Temperature sensors are utilized in diverse applications for example 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 usually measure heat to make sure that a procedure is either; staying in just a certain range, providing safe consumption of that application, or meeting a mandatory condition facing extreme heat, hazards, or inaccessible measuring points.
There are two main flavors: contact and noncontact temperature sensors. Contact sensors include thermocouples and thermistors that touch the object they may be to measure, and noncontact sensors look at the thermal radiation a heat source releases to determine its temperature. The latter group measures temperature coming from a distance and frequently are employed in hazardous environments.
A thermocouple sensors is a pair of junctions which are formed from two different and dissimilar metals. One junction represents a reference temperature along with the other junction is the temperature being measured. They work each time a temperature difference results in a voltage (See beck effect) which is temperature dependent, and therefore voltage is, therefore, converted into a temperature reading. TCs are used as they are inexpensive, rugged, and reliable, usually do not call for a battery, and can be utilized over a wide temperature range. Thermocouples can achieve good performance as much as 2,750°C and could be used for short periods at temperatures up to 3,000°C and as little as -250°C.
Thermistors, like thermocouples, are also inexpensive, readily accessible, simple to operate, and adaptable temperature sensors. They are used, however, to adopt simple temperature measurements rather than for top temperature applications. They are made of semiconductor material using a resistivity that may be especially responsive to temperature. The resistance of any thermistor decreases with increasing temperature so that when temperature changes, the resistance change is predictable. They are widely used as inrush current limiters, temperature sensors, self-resetting overcurrent protectors, and self-regulating heating elements.
Thermistors are different from resistance temperature detectors (RTD) because (1) the information employed for RTDs is pure metal and (2) the temperature response of these two is distinct. Thermistors can be classified into two types; according to the symbol of k (this function means 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, as well as the device is called a positive temperature coefficient (PTC) thermistor. If k is negative, the resistance decreases with increasing temperature, and the device is known as negative temperature coefficient (NTC) thermistor.
As one example of NTC thermistors, we shall examine the GE Type MA series thermistor assemblies intended for intermittent or continual patient temperature monitoring. This application demands repeatability and fast response, specially when used with the proper care of infants and throughout general anesthesia.
The MA300 (Figure 1) makes routine continuous patient temperature monitoring feasible utilizing the ease of the patient’s skin site for an indicator of body temperature. The stainless-steel housing used would work for both reusable and disposable applications, while keeping maximum patient comfort. Nominal resistance values of 2,252, 3,000, 5,000, and 10,000 O at 25°C are offered.
Resistance temperature detectors (RTDs) are temperature sensors having a resistor that changes resistive value simultaneously with temperature changes. Accurate and noted for repeatability and stability, RTDs may be used having a wide temperature cover anything from -50°C to 500°C for thin film and -200°C to 850°C for that wire-wound variety.
Thin-film RTD elements have a thin layer of platinum over a substrate. A pattern is generated that gives an electrical circuit which is trimmed to offer a specific resistance. Lead wires are attached, along with the assembly is coated to guard both the film and connections. In contrast, wire-wound elements are either coils of wire packaged in a ceramic or glass tube, or they are often wound around glass or ceramic material.
An RTD example is Honewell’s TD Series employed for such applications as HVAC – room, duct and refrigerant temperature, motors for overload protection, and automotive – air or oil temperature. Within the TD Series, the TD4A liquid temperature sensor is really a two- terminal threaded anodized aluminum housing. The environmentally sealed liquid temperature sensors are equipped for simplicity of installation, like from the side of any truck, however they are not made for total immersion. Typical response time (for just one 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 they are accurate to ±0.7C° at 20C°-and so are completely interchangeable without recalibration. These are RTD (resistance temperature detector) sensors, and supply 8 O/°C sensitivity with inherently near-linear outputs.
RTDs use a better accuracy than thermocouples and also good interchangeability. Also, they are stable in the long run. With such high-temperature capabilities, they are used often in industrial settings. Stability is improved when RTDs are constructed with platinum, which happens to be not affected by corrosion or oxidation.
Infrared sensors are employed to measure surface temperatures which range from -70 to 1,000°C. They convert thermal energy sent from an object in a wavelength selection of .7 to 20 um into a power signal that converts the signal for display in units of temperature after compensating for any ambient temperature.
When selecting an infrared option, critical considerations include field of view (angle of vision), emissivity (ratio of energy radiated by an item towards the energy emitted by way of a perfect radiator at the same temperature), spectral response, temperature range, and mounting.
A recently announced product, the Texas Instruments TMP006, (Figure 3) is an infrared thermopile sensor inside a chip-scale package. It can be contactless and relies on a thermopile to soak up the infrared energy emitted from the object being measured and uses the corresponding improvement in thermopile voltage to look for the object temperature.
Infrared sensor voltage range is specified from -40° to 125°C make it possible for use in an array of applications. Low power consumption in addition to low operating voltage makes the dexopky90 suited to battery-powered applications. The reduced package height of the chip-scale format enables standard high volume assembly methods, and will come in handy where limited spacing to the object being measured can be obtained.
The application of either contact or noncontact sensors requires basic assumptions and inferences when utilized to measure temperature. So you should read the data sheets carefully and make sure you possess an knowledge of influencing factors so you may be positive that the exact temperature is equivalent to the indicated temperature.