Temperature is one of the most common properties measured in industry. The reason for measuring temperature can be for a very low risk function such as indication functions, general feedback loops and troubleshooting assistance. At the other end of the spectrum, the reason for measuring temperature can be to manage high risk situations such as controlling critical parameters on expensive equipment, monitoring situations where extreme warranty costs may result, and, outside of the industrial process realm, even providing feedback in medical life-support applications.
Yet in each application, a common and widely used temperature-measurement process can become a high level focal point if certain steps are not proactively taken to evaluate and mitigate risks.
Sensor cost will always influence the sensor selected for a given application. Nonetheless, a thorough process will help identify:
- Which sensor type to use.
- The risks associated with that choice.
- How to minimize those risks.
The process outlined in this article is generalized to encompass three major types of industrial temperature sensors: thermistors, RTDs and thermocouples. While basic in nature, the approach can be adapted to applications where more specific elements or characteristics exist.
Establish the Basic Risk Level
There are many ways to evaluate risk, and many risk matrix methods exist; however, this step is at the most basic level. It is not a full risk evaluation but more of a first step in the “risk thought process.” It can be general and tied to an industry, but the essential goal of this step is to determine the risk associated if the component fails.
At one end of the spectrum, many applications have a low severity of failure in which a certain indication may not display correctly or the cost of replacement is minimal. At the other extreme, a temperature sensor failure could cause major damage, create a safety issue, or may incur a high cost of replacement.
A basic evaluation should be performed upfront to establish what could go wrong and the potential impact: low, medium or high risk. The result helps drive the decision about what type of sensor can or should be used.
Determine Sensor Type
To select an industrial sensor, first select which type of sensor is to be used based upon the application parameters. Generally speaking, the temperature range of the environment will be a significant driver for the selection of the proper sensor.
Next, determine what level of accuracy is required for the application, and what impact a reduced accuracy can have. Failure modes can have an impact on accuracy, so considering the level of accuracy required should be part of the failure-mode analysis.
Finally, review common failure modes and determine impact. Each type of sensor is constructed differently and may have both similar and unique failure modes. This step should be general in nature but also should move toward failure modes specific to the application.
When multiple options remain present for an application, other factors such as interface, product history and cost can be more significant factors. One common mistake is to bypass these initial steps and use the same sensor type as that which is currently being used in a different application. Often, this choice is made for expediency or simply because the internal history exists, the part number exists and even the vendor is established.
However, it is at this point in the process where the sensor must be adequately identified independently. Review existing sensor designs of the appropriate style to determine if an existing sensor for a separate application will indeed work for the current application. If this is the case, the existing design can be reviewed based upon the subsequent steps. If not, make the best selection for the application and then continue on the risk-mitigation assessment.
Select the Design Package for the Application
At this point, a more detailed risk analysis can be performed based upon the sensor selection. If more than one type of sensor fits the requirements, a comparison of sensors can be performed. Each failure mode will have a probability, and that will have an impact on design as well as qualification methods.
Sensor design options may include encapsulation materials, metal housings, heat- and moisture-resistant materials of construction, and lead configurations. Each sensor type will have design characteristics specific to that sensor. The overall design should include design parameters specific to the potential failure modes. The design should focus on the sensor package itself but also include the interface where there may be additional protection provided against most common failure modes. Strategies could include safety circuitry, duplication, calibration cycle, process inspections/service schedule, etc. For example, if the most common or likely failure mode is for an open condition to occur, determine the impact of an open-reading error and implement a safe mode when the open condition exists. Design parameters will be specific to the environment such as moisture, thermal shock, extreme heat or cold, chemical exposure, vibration or installation as well as characteristics of the sensing element itself.
Perform Design Qualification
A vital step is to determine how to ensure the design is satisfactory for the intended application throughout the performance lifecycle. Generally, this will require a form of extrapolation of results into gaining confidence that the design and construction of the sensor will perform as required for a predetermined period.
One important function of this step is to have an established goal for the lifecycle of the component. There are many options as to how to gain confidence, and this can be done with a comparison approach between sensor designs as well as with different vendors of similar designs.
Another critical aspect for this step is a detailed risk analysis. A minimal level of qualification can be performed for low risk applications. High risk applications require a high level of qualification Steps should be taken to establish and maintain a high level of confidence in both the design and the continued manufacturing of the sensor. Additionally, the qualification should have an increased level of focus on the higher severity or higher likelihood of potential failure modes.
The design qualification process should be established to gain a high level of confidence in the product selected. This should include communication with the vendor about historical performance in similar applications, internal testing capabilities and any additional pertinent data. (Data may be available from internal applications that are similar.) A clear performance-testing regimen can be established with acceptance limits that can be correlated directly to the application performance.
Qualification can be a combination of external services provided by the vendor, external services provided by a third party and internal qualification. Historical data, risk factors and application design parameters should drive the specifics of the qualification.
Once the parameters are established, test regimens can be established. Highly accelerated life testing (HALT) can be performed to establish maximum performance properties and establish potential design weaknesses. Design improvements can be made based upon this testing, or the information can be used to ensure product reliability through the manufacturing process. The qualification should begin with the design, continue into initial production of the sensor with a qualification of the sensor itself and conclude with an evaluation as a system or equipment package.
Additional potential qualification requirements may include:
- Vendor component qualification.
- Internal component qualification.
- Outside services such as HALT.
- Vendor quality program evaluation.
- Production part approval process (PPAP).
- Vendor process control and in-process testing.
- First article evaluation (internal).
- Lot qualification (internal or external).
- Preproduction runs and beta testing (internal).
- Certifications, qualifications and registration (external).
- Periodic inspection, qualification and calibration.
Using a basic approach to temperature sensor selection can help mitigate risk for each application. By using a process and instituting steps that are directly related to the application, the selection of the sensor becomes much easier.
Cost of the sensor and potential qualification of that sensor will have an impact; however, these must be compared with the cost associated with sensor failure. Working through the selection process with attention to potential failures as well as utilizing and maximizing the less expensive qualification options improves the odds for success in the application. A measured approach can increase the confidence while minimizing the risk and cost associated by choosing the correct sensor design.