Concern increases daily with various questions arising about the Types of temperature sensors in order to make the best choices. There’s a truckload of things you need to know about this various types of sensors, their applications, pros and cons, and affordability. This article aims to equip you with all the knowledge you seek. Stay glued and keep reading to learn more.
It’s quite plain to delve into the subject matter without an understanding of the subject matter itself. This makes it necessary to exhaustively examine what Temperature sensors are. From the name “temperature sensors”, it can be deduced that this equipment is used for measurements, particularly temperature related measurements.
A temperature sensor is an equipment that measures temperature through an electrical signal through a thermocouple or alternatively an RTD. A Thermocouple on the other hand is manufactured from two metals which are dissimilar in nature and produce in commensurate proportion to temperature changes, voltage that is electric in nature.
Without Undue prejudice, it should be kept at heart that one of the most widely evaluated parameter is temperature as it cuts across a wide range of laboratories and Industries. Exact and precise measurements are very cogent determinants of success and are required for a range of applications which include laboratory research, study of geology, study of electronic components, and most importantly medical tests.[toc]
What is Thermocouple and How Does it Work?
As earlier mentioned, a thermocouple is manufactured from two metal wires which are dissimilar. However, these dissimilar wires are attached to each other on one end to serve as a hot measuring junction while the other end popularly known as cold reference junction is connected to an electronic digital indicator.
Based on this structure, the Thermocouple generates a signal of measurement in response to temperature differences between the hot junction and cold junction and not in response to actual temperature as expected.
However, to induce accuracy, a tiny ambient sensor is attached to the electronic digital indicator (close to the cold junction) to be added by the electronic digital indicator to the thermocouple temperature difference in order to calculate and display the actual temperature on the screen.
What is an RTD?
Fully known as Resistor Temperature Detector. All electrical conducting devices to a certain degree, whether greater or lesser have resistance to the electricity flow. This resistance to electric voltage varies based on the Conductor’s temperature which can be evaluated to match a specific temperature. Also, the material which is commonly utilized in an RTD is platinum as a result of its stability, purity over a wide temperature range.
With the information above, it can be deduced that just two copper wires are required to connect an RTD to a circuit. Nevertheless, these wires are susceptible to resistance changes as a result of environment’s temperature. This makes it important for an extra wire to be built into many RTDs to augment and aid the digital controller’s accuracy for all the variations.
Well, since you’re now intimate with a bit of the technical scope of a temperature sensor alongside its composition and functions. We shall exhaustively examine the various types of sensors. The Thermometer is the most common and widely utilized kind of temperature sensor and used for solid, liquid, and gas Temperature measurement. Its wide use stems from its slight chances of not being so accurate. As a result, it is used for non-scientific purposes.
The types of Temperature sensors are distinguished from each other based on sensing capacity over their application range. The types of Temperature Sensors are:
Types of Temperature Sensor
- Resistor Temperature Detectors(RTD)
- Infrared sensors
Surprised the Thermocouples and Resistor Temperature Detectors surfaced as part of the much anticipated types of temperature sensors. What we examined earlier was an overview of it’s operation. Now, we shall consider these types of sensors exhaustively to give you tips on how to pick the best amongst others.
Thermocouple sensor, usually abbreviated as TC is a very desirable temperature sensor. These type of temperature sensors are affordable, durable and used in a wide range of applications such as Industrial, consumer and automotive. Thermocouples are independently powered, operate over a wide range of temperature with quick response duration, and do not require excitation.
Usually, thermocouples consist of two distinct metals known as Opened and closed metals. These metals work on the thermo electric effect principle to cause a seebeck effect( a situation in which distinction in temperature of two dissimilar conductors causes a difference in voltage between two substances and the voltage difference can be measured to calculate temperature).
Despite the overview done in the introductory paragraph, there are numerous kinds of thermocouples that are produced from a wide variety of materials which induces and aids distinct range of temperature and distinct levels of sensitivity which we didn’t mention in the introduction. These distinct types have letter designations for easy identification and effective enumeration.
For a detailed understanding, these types shall be extrapolated with their different code type, conductors alloys, sensing temperature and sensitivity.
The most commonly used thermocouple is the K type with Nickel Chromium or Nickel Aluminium conductor alloys, -180 to 1300°C sensing temperature and 41°C sensitivity . Others are the E type( with Nickel Chromium or constantan conductor alloys, -40 to 900°C sensing temperature and 68°C sensitivity),
J type (with Iron or Constantan conductors alloys, -180° to 800°C sensing temperature, 55°C sensitivity), N type (with Nicrosil or Nisil conductor alloys, -270°C to 1300°C and 39°C sensitivity), T type (with Copper or Constantan conductors, -250 to 400°C sensing temperature, 43°C sensitivity), R/S type (with Copper or Copper Nickel Compensating Conductor alloys, -50 to 1750°C sensing temperature and 10°C sensitivity) and
the B type (with platinum rhodium conductor alloys, 0 to 1820°C sensing temperature and 10°C sensitivity). Without prejudice, the J and T types also enjoy a wide range of utilisation and are available in ready-made forms.
