, you can begin fine-tuning the process parameters to achieve greater uniformity. Presented in chart or graph format, the time-stamped temperature data also serves as a QA report for vendors and customers. It can help prove that products have passed through the process at the proper temperature for the proper duration.
Many oven loggers also offer temperature profile software packages designed exclusively for the paint and finishing industry. These programs enable users to store preprogrammed cure data, to display calculated cure percentages and other desired features.
By using different types of temperature sensors to record the temperature data of an oven’s interior, it is possible to equip yourself with the high level of data needed to create a detailed temperature profile. Whether you are curing parts or replicating a batch of perfectly roasted potatoes, temperature profiling can serve as a guide needed to stabilize and optimize the process.