Probes & Assemblies Engineering Notes

Designing Temperature Probes and Assemblies

Step 1: Understand the Application Requirements

In 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 10kW at 25°C thermistor with a temperature coefficient of - 4.4%/°C, then in an ideal system, any 1°C change in temperature would immediately result in a corresponding change of exactly 440W. Unfortunately, real world applications do not exist under “ideal” conditions.

Remote temperature sensors require some form of packaging to allow them to withstand conditions such as rough handling, vibration and moisture. Many temperature sensors are so fragile that even normal installation can be harmful. But the very materials used to protect the sensor often prevent it from sensing temperature accurately.

Temperature probe design is based on performance trade-offs. Before you can begin the design process, it’s helpful to answer the following questions:

1. What is the minimum and maximum operating temperature for this sensor?
2. Within that temperature span, what temperature (or temperature range) is most important?
3. What is the desired accuracy at that temperature (or temperature range) in ±°C or ±°F.
4. How quickly should the sensor be able to respond to a change in temperature? Keep in mind that the definition of thermal time constant is the amount of time (in seconds) required for a sensor to change by 63.2% when subjected to a step function change in temperature. For example, if a sensor is at 0°C and is then plunged in a 100°C. bath, one time constant is the amount of time it takes the sensor to reach 63.2°C.
5. How will the sensor be coupled to the medium to be measured? There can be a significant amount of thermal resistance between the sensor and the medium which would result in a large temperature gradient, regardless of the rated accuracy of the sensor.
6. Are there any special requirements for isolation, either electrical (HIPOT) or noise (EMI/RFI)?
7. Are there any operating conditions that could adversely affect the sensor (severe vibration, temperature cycling, moisture, corrosive materials)?

Of course, price is always a major consideration and is based on usage quantity as well as the performance requirements noted above.

Step 2: Review Materials That Are Compatible With The Requirements

A typical temperature probe consists of a sensor in a housing with flying leads and, in some cases, a connector or terminals. There are an infinite number of variations of these materials, but having answers to the seven questions in Step 1 can greatly simplify the selection process. The following chart illustrates what information is necessary for selecting materials:

We have devoted several pages to the types of sensors, housings, wires and connectors that are considered to be "standards" at ATP. While it can be less expensive to use common materials, we welcome the opportunity to develop other "non-standard" products to meet your specific needs.

Step 3: Select A Temperature Sensor

For some applications, this process is very easy. Designing a temperature probe to be used in large quantities at temperatures below 300°C would almost certainly dictate using thermistors because of their low cost, accuracy, and ease of use. However, if you are designing an instrument to be used for precision temperature measurement, a platinum RTD would be the proper choice. Other applications have requirements for noise immunity that would indicate a current to temperature I.C. sensor.

There are many factors that go into the sensor selection process. ATP engineering has helped a multitude of customers choose a sensor to meet their requirements. If you can answer most of the questions in Step 1, we can recommend a temperature sensor that will work in your application.

Step 4: Evaluate Probe Construction

Once you have selected the sensor, housing, and wire or cable, it's 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 might be present in the housing (including condensation), how will the sensor and connections be protected from the moisture?
• Epoxy - If the sensor is to be held in place with epoxy or some other filler, what type of epoxy will be used? Is it compatible with the thermal expansion characteristics of the sensor and/or connections? Does the epoxy have to be thermally conductive?

These are some of the questions that are evaluated for every new design at ATP. We can guide you through this selection process to build a probe assembly that will provide years of service in your application.

Step 5: Design Validation

Once the preliminary design is complete, it’s important to validate the design by building and testing prototypes. This can be done by either testing the probe assembly in the actual application, or by developing a test procedure that includes measurements in temperature controlled oil baths, followed by long term stability or thermal cycle testing, hipot measurements (where applicable), and then re-testing in oil baths to see if any changes have occurred. ATP can assist customers in this phase of the design by helping develop a test plan and/or provide facilities and test equipment needed to accurately test the product.

Sensors

Temperature and Measurement

Temperature was one of the first physical parameters to be measured and, over the years, has been sensed in just about every way imaginable. Virtually any 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 that are still used in industry today are thermistors, resistance temperature detectors (RTDs), thermocouples (T/Cs), silicon PTCs, I.C. temperature to voltage transducers, temperature to current transducers, digital temperature sensors, infrared (IR) devices and others. Each sensing element has characteristics that make it better suited to certain types of applications. What follows is a sampling of the standard components that ATP uses to produce “custom” probe assemblies.

NTC Thermistors

Metal oxide ceramic semiconductors that decrease in resistance as temperature increases. NTCs have a well defined resistance versus temperature characteristic and are very sensitive to even the smallest temperature change. A typical temperature coefficient at 25°C is around -4.4%/°C.

NTCs can be precision trimmed to close tolerances. Although the NTC curve is non-linear, advances in A/D converters and microprocessors allow for simple circuit design. The inexpensive NTC is now the preferred sensor for applications from -50°C to 300°C.

Silicon PTC Thermistors

Silicon planar temperature sensors which provide a nearly linear response of +0.7%/°C. They have a useful temperature range from -55°C to +175°C.

Integrated Circuit (Temp to Voltage) Sensors

Integrated circuit temperature to voltage transducers generally provide a voltage output that is directly proportional to temperature. One class of sensors is calibrated in the Fahrenheit scale and provides an output of 10mV/°F and would have as an example an output voltage of 770mV at 77°F. The operating temperature range for these parts is from -50°F to 300°F. Another class of sensors is calibrated on the Celsius scale and provides an output of 10mV/°C and would have an output signal of 250mV at 25°C. The operating temperature range for these parts is from -55°C to 150°C.

