What is an RTD?
Resistance Temperature Detectors (RTDs) are temperature sensors that contain a resistor that changes resistance value as its temperature changes. They have been used for many years to measure temperature in laboratory and industrial processes, and have developed a reputation for accuracy, repeatability, and stability.
Why use an RTD instead of a thermocouple or thermistor sensor?
Each type of temperature sensor has a particular set of conditions for which it is best suited.
RTDs offer several advantages:
• A wide temperature range (approximately -200 to 850°C)
• Good accuracy (better than thermocouples)
• Good interchangeability
• Long-term stability
With a temperature range up to 850°C, RTDs can be used in all but the highest-temperature industrial processes. When made using metals such as platinum, they are very stable and are not affected by corrosion or oxidation.
Other materials such as nickel, copper, and nickel-iron alloy have also been used for RTDs. However, these materials are not commonly used since they have lower temperature capabilities and are not as stable or repeatable as platinum.
RTD standards
There are two standards for platinum RTDs: the European standard (also known as the DIN or IEC standard) and the American standard.
The European standard, also known as the DIN or IEC standard, is considered the world-wide standard for platinum RTDs. This standard, DIN/IEC 60751 (or simply IEC751), requires the RTD to have an electrical resistance of 100.00 Ω at 0°C and a temperature coefficient of resistance (TCR) of 0.00385 Ω/Ω/°C between 0 and 100°C.
There are four resistance tolerances for Thin Film RTDs specified in IEC60751:
Class C = ±(0.6 + 0.1*t)°C or 100.00 ±0.24 Ω at 0°C (-50 to 600°C)
Class B = ±(0.3 + 0.005*t)°C or 100.00 ±0.12 Ω at 0°C (-50 to 500°C)
Class A = ±(0.15 + 0.002*t)°C or 100.0 ±0.06 Ω at 0°C (-30 to 350°C)
Class AA (Formerly 1 ⁄3B) = ±(0.1 + 0.0017*t)°C or 100.0 ±0.04 Ω at 0°C (0 to 150°C)
The combination of resistance tolerance and temperature coefficient define the resistance vs. temperature characteristics for the RTD sensor. The larger the element tolerance, the more the sensor will deviate from a generalized curve, and the more variation there will be from sensor to sensor (interchangeability). This is important to users who need to change or replace sensors and want to minimize interchangeability errors.
The following interchangeability table shows how the tolerance and temperature coefficient affect the indicated temperature of the sensor in degrees Celsius:
The American standard, used mostly in North America, has a resistance of 100.00 ±0.10 Ω at 0°C and a temperature coefficient of resistance (TCR) of 0.00392 Ω/Ω/°C nominal (between 0 and 100°C). Section Z also includes a resistance vs. temperature curve from -100 to 457°C, with resistance values given every one degree Celsius.
RTD elements can also be purchased with resistances of 200, 500, 1000, and 2000 Ω at 0°C. These RTDs have the same temperature coefficients as previously described, but because of their higher resistances at 0°C, they provide more resistance change per degree, allowing for greater resolution.
RTD Element Construction Platinum
RTD elements are available in two types of constructions: thin film and wire wound.
Thin Film
Thin-film RTD elements are produced by depositing a thin layer of platinum onto a substrate. A pattern is then created that provides an electrical circuit that is trimmed to provide a specific resistance. Lead wires are then attached and the element coated to protect the plat
inum film and wire connections.
Wire Wound
RTD elements also come in wire-wound constructions. There are two types of wire-wound elements: those with coils of wire packaged inside a ceramic or glass tube (the most commonly used wire-wound construction), and those wound around a glass or ceramic core and covered with additional glass or ceramic material (used in more specialized applications).
Probe Construction
Once the RTD element is selected, the wiring and packaging requirements need to be determined. There are a number of ways to wire the sensors, along with an unlimited number of probe or sensor constructions to choose from.
Wiring Arrangement
In order to measure temperature, the RTD element must be connected to some sort of monitoring or control equipment. Since the temperature measurement is based on the element resistance, any other resistance (lead wire resistance, connections, etc.) added to the circuit will result in measurement error. The four basic wiring methods are shown below.
Except for the 2-wire configuration, each of the above wiring arrangements allows the monitoring or control equipment to factor out the unwanted lead wire resistance and other resistances that occur in the circuit. Sensors using the 3-wire construction are the most common design, found in industrial process and monitoring applications. The lead wire resistance is factored out as long as all of the lead wires have the same resistance; otherwise, errors can result.
Sensors using the 4-wire construction are found in laboratories and other applications where very precise measurements are needed. The fourth wire allows the measuring equipment to factor out all of the lead wire and other unwanted resistance from the measurement circuit. In the 2-wire with loop construction, the sensor resistance measurement includes the lead wire resistance. The loop resistance is then measured and subtracted for the sensor resistance. The 2-wire construction is typically used only with high resistance sensors, when lead lengths will be very short, or when tight measurement accuracy is not required.
