subject: Temperature Measuring - What sensor is right for you! [print this page] Temperature Measuring - What sensor is right for you!
I'll talk about 3 basic temperature sensors: Thermocouples, NTC and Pt100 sensors.
Thermocouples
Temperature measurement using thermocouples is based on the thermoelectric effect. The thermoelectric effect is the direct conversion of temperature differences to electric voltage and vice versa. A thermoelectric device creates a voltage when there is a different temperature on each side. Conversely when a voltage is applied to it, it creates a temperature difference (known as the Peltier effect). At atomic scale (specifically, charge carriers), an applied temperature gradient causes charged carriers in the material, whether they are electrons or electron holes, to diffuse from the hot side to the cold side, similar to a classical gas that expands when heated; hence, the thermally induced current.One thermocpuple (T-Type)is made from copper/cupronickel (Cu-CuNi). . The cold end (reference point) of athermocouple always refers to 0 C. With electronic thermometers, this reference is calculated electronically by measuring the ambient temperature inside thethermometer. Because the tempera-ture sensor at the reference point needs some time to adjust to the ambient tempera-ture, care should be taken when measuringin a cold environment to allow sufficient time for the thermometer to adjust. This may happen for example if the thermometer has been left in a car overnight (temperature around +5 C) and is then taken into a room at +25 C. Conversely, it is also important to give the thermometer a few minutes to adjust when moving from a warm room (e.g. +10 C) to a cold one. Enough time must be allowed for a stable measured value to be reached.
Platinum resistance sensors (Pt100)
This method of measuring differs in principle from thermocouple probes. Platinum resistance sensors work by what is called the "PTC resistor" effect. Metals have the property of increasing their electrical resistance as the temperature rises. This property can be used to measure temperatures. A wafer-thin wire of the high-purity metal, platinum, is wound around a slender glass tube (one possible construction) until a specified resistance is reached. The commonest resistance value today is 100 ohms (unit of electrical resistance) at 0 C. This is known internationally as a Pt100 sensor. Another version of a Pt100 sensor is a ceramic base material (small leaf) on to which a platinum structure is etched. Sensors made in this way are extremely compact. Probes with platinum resistance sensors are among the most accurate sensors available. They are long-lasting and non-ageing. On the other hand, their response time is quite a bit longer than with thermocouples, depending on the design. Platinum resistance sensors are used in food technology, mainly in the laboratory.
Thermistor sensor (NTC)
Thermistors are modern, low-price temperature sensors made from mixed oxide ceramic. Their resistance fallsas the temperature rises. They thus behave in exactly the opposite way to Pt100 sensors. NTC sensors are inexpensive, accurate and suitable for many applications in the food industry. Thermometers using NTC sensors do not require reference point adjustment (see thermocouples). This makes them ideal for use in cold rooms and refrigeration chambers.
