How beneficial are thermocouple sensors in the automotive industry?

The automotive industry can benefit significantly from a wide range of temperature measurement solutions including a thermocouple sensor, infrared camera, pyrometers, and temperature controllers. This broad range of products can all provide the best solution for all type of automotive needs. In this article, we’ll explore how useful a thermocouple and temperature related solutions are within the automotive industry, so you can gain a better understanding of how they could help you if your operations are in this sector.

Thermocouples and general automotive testing

In the automotive testing process, measuring the temperature of various components is key. When it comes to measuring thermocouples in brakes, a thermocouple wire bundle can quickly get to a stage where the diameter starts to affect the structural integrity.

To effectively deal with this issue, you could get an extra thin and highly accurate type K thermocouple cable. Our thermocouple wires at TRM allow high performance temperature measurements that are consistent and reliable. These thermocouples are designed to handle rigorous conditions, which makes them ideal for use in automotives.

Brake block and disc temperature measurement

A major application for temperature measurement technology in the automotive industry includes measuring the temperature of brake system elements. The surface temperature of the disc has a direct impact on braking performance, which is why temperature measurement systems are essential in the manufacture of efficient braking systems as well as for regular monitoring in the finished product.

The measuring system needs to be able to record the wide range of temperatures that can be found on a brake disc and pad. This is typically done by fitting thermocouples to the disc and pad, and using collector rings in the circuit. Optical measurement systems like thermal cameras and scanners, are also used during brake tests. Thermocouple systems are useful in determining surface pressure distribution within brake pads too.

Exhaust gas temperature measuring

A high-quality thermocouple probe with wide temperature ranges and low response times are very beneficial for applications where surface contact is required, such as for monitoring automotive exhaust temperature. Probes can come in all the common thermocouple types for various applications (K, T, and J).

Turbo chargers are an important part of modern engines, with high rotational speeds and their versatility in coming in different shapes and sizes, a turbo charger is a complex subsystem in itself. Shielding the turbo from excessive temperatures is vital as it is regularly exposed to the high temperature exhaust stream.

This means a reliable thermocouple sensor with a fast response will play an essential role in the control loop. An example of this is a mineral insulated thermocouple cable. It’s thin, strong, and durable enough to last the full lifetime of the car, without compromising on mechanical strength.

Simulated exhaust temperature measurement

Manufacturers are required to test all components to their limits during automotive testing to see how they perform in conditions they are likely to encounter during the service life of the vehicle. Many polymer components that are found in modern automobiles have gone through heat stress tests if they are within close proximity to a heat source.

For example, the bumper is positioned close to the engine exhaust gas, where temperatures can get very high. This means the bumper material needs to be tested to make sure that it’s not negatively affected by the higher temperature of the exhaust, leading to thermal degradation of the polymer, or potentially even a fire, in the worst case.

In this modern testing process, the exhaust system is exposed to simulated heat from a custom electrical heater. A number of thermocouples are recommended for this to measure the temperature at different heat-vulnerable stress points.

Usually, complete accuracy is not important to this application, so thermocouple wires are chosen to ensure the application is cost-effective. This will be helpful in saving money as these heat tests often involve monitoring a large quantity of thermocouples in the entire route of the exhaust system.

 

Contact TRM today for an expertly engineered solution to all your temperature measuring and thermocouple needs within the automotive industry.

What is a thermocouple and where is it used?

What is a thermocouple?

A thermocouple is a sensor that is used for measuring temperature. The sensor has two dissimilar metal wires joined at one end and is connected to a thermometer or another thermocouple-capable device at the other end. When they are correctly configured, they can provide temperature measurements across a broad range of temperatures.

Stable thermocouples are highly versatile as temperature sensors, and so are often used in various applications, from industrial use to a regular thermocouple you can find on utilities and standard appliances. There are many different models and technical specifications for thermocouples, so it’s very important to understand the basics of how it works, its structure, and its ranges to get a better insight into what type of thermocouple and material are right for your application.

How does a thermocouple work?

When two wires made of different metals are joined together at both ends and one of the ends is then heated, there is a consistent current which flows in the thermoelectric circuit. If the circuit breaks at the centre, the net open circuit voltage is a part of the junction temperature and composition of the two metals. This means that when heat or cold is applied to the junction of the metals a voltage is produced that can be linked back to the temperature.

Thermocouple types

Thermocouples can come in multiple different calibrations or combinations of metals. The most commonly used are the base metals referred to as N, T, E, J, and K types. As well as this there are high temperature calibrations called noble metals. These are types R, S, C, and GB.

Where are thermocouples used?