Integrated Circuit (Temp to Current) Sensors

Temperature to current transducers; these two terminal integrated circuit devices produce an output current of 1mA/K from -50°C to 150°C. They exhibit excellent interference rejection and are easy to calibrate for direct conversion to temperature.

Digital Temperature Sensors

Typically convert temperature to a 9-bit digital value. They can measure temperature from -55°C to 125°C in 0.5°C increments and are scalable in Fahrenheit (0.9°F increments). These devices require no external components and can be powered from a data line. Their 1-wire interface requires only one port pin for communication with a processor.

Resistance Temperature Detectors (RTDs)

RTDs operate by exhibiting an increase in resistance with an increase in temperature in the order of 0.385%/°C for a platinum RTD. RTDs are most commonly made from platinum, nickel or copper. The copper and nickel versions are less expensive than platinum but have a lower upper operating temperature limit. Platinum is the most widely used material because of its wide temperature range (-200°C to 850°C), good repeatability and stability, and resistance to chemicals and corrosion.

Platinum RTDs are available in 100W, 500W and 1000W resistance values at 0°C, of which the 100W value is the most popular. There are two basic configurations for RTDs - wirewound and thin film. The wirewound construction consists of a sensing element that is connected to lead wires and supported by an insulator. For a thin film element, the metal is sputtered onto a target and then laser trimmed. The sensors are then covered with a dielectric layer to protect the sensor against mechanical and chemical damage.

PTC Thermistors

Ceramic semiconductors with a switching characteristic. PTCs have a very steep R/T characteristic and switch temperatures can be varied from 0°C to 150°C. Because their R/T characteristic is not well defined, PTC thermistors are normally only used as temperature sensors near their switch temperature.

Thermocouples

A thermocouple is made of two dissimilar metals, joined together at one end, that produces a voltage with a change in temperature. The junction of the two metals, called the sensing junction, is connected to extension wires. The other end of the sensing wires is known as the reference junction. Since the voltage created by a thermocouple is due to the bonding of two dissimilar metals, the introduction of other junctions to the circuit results in voltage changes that are referred to as cold junction errors. If the temperature at the connections is determined these errors can be corrected by a process called cold junction compensation. This is carried out at the receiving device, which is usually a signal conditioner. Generally speaking ATP does not package thermocouples.

Selecting a temperature sensor

The temperature sensor that is selected will be dependent upon the application and other factors including:

• Operating temperature range
• Accuracy needed
• Stability required
• Cost
• Ease of use
• Ability to package efficiently
• Circuitry available

The selection of a temperature sensor is a critical element in the design of the temperature assembly. The reliability, stability and accuracy of the entire system can be no better than that of the sensing element. At ATP we are a manufacturer of temperature sensors but also are knowledgeable of a wide variety of other sensing elements and their advantages and disadvantages. Our goal is to ensure that we provide you with the best possible solution to your application problem. We are always eager to design a custom product for your application that will meet and exceed your requirements.

Probe/Housing Selection

The primary purpose of the housing is to protect the sensing element. The selection of the housing will affect the response time of the sensor but can also 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. This will be affected to some extent by the epoxy/potting materials that are used to encapsulate the sensing element inside the housing. The selection of the housing will also depend upon the type of application, the temperature range needed, the type of sensing element to be used, among other factors. The housing can also protect the sensing element itself from the environment.

Applications for sensing include air sensors, surface sensors and immersion sensors. Housings for air temperature sensors are often simple, inexpensive devices such as molded plastic shells, deep-drawn brass or aluminum cylinders, or even stainless steel tubes. Surface sensors are designed to match the contour of the surface to be sensed, whether it's flat, curved, or round. Mounting holes or clips are often used to simplify installation of the sensor. Immersion sensors are designed so that the sensing element will be surrounded by the medium to be sensed, often times a liquid. The sensing portion will normally protrude through a hole or orifice while a portion of the assembly will remain outside the medium to allow for the signal to be read. Immersion housings are usually threaded to allow them to be screwed into a hole or opening.

The material used for the housing will be dependent upon the temperature range needed, response time, cost, and special considerations such as UL, FDA or NSF requirements in food and/or medical applications. Some of the materials available include stainless steel, copper, plastic, teflon, brass, aluminum, epoxy and heat shrinkable tubing.

Wire and Cable

ATP carries a large inventory of wire and cable, ranging from simple PVC insulated hookup wires to UL/CSA rated plenum cables. These extension leads are available 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 of the assembly.

Standard types of wire include hook-up wire, multi-conductor jacketed cable and twin conductor (zipcord) with stranded or solid conductors. The insulation around the conductor affects the temperature range available as well as the cost. Some insulation materials and their temperature ratings include:

• PVC 105°C
• XLPVC 125°C
• Kynar 175°C
• Teflon 200°C
• Fiberglass 250°C

Terminations/Connections

The simplest termination which is shown on most of the assemblies is a length of extension leads or cable which is cut and stripped. Connectors, plugs or receptacles may be added to the extension leads. Some assemblies may have the connector pins integrated with the housing.

The size and the mass of the connector or terminals should be considered during the design process. If these are too massive for the leads or cable, then a large stress or strain will be created which could cause a failure of the assembly. 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 housing for the sensor.

Typical configurations for the termination include:

• Quick connects (3/16" and 1/4")
• Spade/Lug
• Pin and Socket

ATP has in stock a wider variety of connector/terminals that are manufactured by a number of suppliers including Amp, Molex, Packard and others.