A resistance temperature detector (RTD) also known as resistance thermometer because it helps in measuring temperature which is a measure of the output resistance.
The elementary operation of an RTD is that with decreases or increases in the temperature of an object, the resistance must also decrease or increase proportionally. The main differentiator between a Thermistor and a RTD is that, a thermistor utilises a polymer or ceramic material when it comes to sensing, whereas the sensing element utilised in an RTD is a metal. As platinum is the most widely used metal in the field of manufacturing RTD’s, the device is also called Platinum Resistance Thermometers (PRTs).
RTD Types
RTDs can be classified broadly depending on the different sensing elements that have been used. Copper, Platinum, and Nickel are the most widely used sensing elements. Among these, Platinum is regarded the best as it has the widest temperature range. Platinum type RTD is also well known for its interchange ability when compared with nickel and copper and also offers the highest time stability. It is also possible to use PRTs in unsuitable environments where reducing atmospheric metallic vapours is the priority. PRTs have extensive use cases in industrial applications; as a PRT can help in measuring temperatures that are as high as 600℃, while Nickel and Copper only have a limit of measuring temperatures to a maximum of 400℃.
RTD Styles
RTDs are manufactured and are made available in several options with single, double, or even triple windings, each one separated electrically. To allow two independent measuring circuits to track or measure the same temperature it is important to use more than one winding, this also permits more than one measurement to be done by only installing a single sensor. Nevertheless, to increase both the conduction error along with the response time, extra mass has been introduced to the sensor by the addition of windings along with encapsulating materials, and their associated support. This process of utilising separate sensors provides mechanical independence during maintenance.
RTDs are usually a tip-sensitive, spring-loaded construction that has a 1/4-inch-diameter sheath.
RTD Wiring Arrangements
RTDs are available with either two, three, or four output wires that are required in order to connect to the secondary instrument. The different wiring arrangements are specifically designed in order to reduce or completely get rid of any errors that could have been introduced as a result of resistance changes of the lead wires due to changes in temperature that they too have to experience.
Generally, a three-wire or a four-wire system that has paired lead wires is used in RTDs that are in electrical equipment.
In most cases, Copper is the best choice in terms of lead wires for all the arrangements. For a particular RTD, the lead wires have the same length and are of the same gauge, this helps in ensuring they run within the same conduit.
Power Supply for RTD
There is a need for an electric DC power supply in order to provide current to the resistance measuring circuit. A secondary instrument is generally utilised in applying the required power supply. In some cases, when the secondary instrument happens to be a transmitter providing a current output of (4-20) mA, then the resulting power is generally carried using two output wires of the transmitter.
Transmission of RTD signals
Among all the instruments used for transmitting RTD signals, the transmitter is the most widely used. It is possible to mount a transmitter either locally, or even on an enclosed rack. A local transmitter can be supplied along with it as a complete assembly by simply mounting it on a Thermowell. Among the many popular RTDs used, the “Smart” transmitter is the most widely used RTD transmitter. In the case of a typical “Smart” temperature transmitter its is extraordinarily versatile: it is appropriate for Nickel and Platinum RTDs; 100, 200, or 500 ohm Platinum sensors, etc; 2, 3, or 4 lead wire arrangements. It is possible to use the same instrument as a thermocouple transmitter, which is compatible with any thermocouple combination that is available commercially.
Advantages
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- Sufficiently suitable for measurements that are remote
- Provides high accuracy
- Helps in the measurement of very narrow spans
- Amazing stability along with reproducibility
- Also offers Interchangeability
- Has the capability of matching close tolerances in case of temperature difference measurements.
Disadvantages
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- It can be sensitive to signal noise
- Sensitive in terms of mechanical damage
- It can cost a lot
- Lead wire resistance compensation is needed
- It is sensitive to self-heating errors
- Once damaged, it is not repairable in most cases
- It is not recommended for bare use inside electrically conducting substance
- There is always a need for a Power supply.
With over three decades of expertise in temperature sensor and heater manufacturing, Heatcon Sensors provides a high level of quality, accuracy, and an unbeatable commitment to customer support.
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