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esistance 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 material has an accurate resistance/temperature relationship which is used to provide an indication of temperature. As RTD elements are fragile, they are often housed in protective probes.
Resistance thermometers are constructed in a number of forms and offer greater stability, accuracy and repeatability in some cases than thermocouples. While thermocouples use the Seebeck effect to generate a voltage, resistance thermometers use electrical resistance and require a power source to operate. The resistance ideally varies nearly linearly with temperature per the Callendar–Van Dusen equation.
The platinum detecting wire needs to be kept free of contamination to remain stable. A platinum wire or film is supported on a former in such a way that it gets minimal differential expansion or other strains from its former, yet is reasonably resistant to vibration. RTD assemblies made from iron or copper are also used in some applications. Commercial platinum grades exhibit a temperature coefficient of resistance 0.00385/°C (0.385%/°C) (European Fundamental Interval). The sensor is usually made to have a resistance of 100 Ω at 0 °C. This is defined in BS EN 60751:1996 (taken from IEC 60751:1995). The American Fundamental Interval is 0.00392/°C, based on using a purer grade of platinum than the European standard. The American standard is from the Scientific Apparatus Manufacturers Association (SAMA), who are no longer in this standards field. As a result, the "American standard" is hardly the standard even in the US.
Lead-wire resistance can also be a factor; adopting three- and four-wire, instead of two-wire, connections can eliminate connection-lead resistance effects from measurements (see below); three-wire connection is sufficient for most purposes and is an almost universal industrial practice. Four-wire connections are used for the most precise applications.
• RTD stands for Resistance Temperature Detector.
• RTDs are sometimes referred to generally as
• The American Society for Testing and Materials
(ASTM) has defined the term resistance
thermometer as follows:
• Resistance thermometer, n. - a temperature-
measuring device composed of a resistance
thermometer element, internal connecting wires, a
protective shell with or without means for
mounting a connection head, or connecting wire or
• An RTD is a temperature sensor which measures temperature
using the principle that the resistance of a metal changes with
• In practice, an electrical current is transmitted through a piece
of metal (the RTD element or resistor) located in proximity to
the area where temperature is to be measured.
• The resistance value of the RTD element is then measured by an
• This resistance value is then correlated to temperature based
upon the known resistance characteristics of the RTD element
• TDs work on a basic correlation between metals and
• As the temperature of a metal increases, the metal's resistance to
the flow of electricity increases.
• Similarly, as the temperature of the RTD resistance element
increases, the electrical resistance, measured in ohms (Ω),
COMMON COMPONENTS OF RTD
• RTD platinum resistance element
• RTD Tubing Material
• RTD Process Connection
• RTD Wire Configuration
• RTD cold end termination
RTD PLATINUM RESISTANCE ELEMENT:
• This is the actual temperature sensing portion of the RTD.
Elements range in length from 1/8″ to 3″.
• The standard temperature coefficient is an alpha of .00385 and
the standard resistance is 100 Ω at 0° C.
RTD OUTSIDE DIAMETER
• The most common outside diameter is ¼” in the US or 6mm
(.236″) for non-US applications.
• However, outside diameters range from .063″ to .500″
RTD PROCESS CONNECTION:
Process connection fittings include all standard fittings used with
thermocouples (i.e. compression, welded, spring-loaded, etc.).
RTD WIRE CONFIGURATION
• RTDs are available in 2, 3 and 4 wire configuration.
• 3 wire configurations are the most common for industrial
• Teflon and fiberglass are the standard wire insulation materials.
• Teflon is moisture resistant and can be used up to 400° F.
Fiberglass can be used up to 1000° F.
RTD COLD END TERMINATION
• RTDs can terminate on the cold end with plugs, bare wires,
terminal heads and any of the reference junctions common to
• RTD elements are commonly specified according to their resistance in ohms
at zero degrees Celsius .
• The most common RTD specification is 100 Ω, which means that at 0o C the
RTD element should demonstrate 100 Ω of resistance.
