Rtd Types And Ranges Pdf
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- Resistance Temperature Detector (RTD)
- How Does an RTD Work?
- 4 Most Common Types of Temperature Sensor
- Resistance thermometer
Resistance Temperature Detector (RTD)
A resistance temperature detector RTD can also be called a resistance thermometer as the temperature measurement will be a measure of the output resistance.
The main principle of operation of an RTD is that when the temperature of an object increases or decreases, the resistance also increases or decreases proportionally.
The main difference between a RTD and a Thermistor is that the sensing element used in a RTD is a metal and a thermistor uses ceramic or polymer material. RTD types are broadly classified according to the different sensing elements used.
Platinum, Nickel and Copper are the most commonly used sensing elements. Platinum is considered the best as it has the widest temperature range. This is shown in the resistance versus temperature graph below.
Platinum type RTD is also known for its best interchange ability than copper and nickel. It also has the highest time stability. It can also be used in radioactive environments. In industrial applications, a PRT is known to measure temperatures as high as degree Fahrenheit while copper and Nickel can measure only to a maximum of degree Fahrenheit.
Use of more than one winding enables two independent measuring circuits to measure the same temperature, and also permits more than one measurement to be made with only one sensor installation.
However, the additional mass introduced to the sensor by adding windings and their associated support and encapsulating materials increases both the response time and the conduction error. Using separate sensors provides mechanical independence of the sensors for maintenance. RTDs used for electrical equipment generally use either a three-wire system or a four-wire system having paired lead wires.
Copper lead wires are satisfactory for all the arrangements. For a given RTD, all the lead-wires should be of the same gauge and the same length, and should be run in the same conduit. The four wire system is little affected by temperature induced resistance changes in lead-wires, and, of all the arrangements, it is affected least by stray currents. It, therefore, is used to measure temperature differences and is used generally for making very accurate measurements. The three-wire system is generally satisfactory for industrial measurement using a secondary instrument that is remote, say, more than 3 meters distant from the RTD.
Although the error caused by temperature change in the leads is virtually eliminated in a 3-wire arrangement, a slight non-linearity in the resistance change is introduced with this scheme. The power supply is normally applied through the secondary instrument. If the secondary instrument is a transmitter having a current output of mA, then the power is carried by the two output wires of the transmitter.
Unless a transmitter is mounted on the Thermowell , the sensor should be connected to a connection head generally like that for thermocouples except as follows:. A RTD in a power device, such as a transformer, should be grounded locally; otherwise, RTDs are normally grounded at the power supply.
A power supply and all its associated RTDs should be grounded at only one point. The transmitter is the most commonly used instrument for transmission of RTD signals. A transmitter may be mounted either on an enclosed rack or locally. A local transmitter may be mounted on a Thermowell and supplied with it as a complete assembly. This same instrument can also be used as a thermocouple transmitter, suitable for every thermocouple combination commercially available.
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How Does an RTD Work?
A resistance temperature detector RTD can also be called a resistance thermometer as the temperature measurement will be a measure of the output resistance. The main principle of operation of an RTD is that when the temperature of an object increases or decreases, the resistance also increases or decreases proportionally. The main difference between a RTD and a Thermistor is that the sensing element used in a RTD is a metal and a thermistor uses ceramic or polymer material. RTD types are broadly classified according to the different sensing elements used. Platinum, Nickel and Copper are the most commonly used sensing elements. Platinum is considered the best as it has the widest temperature range. This is shown in the resistance versus temperature graph below.
4 Most Common Types of Temperature Sensor
Resistance thermometers , also called resistance temperature detectors RTDs , are sensors used to measure temperature. Many RTD elements consist of a length of fine wire wrapped around a ceramic or glass core but other constructions are also used. The RTD wire is a pure material, typically platinum, nickel, or copper.
The four most common types of temperature sensors, ranging in responsiveness and accuracy from high to low are:. A thermistor is a thermally sensitive resistor that exhibits a continuous, small, incremental change in resistance correlated to variations in temperature. An NTC thermistor provides higher resistance at low temperatures.
This blog post discusses many useful and practical things to know about the Pt sensors. There's information on RTD and PRT sensors, different Pt mechanical structures, temperature-resistance relationship, temperature coefficients, accuracy classes and on many more. Platinum has a reliable, repeatable and linear temperature-resistance relationship. More details on that later. In an earlier blog post, we discussed thermocouples. Thermocouples are also used as temperature sensors in many industrial applications.
An RTD consists of a resistance element and insulated copper wires. The most common number of wires is 2; however some RTDs have 3 or 4 wires. The resistive element is the temperature sensing element of the RTD. It is usually platinum because as a material it is highly stable over time, it has a wide temperature range, it offers an almost linear relationship between temperature and resistance and it has a chemical inertness. Nickle or copper are also other popular choices of material for the resistive element. An RTD works by using a basic principle; as the temperature of a metal increases, so does the resistance to the flow of electricity. An electrical current is passed through the sensor, the resistance element is used to measure the resistance of the current being passed through it.
Low sensitivity, about +Ω/°C for a Ω Pt RTD. • Requires linearization for wide range; ex. °C to +°C. • Lead wire resistance may introduce.
Sensors are the devices used by electronic, electrical and mechanical equipment to interact with the external environment. These are used for measuring various types of physical phenomena such as voltages, current, acceleration, etc…Sensors make use of various principles to measure these physical quantities. Such as piezoelectric effect is used for measuring voltage and current, Hall effect is used for measuring magnetic density, etc… RTD — Resistance temperature detector, is a temperature detector sensor that uses the relationship between temperature and resistance of the conductor to measure the temperature. This sensor is rapidly replacing thermocouples.
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