Source : Mouser Electronics
The increased automation in our modern world demands more sensing, providing data on which automated decisions can be based. One of the most fundamental, important and commonly measured parameters is temperature. It impacts so many aspects of our lives – our environment, the vehicles we drive and even our own bodies. Temperature changes can signal many things, including impending failure of industrial machines, technology or car engines. Given its importance, it is not surprising that there are several ways to measure it – each of which has different benefits depending on the application. Three of the most often-used approaches are using thermistor, thermocouple and infrared (IR) technology.
Thermistors are basically resistors that alter their resistance value depending on their temperature. As such, they can be placed in free air to measure ambient temperatures, or mounted on a surface or within an object to measure that temperature. Thermistors are constructed from metallic oxides that are pressed during manufacture into a suitable shape for the application – typically a cylinder, disk or bead shape. This is then sealed by encapsulation in a stable material such as epoxy or glass.
There are two primary types of thermistor, defined by how their resistance changes with increasing temperature. Positive thermal coefficient (PTC) devices increase resistance with temperature, while negative thermal coefficient (NTC) devices do the opposite. Generally speaking, NTC devices are most often used for temperature measurement while PTC devices are often used as thermal fuses.
The best things about thermistors are their simplicity, low cost and ruggedness. They also behave predictably in response to temperature changes – albeit a nonlinear curve relationship. Thermistors are noted for their high levels of stability, as well as their sensitivity and precision. The drawbacks are that they are only suited to a relatively narrow range of temperatures and, due to their thermal mass, they can be slow to respond – both of which can limit their suitability for some applications.
The versatility of thermistors means they measure surface temperature by being bonded to a surface, or they can be embedded inside objects, either by molding or being placed in specially created holes. As they are in contact with the object being measured, they can influence the temperature if the size of the object is similar to the thermistor. In practice, this is rarely an issue.
Some thermistors are leaded components while others, such as Murata’s NCU15XH103D60RC, are surface-mount devices (SMD). Murata’s NCU is offered in a tiny 1.0mm x 0.5mm footprint and has a nominal resistance of 10kW with an NTC response. It is suitable for use at temperatures between -40°C and +125°C, can be used in many applications and is especially popular for temperature compensation with devices that can be affected by thermal changes, such as some ICs and oscillators. As battery-powered devices become more popular, this is one key application for thermistors.
Thermistors can be leaded or, more commonly, SMDs – such as Murata’s NCU15XH103D60RC, which is an NTC component in a 1.0mm x 0.5mm footprint. This thermistor has a nominal resistance of 10kW and operates at temperatures between -40°C and +125°C. While it is suitable for a wide range of applications, it is particularly appropriate for temperature compensation in electrical/electronic circuits and temperature-sensitive devices, such as transistors, ICs and oscillators. With the growth in battery-powered technology, thermistors of this type are being utilized to monitor the temperature of rechargeable battery packs during use and also when charging.
Constructed from two wires made from different metals joined at one end and open at the other, thermocouples are an alternate method of measuring temperature. When the temperature changes at the end where the wires are joined (the “hot” end), a small voltage is created at the open end (the “cold” end), which is proportionate to the difference in temperature between the two ends. As this is a relative measurement and not an absolute measurement, the temperature of the cold end must be known to deduce the temperature of the hot end.
It is worth mentioning that the terms “hot” and “cold” are somewhat misleading, as the hot end could easily be at a lower temperature than the cold end – however, this has been accepted in the industry for years. Some manufacturers have begun using “measurement” and “reference” descriptors, which more accurately reflect the role of each end.
The type of wire used to form the thermocouple is the most often-used way of distinguishing between them. This is often denoted by a single letter – of which there are many, although T, K and J are the most common. Two nickel alloys (ChromelÒ and AlumelÒ) are used for type K – these alloys include chromium, aluminum, manganese and silicon.
The Seebeck coefficient defines how the voltage at the cold junction relates to the temperature differential between both ends – this is expressed in mV/°C and varies for each thermocouple type. Typically, S and R types have lower values (<10) and other more popular types, including E, T, J and K, have higher values (>40).
Unlike thermistors, thermocouples can be used over a wide temperature range – typically from -200°C to +2500°C – so they are suited to a very wide range of applications. They are unaffected by shock or vibration, making them good for use in tough environments. As they are passive, there is no chance of a spark, so they are intrinsically safe and can be used in the presence of explosive gases.
As the thermal mass is low, response times are good – in many cases, below one second. The main drawback is the very low level of signal that they produce – which, along with the fact that they are susceptible to pickup, can mean they need complex and sophisticated signal processing/amplification. These challenges can be mitigated to an extent by twisting the wires together or enclosing them in a shielded conduit.
The other issue with thermocouples in some applications is that they are not accurate enough, often having a tolerance of around ±1°C. This limits their suitability for low-temperature applications, but is hardly noticeable in higher-temperature applications, such as measuring the temperature of a furnace. Many thermocouple types have nonlinear outputs. However, the popular J/K types are fairly linear over a significant part of their range, which may explain their popularity.
Using thermocouples with discrete signal processing can be somewhat of a challenge, but help is available in the form of the MCP9600/L00 from Microchip. Capable of direct connection to multiple thermocouple types, the device has inbuilt signal conditioning and correction for nonlinearities and is able to deliver the output digitally via a two-wire I2C bus at 100kHz. The highly energy-efficient device consumes just 2mA when in shutdown mode, and only 300mA when fully operational, making it ideal for battery-powered applications, such as those associated with the IoT.
IR Temperature Measurement
One downside common to both thermistors and thermocouples is that they can be intrusive – meaning that as a result of having to be in contact with what they are measuring, they can affect the temperature. For this reason, infrared (IR) temperature measurement is gaining popularity, especially in medical applications. At the heart of the measurement is the Stefan-Boltzmann law, which defines the principle that energy radiated per unit surface area of a black body is proportional to the fourth power of its temperature.
IR temperature measurement uses a thermopile – a thermally isolated membrane that is connected to several series-connected miniature thermocouples. The low thermal mass of the membrane allows it to heat and cool quickly, making it highly responsive to temperature changes. The thermopile includes a thermistor to calculate the temperature of the cold junction, allowing an absolute temperature to be delivered. It is common for thermopiles to be built on MEMS technology so that they are small enough for inclusion in portable devices.
One popular thermopile-based sensing device is the noncontact MLX90632 miniature IR sensor from Melexis. Calibrated in the factory for the highest accuracy, the device can measure temperatures between -20°C and 200°C for any object located within the 50° field of view (FoV) of the sensor. The MLX90632 can work in ambient temperatures between -20°C and 85°C, using sophisticated compensation algorithms to ensure an accurate reading.
Even if it is considered to be simple, temperature is still one of the most fundamental parameters that we measure on a daily basis. It is important in determining the health of humans, animals and machines, and for many other things as well. There are a number of techniques and devices for temperature measurement and, using the information in this article, engineers should be well placed to select the best approach for their application.
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