• Platinum is the most commonly used metal for RTD elements due to a
number of factors, including its
• Chemical inertness,
• linear temperature versus resistance relationship,
• Temperature coefficient of resistance that is large enough to give readily measurable
resistance changes with temperature and
• Stability (in that its temperature resistance does not drastically change with time).
• Other metals that are less frequently used as the resistor elements in an
RTD include nickel, copper and Balco.
CONFIGURATION OF RTD
CONFIGURATION OF RTD
RTD elements are typically in one of three configurations:
• A platinum or metal glass slurry film deposited or screened onto a
small flat ceramic substrate known as "thin film" RTD elements, and
• Platinum or metal wire wound on a glass or ceramic bobbin and
sealed with a coating of molten glass known as "wire wound" RTD
• A partially supported wound element which is a small coil of wire
inserted into a hole in a ceramic insulator and attached along one
side of that hole. Of the three RTD elements, the thin film is most
rugged and has become increasingly more accurate over time
• Platinum RTDs are the most common type of RTD used in
• This is because platinum has excellent corrosion resistance,
excellent long-term stability, and measures a wide range of
• Nickel RTDs are less expensive than platinum and have good
• However, nickel ages more rapidly over time and loses accuracy
at higher temperatures.
• Nickel is limited to a measurement range of -80…+260°C.
• Copper RTDs have the best resistance to temperature linearity of
the three RTD types, and copper is a low cost material.
• However, copper oxidizes at higher temperatures.
• Copper is limited to a measurement range of -200…+260°C
WIRE WOUND RTD
• In a wire wound RTD, a resistance wire
is wound around a non-conducting core,
which is usually made of ceramic.
• The sensor maker carefully trims the
length of resistance wire to achieve the
specified resistance at 0°C. This is called
the “R0” resistance.
• Next, lead wires are attached to the
resistance wire, and then a glass or
ceramic coating is applied over the wire
• As temperature increases, the length of
resistance wire increases slightly.
• Care must be taken in the design to ensure that the resistance
wire does not twist or otherwise deform as temperature
• This is because mechanical strain causes a change in wire
• Laboratory-grade RTDs used by calibration and standards
laboratories eliminate this source of error by loosely winding
resistance wire around a non-conducting support structure.
• This type of RTD can be extremely accurate, but is fragile and not
suited for most industrial applications.
COILED ELEMENT RTD
• In a coiled element RTD, the resistance
wire is rolled into small coils, which
loosely fit into a ceramic form that is
then filled with non-conductive
• The resistance wire is free to expand
and contract as temperature changes,
minimizing error caused by
• The powder increases the rate of heat transfer into the coils,
thereby improving the response time.
• Coiled element RTDs are usually protected by a metal sheath and
are used in industrial applications
THIN FILM RTDS
• Thin film RTDs are mass-produced and
cost less than the other RTD types.
• They are smaller, and have a faster
response time than the others, which is
desirable in many applications.
• They are made by depositing a thin
pathway of platinum on a ceramic base.
• The manufacturer adjusts the
resistance at 0°C by opening parallel
shunts in the pathway with a laser
• The more shunts are opened, the
higher is the resistance at 0°C
Thin film RTDs are not as accurate as the other types because:
The R0 resistance cannot be adjusted as precisely as in the other
The ceramic base and platinum coating have slightly different
This creates a strain error at higher temperatures.
Because thin film RTDs are smaller, the RTD excitation current
causes a slightly higher error due to RTD self-heating.
RTD RESISTANCE RATIO
• The term “resistance ratio” describes the average slope of
temperature vs. resistance as the RTD temperature changes from 0°C
to +100°C. The expression for resistance ratio is
(R100-R0) / R0
• R100 RTD Resistance at 100°C.
• R0 = RTD Resistance at 0°C.
Resistance ratio is affected by the type and purity of the metal used to
make the RTD
CHARACTERISTICS OF RTD ELEMENT
Temperature Range of Application
Physical Dimensions or Size
MATERIAL OF RESISTANCE ELEMENT
• Several metals are quite common for use in resistance elements
and the purity of the metal affects its characteristics.
