What are Thermocouples?

Thermocouples are a group of highly versatile devices used for measuring very high or low temperatures. These types of sensors are very durable and fast reacting. Thermocouples are constructed from a pairing of two metals whose standardized pairings are categorized by letters, and differ in character based on their constituent metals.

Thermocouples are based on the principle that when two dissimilar metals are fused together, their junction produces a thermoelectric voltage. This voltage is affected by temperature. When measuring temperature, the hot junction is placed in the process, where the unknown temperature is. The connection wires at the other end of the thermocouple is where the reference, or ambient temperature is. When the two metals, one that is negative and one that is positive, form a circuit, they allow a current to flow between them. It is the temperature difference between the hot junction, and the terminal connection that create an imbalance in the voltages, and thus, a current is formed that can be transmitted as a temperature read

 What are RTD’s?

RTD’s or Resistance Temperature Detectors, are a group of widely used devices that are able to measure temperature with very high accuracy, whithin + or – 0.1 of the exact temperature. Though they cannot withstand temperatures as high as some thermocouples can, they are able to measure up to 500 C.

 How do RTD’s Work?

RTD’s are based on the principle that as a metal heats up, it’s resistance increases. The linear correlation between resistance and temperature is used when converting the reading from the sensor, in ohms, into a temperature reading. Platinum, the metal used most commonly in RTD’s (called the Pt100), allows for the extreme accuracy and stability that RTD’s are known for.

 What are Thermistors?

Thermistors are a group of economic temperature sensors. They are similar to RTD’s in that they measure temperature through resistance. They are very accurate and fast-reacting, but function within a much more limited temperature range that RTD’s.

 How do Thermistors Work?

Thermistors function similarly to RTD’s in that they are also resistors. However, due to their defining characteristics, the resistance in thermistors increases exponentially as temperature is added. This non-linearity accounts for the extremely quick reaction time in thermistors, but restricts their functional temperature range.

 What is Ageing?

Ageing is the process by which temperature sensors lose their accuracy due to exposure to harsh process environments and repeated, extreme temperature changes. After a time, the electro and chemical properties of the sensor change, causing a loss in accuracy of temperature readings. Thermocouples, are normally more prone to ageing than RTD’s, however, conditions are a major factor in this equation. Besides choosing the appropriate sensor for each application, there is not much to prevent the inevetibility of ageing.

 Thermocouple Types