Temperature & Thermocouple Sensors

 Temperature & Thermocouple Sensors

 Temperature sensor

A temperature sensor is an electronic device that measures the temperature of its environment and converts the input data into electronic data to record, monitor, or signal temperature changes. 

Introduction

Temperature is the most often-measured environmental quantity. This might be expected since most physical, electronic, chemical, mechanical, and biological systems are affected by temperature. Certain chemical reactions, biological processes, and even electronic circuits perform best within limited temperature ranges. Temperature is one of the most commonly measured variables and it is therefore not surprising that there are many ways of sensing it. Temperature sensing can be done either through direct contact with the heating source or remotely, without direct contact with the sou


rce using radiated energy instead. There are a wide variety of temperature sensors on the market today, including Thermocouples, Resistance Temperature Detectors (RTDs), Thermistors, Infrared, and Semiconductor Sensors.

Temperature sensor types

  • Resistance temperature detector (RTD)
  • Negative temperature coefficient (NTC) Thermistor 
  • Thermocouple
  • Semiconductor based sensor 

Resistance temperature detector (RTD)

The RTD is a temperature-sensing device whose resistance changes with temperature. Typically built from platinum, though devices made from nickel or copper are not uncommon, RTDs can take many different shapes like wire wound, thin film. To measure the resistance across an RTD, apply a constant current, measure the resulting voltage, and determine the RTD resistance. RTDs exhibit fairly linear resistance to temperature curves over their operating regions and any nonlinearity is highly predictable and repeatable. The PT100 RTD evaluation board uses surface mount RTD to measure temperature. An external 2, 3, or 4-wire PT100 can also be associated with measure temperature in remote areas. The RTDs are biased using a constant current source. To reduce self-heat due to power dissipation, the current magnitude is moderately low. The circuit shown in the figure is the constant current source uses a reference voltage, one amplifier, and a PNP transistor.

 Thermistor (Negative temperature coefficient)

Similar to the RTD, the thermistor is a temperature-sensing device whose resistance changes with temperature. Thermistors, however, are made from semiconductor materials. Resistance is determined in the same manner as the RTD, but thermistors exhibit a highly nonlinear resistance vs. temperature curve. Thus, in the thermistors operating range, we can see a large resistance change for a very small temperature change. This makes for a highly sensitive device, ideal for set-point applications.

Thermocouple

It is a type of temperature sensor, which is made by joining two dissimilar metals at one end. The joined end is referred to as the HOT JUNCTION. The other end of these dissimilar metals is referred to as the COLD END or COLD JUNCTION. The cold junction is formed at the last point of thermocouple material. If there is a difference in temperature between the hot junction and cold junction, a small voltage is created. This voltage is referred to as an EMF (electro-motive force) and can be measured and in turn used to indicate temperature.

Semiconductor based sensor

They are classified into different types like Voltage output, Current output, Digital output, Resistance output silicon, and Diode temperature sensors. Modern semiconductor temperature sensors offer high accuracy and high linearity over an operating range of about 55°C to +150°C. Internal amplifiers can scale the output to convenient values, such as 10mV/°C. They are also useful in cold-junction compensation circuits for wide temperature range thermocouples.

Temperature Sensor Applications

There are Temperature Sensor applications in many industries including 

  • medical, 
  • motorsport, 
  • HVAC, 
  • agriculture, 
  • industrial, 
  • aerospace and 
  • automotive. 

Here are some of the specific temperature sensor applications which we have come across.

  • Motors– there are many different aspects of motors and most of these require temperature measurement to ensure the motor itself does not overheat.
  • Surface plates – ring terminal temperature sensors are often used on surface plates as they can be mounted onto a flat surface and measure temperature effectively.
  • Home appliances – kettles, toasters, washing machines, dishwashers and coffee machines will all contain temperature sensors.
  • Computers– within computers there are temperature sensors to ensure the system does not overheat
  • Industrial equipment – temperature sensors used within these applications will need to be robust as the environment can be very demanding.
  • Warming Electrical Radiators – NTC thermistors are used to control the heat on electric radiators.
  • Exhaust Gas Monitoring on Motorsport Vehicles – Motorsport temperature sensors need to be highly reliable and durable to ensure performance is not compromised in this harsh environment.
  • Food Production; 3D printed chocolates – temperature sensors are used to monitor the temperature of the melted chocolate for 3D printing.
  • Alcohol breathalyser – thermistors are used within alcohol breathalysers to measure the temperature of the subject's breath.