Wire Materials
When specifying the lead wire materials, care should be taken to select the right lead wires for the temperature and environment the sensor will be exposed to in service. When selecting lead wires, temperature is by far the primary consideration, however, physical properties such as abrasion resistance and water submersion characteristics can also be important. Below is a table listing the capabilities of the three most popular constructions:
Configuration
Once the RTD element, wire arrangement, and wire construction are selected, the physical construction of the sensor needs to be considered. The final sensor configuration will depend upon the application. Measuring the temperature of a liquid, a surface, or a gas stream requires different sensor configurations.
Liquid Measurements
Probe-type sensor styles are normally used for
measuring liquids. They can be as simple as our
general purpose PR-10 and PR-11 constructions,
or as involved as our PR-12, 14, 18, or 19—with
connection heads and transmitters. A popular choice
is the quick-disconnect sensor. This can be used as
is, with compression fittings for flexible installation, or
with our PRS plastic handle for a handheld probe.
When measuring the temperature of harsh environments such as plating baths or highly pressurized systems, sensors can be coated with a material like PFA Teflon®, or they can be housed in a thermowell to protect the sensor from extreme conditions. Speak to our application engineers if you have any special measurement challenges.
Air and Gas Stream Measurements
Air and gas stream measurements are a challenge
because the rate of transfer of temperature from the
fluid to the sensor is slower than for liquids. Therefore,
sens
ors specifically designed for use in air or gas place
the sensing element as close to the media as possible. With a
housing design containing
slots that allow the air to
flow past the element,
this construction is very
popular in measuring air temperature in laboratories,
clean rooms, and other locations.
When the situation requires a little more protection for
the sensor, an option is to use a design similar to the
RTD-860. This design
has a small diameter
probe with a flange
for mounting. The
configuration will be a
little slower to respond
to changes in the air
stream, but it will provide
improved protection for
the sensor.
Surface Temperature Measurements
Surface measurements can be one of the most difficult to make accurately. There are a wide variety of styles to choose from, depending on how you want to attach the sensor, how sensitive to changes in temperature the sensor has to be, and whether the installation will be permanent. The most accurate and fastest-responding surface RTD is our SA1-RTD sensor.
When applied to a surface, it becomes virtually a part of the surface it is measuring. Surface sensors can also be bolted, screwed, glued, or cemented into place. The RTD-830 has a pre-machined hole in the housing to allow for easy installation with a #4 screw. The RTD- 850 has a housing with threaded tip that allows it to be installed into a standard #8-32 threaded hole. This RTD is handy for measuring the temperature of heat sinks or structures where screw holes may already exist.
Resistance Temperature Detectors (RTDs) are temperature sensors that contain a resistor that changes resistance value as its temperature changes. They have been used for many years to measure temperature in laboratory and industrial processes, and have developed a reputation for accuracy, repeatability, and stability.
Why use an RTD instead of a thermocouple or thermistor sensor?
Each type of temperature sensor has a particular set of conditions for which it is best suited.
RTDs offer several advantages:
• A wide temperature range (approximately -200 to 850°C)
• Good accuracy (better than thermocouples)
• Good interchangeability
• Long-term stability
With a temperature range up to 850°C, RTDs can be used in all but the highest-temperature industrial processes. When made using metals such as platinum, they are very stable and are not affected by corrosion or oxidation.
Other materials such as nickel, copper, and nickel-iron alloy have also been used for RTDs. However, these materials are not commonly used since they have lower temperature capabilities and are not as stable or repeatable as platinum.
RTD standards
There are two standards for platinum RTDs: the European standard (also known as the DIN or IEC standard) and the American standard.
The European standard, also known as the DIN or IEC standard, is considered the world-wide standard for platinum RTDs. This standard, DIN/IEC 60751 (or simply IEC751), requires the RTD to have an electrical resistance of 100.00 Ω at 0°C and a temperature coefficient of resistance (TCR) of 0.00385 Ω/Ω/°C between 0 and 100°C.
There are four resistance tolerances for Thin Film RTDs specified in IEC60751:
Class C = ±(0.6 + 0.1*t)°C or 100.00 ±0.24 Ω at 0°C (-50 to 600°C)
Class B = ±(0.3 + 0.005*t)°C or 100.00 ±0.12 Ω at 0°C (-50 to 500°C)
Class A = ±(0.15 + 0.002*t)°C or 100.0 ±0.06 Ω at 0°C (-30 to 350°C)
Class AA (Formerly 1 ⁄3B) = ±(0.1 + 0.0017*t)°C or 100.0 ±0.04 Ω at 0°C (0 to 150°C)
The combination of resistance tolerance and temperature coefficient define the resistance vs. temperature characteristics for the RTD sensor. The larger the element tolerance, the more the sensor will deviate from a generalized curve, and the more variation there will be from sensor to sensor (interchangeability). This is important to users who need to change or replace sensors and want to minimize interchangeability errors.
The following interchangeability table shows how the tolerance and temperature coefficient affect the indicated temperature of the sensor in degrees Celsius:
The American standard, used mostly in North America, has a resistance of 100.00 ±0.10 Ω at 0°C and a temperature coefficient of resistance (TCR) of 0.00392 Ω/Ω/°C nominal (between 0 and 100°C). Section Z also includes a resistance vs. temperature curve from -100 to 457°C, with resistance values given every one degree Celsius.
