Temperature sensors are utilized in diverse applications like food processing, HVAC environmental control, medical devices, chemical handling and automotive within the hood monitoring (e.g., coolant, air intake, cylinder head temperatures, etc.). Temperature sensors tend to measure heat to ensure that a procedure is either; staying in a certain range, providing safe use of that application, or meeting a mandatory condition when confronted with extreme heat, hazards, or inaccessible measuring points.
The two main main flavors: contact and noncontact temperature sensors. Contact sensors include thermocouples and thermistors that touch the object they are to measure, and noncontact sensors measure the thermal radiation a source of heat releases to determine its temperature. The latter group measures temperature from the distance and often are being used in hazardous environments.
A temperature sensor thermocouple is a couple of junctions which are formed from two different and dissimilar metals. One junction represents a reference temperature and also the other junction is definitely the temperature to be measured. They work when a temperature difference causes a voltage (See beck effect) that may be temperature dependent, and this voltage is, subsequently, transformed into a temperature reading. TCs are used since they are inexpensive, rugged, and reliable, will not need a battery, and can be utilized over a wide temperature range. Thermocouples can achieve good performance approximately 2,750°C and can even be employed for short periods at temperatures approximately 3,000°C and as little as -250°C.
Thermistors, like thermocouples, will also be inexpensive, easily accessible, user friendly, and adaptable temperature sensors. They are utilized, however, for taking simple temperature measurements rather than for top temperature applications. They are made of semiconductor material by using a resistivity which is especially responsive to temperature. The resistance of any thermistor decreases with increasing temperature to ensure when temperature changes, the resistance change is predictable. These are popular as inrush current limiters, temperature sensors, self-resetting overcurrent protectors, and self-regulating heating elements.
Thermistors differ from resistance temperature detectors (RTD) because (1) the material employed for RTDs is pure metal and (2) the temperature response of the two is different. Thermistors may be classified into 2 types; dependant upon the indication of k (this function signifies the Steinhart-Hart Thermistor Equation to convert thermistor potential to deal with temperature in degrees Kelvin). If k is positive, the resistance increases with increasing temperature, and also the device is named a positive temperature coefficient (PTC) thermistor. If k is negative, the resistance decreases with increasing temperature, and the device is called a negative temperature coefficient (NTC) thermistor.
For example of NTC thermistors, we shall examine the GE Type MA series thermistor assemblies created for intermittent or continual patient temperature monitoring. This application demands repeatability and fast response, specially when used with the care of infants and through general anesthesia.
The MA300 (Figure 1) makes routine continuous patient temperature monitoring feasible by using the simplicity of the patient’s skin site as an indicator of body temperature. The stainless housing used would work both for reusable and disposable applications, while maintaining maximum patient comfort. Nominal resistance values of 2,252, 3,000, 5,000, and ten thousand O at 25°C can be found.
Resistance temperature detectors (RTDs) are temperature sensors with a resistor that changes resistive value simultaneously with temperature changes. Accurate and recognized for repeatability and stability, RTDs may be used with a wide temperature cover anything from -50°C to 500°C for thin film and -200°C to 850°C for your wire-wound variety.
Thin-film RTD elements have got a thin layer of platinum on the substrate. A pattern is created that offers an electric circuit that may be trimmed to offer a unique resistance. Lead wires are attached, along with the assembly is coated to protect the two film and connections. In contrast, wire-wound elements may be coils of wire packaged in the ceramic or glass tube, or they are often wound around glass or ceramic material.
An RTD example is Honewell’s TD Series utilized for such applications as HVAC – room, duct and refrigerant temperature, motors for overload protection, and automotive – air or oil temperature. Throughout the TD Series, the TD4A liquid temperature sensor is actually a two- terminal threaded anodized aluminum housing. The environmentally sealed liquid temperature sensors are equipped for simplicity of installation, like in the side of the truck, but they are not made for total immersion. Typical response time (first time constant) is four minutes in still air and 15 seconds in still water.
TD Series temperature sensors respond rapidly to temperature changes (Figure 2) and therefore are accurate to ±0.7C° at 20C°-and therefore are completely interchangeable without recalibration. These are RTD (resistance temperature detector) sensors, and supply 8 O/°C sensitivity with inherently near-linear outputs.
RTDs have got a better accuracy than thermocouples in addition to good interchangeability. Also, they are stable over the long term. By using these high-temperature capabilities, they are utilized often in industrial settings. Stability is improved when RTDs are constructed with platinum, which happens to be not impacted by corrosion or oxidation.
Infrared sensors are widely used to measure surface temperatures ranging from -70 to 1,000°C. They convert thermal energy sent from an object in a wavelength array of .7 to 20 um into an electric signal that converts the signal for display in units of temperature after compensating for almost any ambient temperature.
When choosing an infrared option, critical considerations include field of view (angle of vision), emissivity (ratio of energy radiated by a physical object for the energy emitted from a perfect radiator at the same temperature), spectral response, temperature range, and mounting.
A recently announced product, the Texas Instruments TMP006, (Figure 3) is surely an infrared thermopile sensor inside a chip-scale package. It really is contactless and works with a thermopile to absorb the infrared energy emitted in the object being measured and uses the corresponding alteration of thermopile voltage to discover the object temperature.
Infrared sensor voltage range is specified from -40° to 125°C to permit use in a variety of applications. Low power consumption along with low operating voltage helps to make the dexopky90 appropriate for battery-powered applications. The reduced package height of the chip-scale format enables standard high volume assembly methods, and may come in handy where limited spacing for the object being measured is available.
Using either contact or noncontact sensors requires basic assumptions and inferences when employed to measure temperature. So it is essential to look at the data sheets carefully and be sure you possess an idea of influencing factors so you will end up positive that the particular temperature is equivalent to the indicated temperature.