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In this article We will Discuss in detail about RTD advantages and disadvantages But before Starting about RTD advantages and disadvantages We will go through in brief to what is RTD and how does it works. Resistance Temperature Detectors, or RTDs, use resistance to measure temperature; as the temperature rises, the resistance value rises as well.
Typically, copper, nickel, or platinum are used to make RTDs. Base resistance and temperature coefficient of resistance (or TCR) values are included in RTD standards. The base resistance value, which commonly ranges from 10 to several thousands of Ohms depending on the material used, is the nominal resistance of the RTD at 0°C for nickel and platinum constructions or 25°C for copper constructions. The most common number is 100 ohms.
The measuring instrument that calculates temperature changes needs to know the coefficient of resistance. The relationship between resistance and temperature is established by the TCR, which calculates a rise in temperature (in degrees Celsius) based on an increase in resistance (measured in ohms).
In comparison to thermocouples, generally RTD has higher stability, precision, and repeatability. Due to its great accuracy and appropriateness for precision applications, it is slowly replacing traditional thermometers in many industrial applications.
The majority of RTD components are created by wrapping a ceramic or glass core with a length of finely coiled wire made of a pure material, often platinum, nickel, or copper.
As the temperature changes, the material’s resistance changes linearly.
RTD functions by linking the element’s resistance to temperature.
Metal has a higher resistance the hotter it gets.
Usually, platinum is utilized because
- platinum has linear resistance to temperature.
- Chemically unaffected
- Consistent with temperature
In contrast to thermocouples, RTDs only need a tiny amount of current.
Temperature and the measured resistance are correlated.
Measurement error may be influenced by lead resistance, particularly as wire lengthens.
Several types of RTD:
There are Different types of RTD. These are..
2- Wire RTD: Advantages and Disadvantages
Used when high accuracy is not necessary.
The simplest RTD circuit design is the two wire RTD arrangement. A single lead wire serves as the connection between the RTD element’s ends and the monitoring device in this serial setup. The result will always have some degree of error because the resistance computed for the circuit includes the resistance in the lead wires and connectors as well as the resistance in the RTD element.
3-Wire RTD: Advantages and Disadvantages
The two sensor leads are on adjacent arms. For as long as the two lead resistances are the same, the resistance is cancelled out by the lead resistance in each arm of the bridge.
The most popular RTD circuit design is the three wire RTD layout, which is employed in industrial process and monitoring applications. In this setup, two wires connect the sensing element to the monitoring device on one side and one wire connects it to the other.
3-Wire RTD gives more accurate readings and its more reliable.
4-Wire RTD: Advantages and Disadvantages
In RTD temperature measuring system using four wires is the most accurate. In both sets of leads, the apparatus measures and reduces lead resistance. Although it takes the longest and costs the most money to install, this setup yields the most precise results. The RTD resistance can be inferred indirectly from the bridge output voltage. Three resistors with a zero temperature coefficient, an external source, and four connection wires are needed for the bridge. The RTD sensor is isolated from the bridge by a pair of extension wires in order to prevent exposing the three bridge-completion resistors to the same temperature as the RTD sensor.
The issue we first encountered—that the impedance of the extension wires influences the temperature reading—is recreated by these wires. Utilizing a three-wire bridge design helps reduce this effect: Two wires connect the sensing element to the monitoring device on either side of the sensing element in a 4-wire RTD setup. The measurement current is delivered by one set of wires, while the voltage drop across the resistor is measured by the other set.
Why Platinum is most commonly used in RTD Construction:
RTDs are also made from various materials. Copper and Nickel are both easily workable materials for RTD, but because to non-linearity and Copper’s issues with wire oxidation, these materials have limited operating ranges. Platinum is the chosen material for precision measurement because, in its purest form, the temperature coefficient of resistance is virtually linear, making it possible to detect temperatures with an accuracy of + or – 1 degree Celsius using equipment that is reasonably priced.
The resistance temperature detector or RTD has a number of benefits, some of which are listed below.
An RTD provides a number of benefits over thermocouples, chief among them being stability, precision, and repeatability.
A sensor’s stability is how accurately it can monitor temperature over a specific period of time.
The capacity of a temperature sensor to maintain the same physical behaviour for any given temperature even after being used and subjected to various temperatures is known as its repeatability qualities. Its capacity to maintain stability even after numerous heating and cooling cycles. Some other RTD advantages are as given below.
It is simple to install and replace the RTD.
It comes in a variety of options.
It is possible to monitor differential temperature using the RTD.
They are appropriate for distant indication.
Maintaining stability over a long period of time.
No need to compensate for temperature.
Drawbacks of RTD:
RTD (resistance temperature detector) has some drawbacks, some of which are listed below.
It is difficult to determine whether RTDs or thermocouples are superior in general. The performance of RTDs and thermocouples should instead be compared by taking into account other aspects such measuring range, accuracy, cost, and response time.
Range of temperatures –RTDs are suitable for temperatures between -200°C and +600°C. RTDs are not the greatest option outside of this temperature range, though.
Response time- RTDs typically respond more slowly than thermocouples do.
Costs-Thermocouples are typically less expensive than RTDs. However, it takes more expertise to properly weld the RTD element to the bare conductors when RTDs are used in MI cables. An RTD is more expensive, depending on the viewpoint you’re going for.
The element’s size- Thermocouples can be produced with 0.25 mm diameters. Even though thin film RTD elements are extremely thin, it might be difficult to accommodate them in MI cables with small diameters. Some more disadvantages of RTD are below.
A more complicated measurement circuit is needed for RTD.
Shock and vibration have an impact on it.
Power supply requires a bridge circuit.
greater lag in response than a thermocouple.
the potential for self-heating.
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