Other Temperature Sensor Applications Include:

  • Transit
  • HVAC
  • Power and Utilities
  • Calibration and Instrumentation
  • Heat Exchangers
  • Industrial Processes
  • Drilling
  • Heating/cooling systems
  • Laboratory
  • Energy

 Thermocouple Sensors

A thermocouple is a junction between two different metals that produces a voltage related to a temperature difference.

A thermocouple is a temperature-measuring device consisting of two dissimilar conductors that contact each other at one or more spots. It produces a voltage when the temperature of one of the spots differs from the reference temperature at other parts of the circuit.

Principle of Operation

Thermocouples are based on the principle that two wires made of dissimilar materials connected at either end will generate a potential between the two ends that is a function of the materials and temperature difference between the two ends (also called the Seebeck Effect).

Types of Thermocouple

Before discussing the various types of thermocouples, it should be noted that a thermocouple is often enclosed in a protective sheath to isolate it from the local atmosphere. This protective sheath drastically reduces the effects of corrosion.

Type K Thermocouple (Nickel-Chromium / Nickel-Alumel): 

The type K is the most common type of thermocouple. It’s inexpensive, accurate, reliable, and has a wide temperature range. The type K is commonly found in nuclear applications because of its relative radiation hardness. Maximum continuous temperature is around 1,100C.

Type K Temperature Range:

Thermocouple grade wire, –454 to 2,300F (–270 to 1260C)

Extension wire, 32 to 392F (0 to 200C)

Type K Accuracy (whichever is greater):

Standard: +/- 2.2C or +/- .75%

Special Limits of Error: +/- 1.1C or 0.4%

Type J Thermocouple (Iron/Constantan): 

The type J is also very common. It has a smaller temperature range and a shorter lifespan at higher temperatures than the Type K. It is equivalent to the Type K in terms of expense and reliability.

Type J Temperature Range:

Thermocouple grade wire, -346 to 1,400F (-210 to 760C)

Extension wire, 32 to 392F (0 to 200C)

Type J Accuracy (whichever is greater):

Standard: +/- 2.2C or +/- .75%

Special Limits of Error: +/- 1.1C or 0.4%

Consideration for bare wire type J thermocouple applications:

The Type J Is Well Suited To Oxidizing Atmospheres

Type T Thermocouple (Copper/Constantan): 

The Type T is a very stable thermocouple and is often used in extremely low temperature applications such as cryogenics or ultra low freezers. It is found in other laboratory environments as well. The type T has excellent repeatability between –380F to 392F (–200C to 200C)..

Type T Temperature Range:

Thermocouple grade wire, -454 to 700F (-270 to 370C)

Extension wire, 32 to 392F (0 to 200C)

Type T Accuracy (whichever is greater):

Standard: +/- 1.0C or +/- .75%

Special Limits of Error: +/- 0.5C or 0.4%

Consideration for bare wire type T thermocouple applications:

The Type T Is Well Suited To Oxidizing Atmospheres

Type E Thermocouple (Nickel-Chromium/Constantan): 

The Type E has a stronger signal & higher accuracy than the Type K or Type J at moderate temperature ranges of 1,000F and lower. The type E is also more stable than the type K, which adds to its accuracy.

Type E Temperature Range:

Thermocouple grade wire, -454 to 1600F (-270 to 870C)

Extension wire, 32 to 392F (0 to 200C)

Type E Accuracy (whichever is greater):

Standard: +/- 1.7C or +/- 0.5%

Special Limits of Error: +/- 1.0C or 0.4%

Consideration for bare wire type E thermocouple applications:

In oxiding or inert atmospheres the operating range is roughly –418F to 1,652F (–250C to 900C).

Type N Thermocouple (Nicrosil / Nisil): 

The Type N shares the same accuracy and temperature limits as the Type K. The type N is slightly more expensive. The type N has better repeatability between 572F to 932F (300C to 500C) compared to the type K.