Thermocouples are the most commonly used temperature sensors in the world because they can measure a wide range of temperatures, are durable, and are relatively inexpensive. When it comes to high temperatures, fast response, small temperatures, and a high vibration, you will likely find a thermocouple wire collecting the temperature measurements. Below we’ll be looking at just a few examples of where these sensors are used, so you will know how they can be applied in your operations.

Food applications

Thermocouples are used in many different types of applications within the food and drink industry, such as:

  • Clean-in-place sensors
  • Penetration probes
  • Oven control
  • Food chain monitoring
  • Hotplate control and monitoring
  • Steam kettle temperature control

Extruders

Extruders need high temperatures and pressures. Also, they have a unique thread adapter that works to position the tip of the sensor in the molten plastic under the high-pressure conditions located there.

Measuring low temperature

Type E, K, T, and N thermocouples can all be used to record low temperatures, as low as -200C. However, the alloys used need to be specially chosen for use at these temperatures to meet the published accuracies.

Many manufacturers ensure their alloys are calibrated for use from 0C and above. The same alloys can be used to measure down to -200C, but the accuracy might change slightly from the established values. If you purchase individual calibrations, you can determine offset values.

Furnaces

The right thermocouple for this application will depend on the furnace conditions it will be exposed to. When deciding on the right thermocouple, some factors that will need to be considered are:

  • The temperature capabilities of the thermocouple cables
  • Temperature capabilities of the sheath or protective coating (metal or ceramic)
  • The environment it will be used in (air, reducing, oxidising)
  • The configuration for mounting

Molten metal

It is difficult to measure the temperature of molten metal due to the high temperatures and harsh conditions involved. The only option for getting contact measurements in this area is to use Type K or N Base Metal thermocouples, or Types R, S, and B Platinum thermocouples.

When base metal thermocouples are used, the wires are generally large in diameter, with ceramic insulators and ceramic and/or metal protection tubes. The increased diameter of the Type K or N wires degrade slower to enable enough time for measurements to be taken before the high temperature conditions break down the wires.

Platinum thermocouple wires, unlike the base metal Type K and N, will become soft instead of becoming corroded, due to long term annealing and ultimately fail because of grain growth.

Contact TRM today for help with temperature measurement solutions for your business’ operations.

The Green Rot Phenomenon

MICC Ltd recommendations on green rot phenomenon exhibited in Type K thermocouples used in heat treatment applications with hydrogen atmosphere and operating at high temperatures exceeding 900 °C

Issue

Thermocouples recovered from a customer site were exhibiting large negative drift, open circuit and upon inspection had become very brittle, in places broken and exhibited so-called “green rot”.

The application was an annealing furnace operating at 1050 °C with a hydrogen atmosphere.

Green rot corrosion

Green rot is a type of corrosion that occurs on the positive leg of a type K thermocouple (and type E) if two conditions are met:-

  1. application temperature of approximately 800 – 1100 °C and
  2. the thermocouple is exposed to the low concentration of oxygen environment (for example, steam, reducing or cyclically reducing and oxidising atmospheres, or stagnant atmosphere within a protection tube)

The name “green rot” is derived from the greenish shimmering colour forming on the positive leg.

The cause of it is the selective oxidation of chromium, and related to it, chromium depletion in the surrounding area, which occurs in the NiCr alloy of the positive leg of the thermocouple. The negative leg material is not affected.

The consequence of this change in chemical composition of the positive leg of the thermocouple is a drift of the measured value caused by decreasing thermoelectric voltage. This effect is accelerated if there is a shortage of oxygen (reducing atmosphere), since a complete oxide layer, which would protect it from further oxidation of the chromium, cannot be formed on the surface of the thermocouple.

The problem sets in slowly as the wire degrades. If it is not discovered in time, measuring errors of tens of degrees can occur. In extreme cases, assuming the wire does not break, negative measurement errors of 50°C or more may be found. The thermocouple calibration is permanently disrupted by this phenomenon.

In addition to the negative measurement errors the thermocouple wire will exhibit brittleness and eventual failure.

Recommendations

In order to provide a solution for the customer the following solutions were recommended (set out in order of cost, technical solution and ease of implementation)

  1. keeping of the dewpoint in hydrogen or cracked ammonia atmospheres below -42℃
  2. changing to the use of type N for the thermocouple pair. The type N (NiCrSi-NiSi) thermocouple has an advantage in this regard due to its Silicon content. A more robust protective oxide layer forms on its surface under the same conditions due to the silicon..
  3. the use of Type K thermocouples which incorporate sacrificial titanium wires to delay the reaction
  4. introduction of additional oxygen into the Type K protection tube through the positive overpressure from a suitable air supply
  5. introduction of an inert atmosphere into the Type K protection tube through the positive overpressure from a suitable inert gas supply
  6. the use of Type S thermocouple but it will require a ceramic sheath

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