Calibrating industrial temperature sensors – gilautomation

In the realm of process control, industrial temperature measurement and control stand out as highly dynamic subsets. Sensors of various sizes, shapes, lengths, and accuracies flood the market, catering to diverse temperature applications. However, in many instances, the selection of probes and probe assemblies isn’t solely based on their suitability for the task at hand but rather influenced by factors like price, expediency, or availability.

During the early design phase of a process, it’s crucial to meticulously plan the calibration of industrial temperature sensors. This proactive approach ensures a more precise alignment of the sensor with the application, resulting in enhanced overall accuracy and diminished intrinsic uncertainty.

To conduct a proper calibration of a temperature sensor, whether in use or pre-deployment, four pivotal factors demand attention:

Grasping the dynamics of the entire process.

Opting for the sensor most compatible with the application.

Calibrating the system to closely mimic the process.

Supervising the sensor’s recalibration to uphold quality assurance.

In today’s industrial landscape, the significance of precise industrial process temperature measurements has soared. The drive to refine the quality or efficiency of industrial operations has spurred a rapid proliferation of temperature sensors within these systems, accompanied by heightened demands for measurement accuracy. Amidst myriad sources of measurement error, this discourse zeroes in on the sensors themselves. Temperature sensors, typically tailored for specific measurement applications, often lack consideration for ease of calibration or support, leading to a diverse array of shapes, sizes, and types that may compromise calibration accuracy and exacerbate support challenges. Consequently, sensors chosen for a given application may not always be the optimal choice, introducing additional measurement errors.

To mitigate measurement errors to acceptable levels, every facet of the process measurement and its traceability warrants scrutiny. This entails evaluating the sensor’s suitability for the intended measurement (its compatibility with the process), alongside sensor calibration and stability in the operational environment. Optimal outcomes are achieved when these considerations are addressed early in the process design or prior to implementing the measurement scheme. Rectifying an inadequate measurement system post-implementation is often more arduous and costly than installing a satisfactory one from the outset.

Towards this objective, four critical factors significantly influence the measurement system’s capability:

Understanding the dynamics of the entire process under scrutiny.

Selecting a sensor that aptly suits the application.

Calibrating the sensor to closely replicate the process.

Overseeing sensor calibration to ensure quality assurance.

Process Evaluation

The initial step in devising a temperature monitoring or control solution entails a comprehensive examination of the process, particularly its dynamic thermal properties. Key queries to address include:

Identifying the heat source.

Understanding the mode of heat transmission.

Assessing whether the process is static or dynamic.

Clarifying the purpose of monitoring.

Determining the required accuracy.

Evaluating potential exposure to harsh environments, contaminants, or chemicals.

Anticipating the implications of measurement errors.

For instance, consider monitoring a standard autoclave independently of its internal controls. Understanding the thermal dynamics within the unit involves probing into various aspects such as heat generation, circulation within the sterilization area, heat transfer to the labware, impact of gas purges or vacuum on sensors, and potential effects of contaminants released from treated items.

Armed with insights into the internal heat dynamics of the sterilizer, one can aptly determine the type and placement of sensors required. Different locations may yield varying temperature profiles, necessitating multiple tests to comprehensively grasp temperature variations before devising an optimal monitoring strategy.

Sensor Selection and Placement

The selection and positioning of sensors emerge as critical determinants of measurement accuracy, often exerting more influence than any other factor. While seemingly basic, this aspect is frequently overlooked. The chosen sensor must seamlessly integrate with the application, such as opting for an air probe for gas measurements and an immersion probe for liquid measurements. Considerations extend to stem effect when specifying probe length and diameter, assessing the sensor’s susceptibility to installation-induced effects, and ensuring robustness of wire and transition junctions against extreme temperatures and moisture ingress if required.

The choice of sensor type (e.g., PRT, thermistor, or thermocouple) hinges on factors like temperature range, accuracy requirements, calibration needs, sensitivity, size, and compatibility with electronics. PRTs excel in high-accuracy applications across a wide temperature range, whereas thermistors offer high accuracy but within a narrower temperature band. Thermocouples find favor in low-accuracy or high-temperature scenarios, or where harsh environments are encountered.

Moreover, the impact of exposure to the process on sensor performance cannot be underestimated. For instance, certain industrial ovens and autoclaves employ exposed type K thermocouples for control sensing. While suitable for control purposes, type K wire rapidly deteriorates in such environments if not shielded from product gases or vacuum, leading to erroneous measurements. Hence, it’s imperative to select sensors or protective measures that align with the operational environment to ensure accurate readings.

Additionally, the stress imposed by the process on sensors warrants consideration. Sensors operating near their operational limits are more prone to drift, with vibration exacerbating this phenomenon. Hence, selecting sensors capable of withstanding process-induced stresses is paramount, lest measurement accuracy suffers.

Sensor placement is another critical factor often overlooked. Placing the sensor’s sensing area in the critical temperature zone is imperative for accurate readings. While this may entail inconvenience or disruption to the process, the importance of precise measurements outweighs such concerns.

For instance, when measuring fluid temperature in a pipe, placing the sensor inside the pipe yields more accurate readings than attaching it to the exterior surface. Balancing sensor type, placement, and desired accuracy is essential, with certain sensor types better suited for specific installations. For instance, thermistors may be preferred for precise measurements in confined spaces, where accuracy trumps sensor type.

Similarly, considerations extend to scenarios involving thermowells. For instance, when measuring the temperature of an abrasive slurry in a pipe, the effect of the thermowell on measurement accuracy may necessitate the use of a more accurate sensor or accept lower accuracy within cost constraints. Thus, sensor type, placement, and accuracy requirements should be harmonized to arrive at an optimal solution.

Sensor Calibration

Temperature sensors, being transducers, require calibration and periodic recalibration to uphold measurement accuracy. However, the diverse array of sensor types, shapes, sizes, and unique characteristics poses a challenge to sensor calibration in process temperature applications. Achieving a calibration that accurately reflects the installation conditions demands meticulous attention to sensor selection and calibration methodology.

One approach involves selecting sensors amenable to easy calibration, with characteristics facilitating validity in the installed system. For instance, when dealing with a 6-inch diameter pipe, opting for a small diameter probe of 6 or 8 inches with thin wall construction enhances calibration accuracy, owing to the smaller diameter facilitating high accuracy with minimal immersion, while the additional length eases calibration.

Alternatively, calibration should mirror the thermal characteristics of the process as closely as possible. Ideally, sensors should be calibrated in situ by comparison to reference equipment. If in situ calibration isn’t feasible and the sensor must be removed, calibration should be tailored to the application. For instance, immersion probes should be calibrated through immersion, with immersion depth matching operational conditions. Similarly, surface temperature measurement necessitates calibration on a surface calibrator, with considerations for readout device compatibility to mitigate errors arising from different excitation currents.

In many cases, calibrating the sensor and readout as a system yields optimal results, particularly when field-deployable calibrators are utilized in tandem with an understanding