Type N Temperature Range:

Maximum continuous operating temperature: up to 2,300F (1,260C)

Short term use: 2,336F (1,280C)

Thermocouple grade wire, -454 to 2300F (-270 to 1,260C)

Extension wire, 32 to 392F (0 to 200C)

Type N Accuracy (whichever is greater):

Standard: +/- 2.2C or +/- .75%

Special Limits of Error: +/- 1.1C or 0.4%

Consideration for bare wire type E thermocouple applications:

The type N holds up better to oxidation at high temperatures when compared to the type K.

Type S Thermocouple (Platinum Rhodium - 10% / Platinum): 

The Type S is used in very high temperature applications. It is commonly found in the BioTech and Pharmaceutical industries. It is sometimes used in lower temperature applications because of its high accuracy and stability. The type S is often used with a ceramic protection tube.

Type S Temperature Range:

Maximum continuous operating temperature: up to 2,912F (1600C)

Short term use: up to 3,092F (1,700C)

Thermocouple grade wire, -58 to 2700F (-50 to 1480C)

Extension wire, 32 to 392F (0 to 200C)

Accuracy (whichever is greater):

Standard: +/- 1.5C or +/- .25%

Special Limits of Error: +/- 0.6C or 0.1%

Consideration for bare wire type J thermocouple applications:

The Type S can be used in inert and oxidizing atmospheres up to 2,912F (1600C) continuously and up 3,092F (1,700C) for short term use.

Type R Thermocouple (Platinum Rhodium -13% / Platinum): 

The Type R is used in very high temperature applications. It has a higher percentage of Rhodium than the Type S, which makes it more expensive. The Type R is very similar to the Type S in terms of performance. It is sometimes used in lower temperature applications because of its high accuracy and stability. Type R has a slightly higher output and improved stability over the type S.

Type R Temperature Range:

Thermocouple grade wire, -58 to 2700F (-50 to 1480C)

Extension wire, 32 to 392F (0 to 200C)

Accuracy (whichever is greater):

Standard: +/- 1.5C or +/- .25%

Special Limits of Error: +/- 0.6C or 0.1%

Type B Thermocouple (Platinum Rhodium – 30% / Platinum Rhodium – 6%): 

The Type B thermocouple is used in extremely high temperature applications. It has the highest temperature limit of all of the thermocouples listed above. It maintains a high level of accuracy and stability at very high temperatures. The type B has a lower output than the other noble metals (type R & type S) at temperatures below 1,112F (600C).

Type B Temperature Range:

Thermocouple grade wire, 32 to 3100F (0 to 1700C)

Extension wire, 32 to 212F (0 to 100C)

Accuracy (whichever is greater):

Standard: +/- 0.5%

Special Limits of Error: +/- 0.25%

Advantages

It is rugged in construction

Covers a wide temperature range

Using extension leads and compensating cables, long transmission distances for temperature measurement possible. This is most suitable for temperature measurement of industrial furnaces

Comparatively cheaper in cost

Calibration can be easily checked

Offers good reproducibility

High speed of response

Satisfactory measurement accuracy

Limitations

For accurate temperature measurements, cold junction compensation is necessary

The emf induced versus temperature characteristics is somewhat nonlinear

Stray voltage pickup is possible

In many applications, amplification of signal is required

Applications

  • Type B, S, R and K thermocouples are used extensively in the steel and iron industries to monitor temperatures and chemistry throughout the steel making process.
  • Gas-fed heating appliances such as ovens & water heaters.
  • In the testing of prototype electrical and mechanical apparatus.
  • Steel industry
  • Gas appliance safety
  • Thermopile radiation sensors
  • Manufacturing
  • Power production
  • Thermoelectric cooling
  • Process plants
  • Thermocouple as vacuum gauge


Comments

  1. I appreciate your efforts which you have put into this article. This post provides a good idea about Thermistors. Genuinely, it is a useful article to increase our knowledge. Thanks for sharing such articles here.
    Custom Thermistors Manufacturers in USA

    ReplyDelete
  2. I really impressed by your writing style and knowledge in post which you have shared here. This post gives a interesting information about Thermistors. It is useful article for us. Surgical Devices Linear Motor

    ReplyDelete

Post a Comment