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In your daily life and different branches of industries, there are many cases in which it’s required to know the temperature of the environment, inside a reactor, winding of an electrical machine, and so on.
Most likely you’ve heard about “RTDs”, “Thermocouples”, “Thermistors”, “Semiconductor” type elements, and so on, which will be addressed here.
let’s see what a “Temperature Sensor” (Temperature Transducer) is and what does a “Temperature Transmitter” mean.
Generally, a sensor or transducer is a physical device that is capable of transforming one type of process variable to my favorite signal type.
Temperature, pressure, flow, etc., are some process variables and actually, they are physical characteristics of our real world.
The temperature sensor is a device that will transform the Temperature into an electrical signal, no matter how tiny the amount of this signal might be!
Based on different sensor technologies, this signal may have different ranges and for industrial applications, I need to have my signals limited to some universally accepted electrical “signal-ranges”.
Today some of these globally accepted electrical signal-ranges are 4-20 mA, 1-5 V, 0-10 V, etc.
A “Temperature Transmitter” is a device that transforms the tiny output of a “Temperature Transducer” to one of these standard signal ranges.
RTD or “Resistance Temperature Detector” is a device the resistance of which varies with the temperature.
Since it is a passive device, an external electrical current should be applied to it and then the voltage drop across it can be measured. This voltage is a good indication of the temperature.
In reality, the distance between the temperature sensing point and measuring system calls for wiring and since the real wiring has its own resistance, some measurement error sneaks in hereby!
Three-wire and Four-wire solutions are developed to remove this error.
One of the most common RTDs is “PT100” which consists of a thin film of Platinum on a plastic film and shows a resistance of 100Ω at 32° F.
Thermistors are temperature-dependent resistors and are widely used in industrial purposes, e.g. over-current protection, self-regulating heating elements, inrush current limiters, and so on. Thermistors can be NTC or PTC.
In NTC (Negative Temperature Coefficient) thermistors, resistance decreases as temperature rises. NTC’s are commonly used as “inrush” current limiters.
And with PTC (Positive Temperature Coefficient) thermistors, resistance increases as temperature increases. PTC thermistors are commonly used as “overcurrent protection” and in resettable fuses.
A thermocouple or simply “TC” is comprised of a couple of specific dissimilar wires joined together, forming the “sensing point” or “junction”.
Based on physical characteristics called “Thermoelectric Effect”, when this junction is placed at different temperatures, different millivolt signals are generated which can be interpreted as an indication of the temperature.
The main restriction of Thermocouples is the “accuracy” which doesn’t make it the best solution for precise applications. Also, Thermocouples need a reference measurement point called “Cold Junction”.
The thermocouple junction is often exposed to extreme environments, while the cold junction is often mounted near the instrument location.
Based on “range” of temperature measurement, “sensitivity” and some other factors in each application, different types of Thermocouples are available, for example, E, J, K, M, N, T, and so on.
“Semiconductor Temperature Sensor” is based on the fact that the junction voltage across a p-n combination of semiconductors, like a diode junction or “base-emitter” junction of regular transistors, is a function of temperature.
If I have to measure electronic PCB and/or IC temperature, silicon-based types are the best choices. Also alongside the technical bonus-malus comparison, sometimes cost is the key factor in selecting the proper device, as they say: “Money Talks!”
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31 мар 2019