It is known that for every great equipment, there are peculiar features and complimentary setbacks to performance. The thermocouple is non linear, that is, the voltage output is not linear to temperature itself, and conversion of voltage output to temperature requires mathematical calculations.
This feature has proven to be a disadvantage of thermocouples. This is as a result of the small voltage output which makes measurement an arduous task as precise amplification vulnerability to external noise over lengthy wires and cold junction( where the wires meet) is required. This creates the seebeck effect earlier mentioned.
Not to worry, Maxim integrated offers solutions to this with its digital output thermocouples which include the MAX 31855 and MAX 31856. These aid signal conditioning by blending a high-resolution analog to digital converter (ADC), low noise precision gain stage, and cold junction compensation sensor.
This equipment aid producers and designers of thermocouple circuits to offer detailed, accurate, and practical solutions to signal conditioning in a tiny affordable package. Interestingly, these devices work with many of the popular types of thermocouples earlier extrapolated.
2. Resistor Temperature Detector
RTD sensors are also widely used and one of the most accurate types of temperature sensors. In a Resistance Temperature Detector, the resistance is proportional to the temperature. As a change in temperature occurs, the resistance of any metal changes as well. The distinctions in resistance is the major determinant by which RTD sensors measure temperature.
The Resistor Temperature Detector is produced from copper metals, platinum, and nickel and has a quite wide range of temperature measurement capabilities. It can be used to ascertain the range between -270°C to +850°C.
The most widely utilized material is platinum and Resistance Temperature Detectors made from this material are referred to as Platinum RTDs (also known as PRTDs). These RTDs often have at their disposal a 100 ohms and 1000 ohms resistance at 0°C which are referred to mostly as PT100 and PT1000 respectively.
The rationale behind the use of Platinum RTDs is not far fetched as they offer a direct response to changes in temperature, they are consistently stable and accurate, provide recurring responses and possess a wide temperature range. This accuracy and frequency facilitates the use of RTDs in precision applications.
However, an external current source is required for RTDs to function properly. Nevertheless, the current source usually generates heat in the resistive element subsequently orchestrating error in the measurement of temperature. The error could be calculated via this formula:
Delta T = P*S
Where T is temperature, P is I squared power generated and S is a Degree C or mill watt.
This is because resistor temperature Detector elements have a greater thermal mass and respond less effectively to temperature changes compared to thermocouples. This makes signal conditioning and flow of excitation current important in resistor temperature Detectors. Once the current which flows through the resistor temperature Detector is known, the resistance can be calculated.
There are different configurations for measuring temperature in resistor temperature Detectors after the current error has been ascertained. These are the two-wire, three-wire, and four-wire options. The two wired option is easy to design and implement.
The current is coerced through the resistor temperature Detector to accurately ascertain the resulting voltage. It is useful when the lead length is so short that resistance doesn’t have a significant effect on the accuracy of the measurement.
The three-wired option shares a certain similarity with the two wired method, however the third wire is a compensation for the lead resistance as it adds a resistor temperature Detector probe that can contain the excitation current. The Four-wired which is the most accurate completely provides compensation for the lead resistance. The current is coerced on just one set of the wires while voltage is sensed effectively by the other set of wires.
Another notable type of temperature sensor is a thermistor which is similar to Resistor Temperature Detectors as changes in temperature cause ascertainable changes in resistance. Thermistors are quite affordable, easy to use with speedy adaptability. However, most Thermistors are produced in two wire configurations and less accurate than Resistor Temperature Detectors.
Thermistors are usually produced from materials such as polymer or ceramics which are made from manganese and oxides of nickel. This makes them vulnerable to damages.
Although the thermistor sensor is less accurate compared to a Resistor Temperature Detector, it offers an higher level of sensitivity compared to resistor temperature Detectors. Also, most thermistors possess a Negative Temperature Coefficient (NTC) which is used for application of temperature.
A Negative Temperature Coefficient’s temperature increases as the resistance decreases. This type of sensors require an important correction to accurately interpret data as they possess an absolutely non-linear temperature resistance relationship.
Also, the common approach to use a thermistor is where a thermistor and a fixed value resistor form a voltage divider with an output digitized by a digital converter (ADC).
Thermometers enjoy a wide range of application especially in the medical line. A lot of persons today know the different types of thermometers and how they work coupled with it’s inclusion in basic school curriculum.
Thermometers are devices used to measure temperature of solids, liquids, or gases. The word thermometer has its origin from a couple of words: thermo which connotes heat and meter which connotes measurement.
The thermometer consists of a liquid, which is either mercury or alcohol in its glass. Its volume is linearly proportional to the temperature. That is, the thermometer’s volume increases when there is a corresponding increase in temperature.
When this liquid is boiled, it causes an expansion inside the thermometer’s small tube. To give accurate readings, the thermometer has a calibrated scale with numbers marked alongside the glass tube to infer the temperature when the mercury line is at that point.
The temperature in thermometers is recorded in Fahrenheit, Celsius or Celsius scales respectively. This makes it necessary to take note of the scale in which the thermometer is calibrated before commencing measurements.