RTD elements can also be purchased with resistances of 200, 500, 1000, and 2000 Ω at 0°C. These RTDs have the same temperature coefficients as previously described, but because of their higher resistances at 0°C, they provide more resistance change per degree, allowing for greater resolution.
RTD Element Construction Platinum
RTD elements are available in two types of constructions: thin film and wire wound.
Thin Film
Thin-film RTD elements are produced by depositing a thin layer of platinum onto a substrate. A pattern is then created that provides an electrical circuit that is trimmed to provide a specific resistance. Lead wires are then attached and the element coated to protect the plat
inum film and wire connections.
Wire Wound
RTD elements also come in wire-wound constructions. There are two types of wire-wound elements: those with coils of wire packaged inside a ceramic or glass tube (the most commonly used wire-wound construction), and those wound around a glass or ceramic core and covered with additional glass or ceramic material (used in more specialized applications).
Probe Construction
Once the RTD element is selected, the wiring and packaging requirements need to be determined. There are a number of ways to wire the sensors, along with an unlimited number of probe or sensor constructions to choose from.
Wiring Arrangement
In order to measure temperature, the RTD element must be connected to some sort of monitoring or control equipment. Since the temperature measurement is based on the element resistance, any other resistance (lead wire resistance, connections, etc.) added to the circuit will result in measurement error. The four basic wiring methods are shown below.
Except for the 2-wire configuration, each of the above wiring arrangements allows the monitoring or control equipment to factor out the unwanted lead wire resistance and other resistances that occur in the circuit. Sensors using the 3-wire construction are the most common design, found in industrial process and monitoring applications. The lead wire resistance is factored out as long as all of the lead wires have the same resistance; otherwise, errors can result.
Sensors using the 4-wire construction are found in laboratories and other applications where very precise measurements are needed. The fourth wire allows the measuring equipment to factor out all of the lead wire and other unwanted resistance from the measurement circuit. In the 2-wire with loop construction, the sensor resistance measurement includes the lead wire resistance. The loop resistance is then measured and subtracted for the sensor resistance. The 2-wire construction is typically used only with high resistance sensors, when lead lengths will be very short, or when tight measurement accuracy is not required.
Wire Materials
When specifying the lead wire materials, care should be taken to select the right lead wires for the temperature and environment the sensor will be exposed to in service. When selecting lead wires, temperature is by far the primary consideration, however, physical properties such as abrasion resistance and water submersion characteristics can also be important. Below is a table listing the capabilities of the three most popular constructions:
Configuration
Once the RTD element, wire arrangement, and wire construction are selected, the physical construction of the sensor needs to be considered. The final sensor configuration will depend upon the application. Measuring the temperature of a liquid, a surface, or a gas stream requires different sensor configurations.
Liquid Measurements
Probe-type sensor styles are normally used for
measuring liquids. They can be as simple as our
general purpose PR-10 and PR-11 constructions,
or as involved as our PR-12, 14, 18, or 19—with
connection heads and transmitters. A popular choice
is the quick-disconnect sensor. This can be used as
is, with compression fittings for flexible installation, or
with our PRS plastic handle for a handheld probe.When measuring the temperature of harsh environments such as plating baths or highly pressurized systems, sensors can be coated with a material like PFA Teflon®, or they can be housed in a thermowell to protect the sensor from extreme conditions. Speak to our application engineers if you have any special measurement challenges.
Air and Gas Stream Measurements
Air and gas stream measurements are a challenge
because the rate of transfer of temperature from the
fluid to the sensor is slower than for liquids. Therefore,
sens
ors specifically designed for use in air or gas place
the sensing element as close to the media as possible. With a
housing design containing
slots that allow the air to
flow past the element,
this construction is very
popular in measuring air temperature in laboratories,
clean rooms, and other locations.
When the situation requires a little more protection for
the sensor, an option is to use a design similar to the
RTD-860. This design
has a small diameter
probe with a flange
for mounting. The
configuration will be a
little slower to respond
to changes in the air
stream, but it will provide
improved protection for
the sensor.Surface Temperature Measurements
Surface measurements can be one of the most difficult to make accurately. There are a wide variety of styles to choose from, depending on how you want to attach the sensor, how sensitive to changes in temperature the sensor has to be, and whether the installation will be permanent. The most accurate and fastest-responding surface RTD is our SA1-RTD sensor.
When applied to a surface, it becomes virtually a part of the surface it is measuring. Surface sensors can also be bolted, screwed, glued, or cemented into place. The RTD-830 has a pre-machined hole in the housing to allow for easy installation with a #4 screw. The RTD- 850 has a housing with threaded tip that allows it to be installed into a standard #8-32 threaded hole. This RTD is handy for measuring the temperature of heat sinks or structures where screw holes may already exist.
Again another great explanation on working of RTDs, this comes in handy especially when you are making a selection of the right RTD to use a given application.
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