OpAmps for Everyone. Simple Thermocouple Amplifier by OP07. Thermocouple application note from Pico Technology. Thermocouples are the most popular temperature sensors. They are cheap, interchangeable, have standard connectors and can measure a wide range of temperatures. The main limitation is accuracy, system errors of less than 1°C can be difficult to achieve.
How they work In 1822, an Estonian physician named Thomas Seebeck discovered (accidentally) that the junction between two metals generates a voltage which is a function of temperature. Thermocouples rely on this Seebeck effect. Although almost any two types of metal can be used to make a thermocouple, a number of standard types are used because they possess predictable output voltages and large temperature gradients. The diagram below shows a K type thermocouple, which is the most popular: Standard tables show the voltage produced by thermocouples at any given temperature, so for example in the above diagram, the K type thermocouple at 300 °C will produce 12.2 mV. Linearisation Thermocouples View the Pico range of thermocouples Noise. Build a Thermocouple Amplifier. Compiled by Lewis Loflin In 1821 Thomas Johann Seebeck discovered that a small voltage is produced when two different metals are joined together and the junction is heated.
By adding a second junction at a lower temperature, the effect was even more pronounced. That difference increases with temperature, and is between 1 and 70 micro volts per degree Celsius (uV / degree C) for various metal combinations. Thermocouples are used at much higher temperatures than thermistors,which change resistance with change in temperature, and don't produce a voltage at all. How the Thermocouple Works on Gas Furnaces A thermocouple (technically called a thermocouple junction) is a device that contains two differing metal wires welded at the ends and placed inside a protective metal (often steel) case.
As the thermocouple heats up, it produces a small amount of voltage and when it gets hot enough, the voltage will open the gas valve by using a solenoid operated by a 24 volt transformer. Thermocouple junction types- Grounded, Ungrounded, Exposed. Thermocouple Sensitivity Type K --- Chromel ( Nickel-Chromium Alloy ) / Alumel ( Nickel-Aluminium Alloy ) This is the most commomly used "general purpose" thermocouples. They are available in the ~ -200 °C to +1200 °C range. The characteristic of the thermocouple undergoes a step change when a magnetic material reaches its Curie point. This occurs for this thermocouple at ~ 352°C. Type J --- Iron / Constantan Limited range (~ -40 to +750 °C) makes type J less popular than type K. Type T --- Copper / Constantan Good for temperature in the ~ -200 to 350 °C range. Type E --- Chromel / Constantan ( Copper-Nickel Alloy ) Type E has a high output ~ 68µV/°C which makes it well suited to cryogenic use.
Type N --- Nicrosil (Nickel-Chromium - Silicon Alloy) / Nisil (Nicke l- Silicon Alloy) High stability and resistance to high temperature oxidation makes type N suitable for high temperature measurements without the cost of platinum (B, R, S) types.