5. Semiconductor sensors
Semiconductor sensors are devices that come in form of ICs and are majorly known as IC temperature sensor. There are two distinct types (without undue prejudice to others) of Semi Conductor sensors which are Local Temperature sensors and remote digital temperature sensor.
Local temperature sensors are ICs which make use of physical transistor properties to measure their temperature while Remote digital temperature sensors quantify the temperature of an external transistor.
Moreover, local temperature sensors can utilize either analog or digital outputs. Analog outputs could be either voltage or current. Local Temperature sensors discern temperature on printed circuit boards or the ambient air surrounding it.
A good example of a very handy local temperature sensor which has a very flexible application range is the Max 31875.
On the other hand, remote digital temperature sensors function in a similar manner with local temperature sensors by making use of a transistor’s physical properties. The distinction lies in the location of the transistor which is far away from the sensor chip. Also, some microprocessors have an inbuilt bipolar sensing transistor to measure the target IC’s temperature.
Other types include the diode temperature sensors and resistance output silicon temperature sensor. The silicon diode temperature sensors have been specifically optimized for the cryogenic range of temperature. They are also devices with a linear configuration where the diode’s conductivity rises in a linear progression in the low cryogenic regions.
In recent times, present semiconductor temperature sensors offer high linearity and a corresponding accuracy over an operating range of about 55°C to +150°C. Regardless, the most common and popularly validated of these kind of sensors are AD590 and LM35.
6. IR sensor
These sensors are commonly referred to as IR sensors for convenience. They are actually Infrared sensors and are electronic devices which can be utilized to infer certain readings and qualities of its present environs by either emission or detection of Infrared radiation. These sensors are peculiar sensors and do not require contact. For instance, simply place your Infrared sensor on your table without any prior or subsequent contact, the sensor detects the desk’s temperature regardless utilizing its radiation feature.
Infrared sensors are classified into two types namely Quantum Infrared sensors and thermal infrared sensors and each has its peculiarities to suit whatever process required.
Things to Look Out For a Selection Amongst The Types of Temperature Sensors?
Asides from detailed and technical explanations given about the temperature sensors. You could still be left in a dilemma concerning the best one for you. This takes us a step further to examine factors you need to look out for before making a purchase of your desired temperature sensor.
It is important to know that the application is the overall determinant. Hence, the factors to be examined would serve as a guide to determine which would best suit a particular application. These factors are extrapolated as follows:
This should be your first consideration when making a choice amongst the various types of sensors for a peculiar application. The various types of sensors are more effective with different ranges. For instance, Negative Temperature Coefficient Thermistors are more effective when applied from -50°C to +250°C. Hence, it is suitable for a wide range of applications.
Thermocouples are also less accurate at relatively lower temperatures but are able to measure temperatures as much as 2000°C. Resistor Temperature Detectors also work within a wide temperature range but lower than the thermocouples unlike Semi-Conductor sensors which possess a very restricted temperature range.
Accuracy is also a very cogent factor which must be considered. For instance, thermocouples are frequently inaccurate compared to Resistor temperature Detectors and thermistor temperature probe even though they cost less.
Negative Temperature Coefficient Thermistors and thermistor probes are the most accurate between -50°C and +150°C. If encapsulated in a glass, they could be extremely accurate at 250°C. However, this thermistors are still not as accurate as of the Resistor Temperature detectors and need a great deal of correction of data to interpret the temperature.
Resistor Temperature Detectors provide accurate and reoccurring measurements. If your application requires a great deal of accuracy, these sensors should be your pick amongst others.
Speed might be important for your application. This would make a Resistor Temperature detector less useful for you. Although it is very accurate, it is slower and requires and excitation and signal conditioning to properly function. A thermocouple, thermistor or thermometer would be more preferable.
The length of your operation must be put into maximal consideration. This takes a toll on your choice of temperature sensors as these sensors could become less effective and efficient overtime subject to their configuration, packaging, production, and quality of materials utilized for their construction.
For instance, over the course of a year, a thermistor can change by 0.02°C to 0.2°C subject to the presence or absence hermetical seal. A thermocouple can also transform by 1°C to 2°C as the year goes on. This makes this sensors preferable to other sensors as the change is relatively smaller.
Packaging or Style
The manner in which a temperature sensor is packaged differs based on the application for which it is required. Threaded studs are suitable for general purpose applications while closed tubes such as thermometers are well suited to liquid measurements application.
Moreover, If you prefer your sensors handy and easy to carry about, a clinical thermometer or Negative Temperature Coefficient Thermistors would be okay. Your best bet is the Semi Conductor sensor which comes in extremely minute packages making mobility easier and efficient for you.
Resistance to Noise
You wouldn’t want your operation delayed for failing to equip yourself well, would you? Before selection, you should check the temperature sensor’s resistance to noise and how effective it can be in your target location.
Negative Temperature Coefficient Thermistors have proven to have a high level of resistance at the initial switch which makes them resistant to lead resistance and electrical noise. Thermocouples are also resistant to lead resistance but are vulnerable to electrical noise due to their small output signal.
I can bet you know best the right choices to make after reading this. You can now explore various applications with the types of temperature sensors exhaustively discussed.