Step 1: Understand the Application RequirementsIn an ideal temperature measurement system, the sensing element would be very small and fully encased or surrounded by the medium to be measured. Even the smallest change in temperature would cause the sensor to respond immediately with a corresponding change in the output parameter. For example, if the sensor is a 10 kohm at 77oF (25oC) thermistor with a temperature coefficient of -4.4 percent/oC, then in an ideal system, any 1oC change in temperature would immediately result in a corresponding change of exactly 440 W. Unfortunately, real-world applications do not allow these ideal conditions.
Temperature probe design is based on performance tradeoffs. Before you can begin the design process, it is helpful to answer the following questions:
- What is the minimum and maximum operating temperatures for this sensor?
- Within that temperature span, what temperature (or temperature range) is most important?
- What is the desired accuracy at that temperature (or temperature range) in +/-oC, +/-oF or +/-K?
- How quickly should the sensor be able to respond to a change in temperature?
- How will the sensor be coupled to the medium to be measured?
- Are there any special requirements for isolation, either electrical (hipot) or noise (EMI/RFI)?
- Are there any operating conditions such as severe vibration, temperature cycling, moisture or corrosive materials that could adversely affect the sensor?
Of course, price is always a consideration, and the performance requirements will help dictate sensor cost.
Step 2: Review Materials that Are Compatible with the RequirementsA typical temperature probe consists of a sensor in a housing with flying leads and a connector or terminals. These materials can vary infinitely, but answering the questions in Step 1 can greatly simplify the selection process. Table 1 illustrates what information is necessary to select materials.
Step 3: Select a Temperature SensorJust about every sensing element that changes some measurable property with temperature has been used as a basis for determining the temperature of an object or process. Among the many methods still used in industry today are thermistors, resistance temperature devices (RTDs), thermocouples, silicon PTCs, IC temperature-to-voltage transducers, temperature-to-current transducers, digital temperature sensors, infrared devices and others.
Selecting a temperature sensor will depend on the application and other factors such as operating temperature range, accuracy needed, stability required, cost, ease of use, ability to package efficiently and available circuitry. Table 2 offers a comparison of typical temperature sensing elements.
The selection of a temperature sensor is critical when designing the temperature assembly. The reliability, stability and accuracy of the entire system can be no better than that of the sensing element. Many factors go into the sensor selection process. If you can answer most of the questions in Step 1, you should be able to select a temperature sensor that will operate effectively in your application.
Step 4: Evaluate Probe ConstructionOnce you have selected the sensor, you will need to review the materials for the housing as well as the wire or cable and terminations or connections.
Housing selection for a temperature sensor is important in that the housing acts as a transfer mechanism between the medium to be measured and the sensor. The housing will affect sensor response and can affect the accuracy of the entire system. It is important that the housing is matched with the sensor to allow for good thermal transfer between the two. Thermal transfer will be affected to some extent by the epoxy/potting materials used to encapsulate the sensing element in the housing. Housing selection also will depend on:
- Type of application.
- Temperature range needed.
- Type of sensing element to be used.
- Response time required.
- Application-specific considerations such as UL, FDA or NSF requirements in food and medical applications.
The housing also can protect the sensing element itself from the environment. Some of the housing materials available include stainless steel, copper, plastic, Teflon, brass, aluminum, epoxy and heat-shrinkable tubing.
When it comes to wire and cable for temperature sensing assemblies, the choices can range from simple PVC-insulated hookup wires to UL/CSA-rated plenum cables. These extension leads are offered in many conductor alloys, gauge sizes, insulations and constructions. The physical dimensions, electrical characteristics and thermal properties of the leads or cable should be given appropriate attention during the development.
Terminations and connectors are another consideration. The simplest termination is a length of extension lead or cable that is cut and stripped. Connectors, plugs or receptacles may be added to the extension leads. Some assemblies may integrate the connector pins with the housing. The size and the mass of the connector or terminals should be considered during the design process. If they are too massive for the leads or cable, then a large stress or strain will be created, which could cause the assembly to fail. Proper strain relief of the leads or cable is an important consideration in any sensor design. This applies not only to the selection of the termination but also to the design of the sensor housing.
Typical termination configurations include quick connects, spade/lug, and pin and socket. The manufacturer can guide you through the selection process to build a probe assembly that will provide years of service in your application.
Once the materials have been selected, it is time to look at how these materials will be assembled. Some of the major issues to address are:
Connections. Will the connections between the sensor and the lead wires be soldered, spliced or welded? Where will the connections be located?
Insulation. How will the sensor be electrically isolated from the housing? What type of insulation will be used over the connections?
Moisture Resistance. If the sensing element is affected by moisture and the application is such that moisture (including condensation) might be present in the housing, how will the sensor and connections be protected from moisture?
Epoxy. If the sensor is to be held in place with epoxy or some other filler, what type will be used? Is it compatible with the thermal expansion characteristics of the sensor and connections? Does the epoxy need to be thermally conductive?