• Platinum is by far the most popular due to its linearity with
• Other common materials are nickel and copper, although most
of these are being replaced by platinum elements.
• Other metals used, though rarely, are Balco (an iron-nickel alloy),
tungsten and iridium
• The temperature coefficient of an element is a physical and
electrical property of the material.
• This is a term that describes the average resistance change per
unit of temperature from ice point to the boiling point of water
Nominal Resistance is the prespecified resistance value at a given
TEMPERATURE RANGE OF APPLICATION
• Depending on the mechanical configuration and manufacturing
methods, RTD’s may be used from -270oC to 850oC.
• Specifications for temperature range will be different, for thin
film, wire wound and glass encapsulated types
The most critical dimension of the element is outside diameter
(O.D.), because the element must often fit within a protective
IEC 751 specifications for Platinum Resistance Thermometers
have adopted DIN 43760 requirements for accuracy
APPLICATIONS OF RTD
• RTD sensor is used in automotive to measure the engine
temperature, an oil level sensor, intake air temperature sensors.
• In communication and instrumentation for sensing the over the
temperature of amplifiers, transistor gain stabilizers, etc
• RTD is used in power electronics, computer, consumer
electronics, food handling and processing, industrial electronics,
medical electronics, military, and aerospace.
APPLICATIONS OF RTD
Food handling and processing,
• Thermistor is special type of resistor, whose resistance varies
more significantly with temperature than in standard
• Generally, the resistance increases with the temperature for
most of the metals but the thermistors respond negatively i.e.
the resistance of the thermistors decrease with the increase
• This is the main principle behind thermistor.
• As the resistance of thermistors depends on the temperature,
they can be connected in the electrical circuit to measure the
temperature of the body.
• Thermistors are mainly used as temperature sensors, inrush current limiters, self-
resetting over-current protectors and self-regulating heating elements.
• A thermistor is made from a semiconductor material.
• It is shaped into a disc, a rod or a bead.
• Bead thermistors may be only a few millimetres in diameter.
• Some bead thermistors have the bead enclosed in a glass capsule.
• In PTC type thermistor, resistance increase with increase in
temperature. Whereas in NTC thermistor, resistance decrease
with increase in temperature.
• Pure metals have positive temperature coefficient (PTC) of
resistance, alloys have nearly equal zero temperature coefficient
of resistance and semi conductors have negative temperature
coefficient (NTC) of resistance.
• PTC thermistors can be used as heating elements in small
temperature controlled ovens. NTC thermistors can be used as
inrush current limiting devices in power supply circuits.
FEATURES OF THERMISTOR
• Thermistors are at least 10 times as sensitive as the platinum
Resistance temperature detector (RTD).
• This high sensitivity of thermistors is very useful for precision
temperature measurement, control and compensation.
• Although thermistors are very sensitive but on the other hand, it
exhibits highly non-linear characteristics of resistance versus
• Thermistors are available in variety of sizes and shapes
• Thermistors are compact and rugged in construction.
• It is widely used in the application where temperature measurements
ranges -60 °C to 15 °C.
• Resistance increase with increase in
temperature for PTC and Resistance
decrease with increase in temperature
• The thermistor exhibits a highly non-
linear characteristic of resistance vs
• There are two fundamental ways to change the temperature of
thermistor internally or externally.
• The temperature of thermistor can be changed externally by
changing the temperature of surrounding media and internally
by self-heating resulting from a current flowing through the
• Thermistors are used in circuits to control temperature.
• They are used to compensate for the effects of temperature on
conductor or circuit performance.
• It is used in the measurement of power at high frequencies.
• Thermistors are used to provide time delay in circuits.
• It is used in the measurement of thermal conductivity.
• It is used in the measurement of composition of gases.
• Thermistors are used for the measurement of level, flow, and
pressure of liquids.
• PTC thermistors were used as timers in the degaussing coil circuit of most
• A degaussing circuit using a PTC thermistor is simple, reliable (for its
simplicity), and inexpensive.
• PTC thermistors used as heater in automotive industry to provide
additional heat inside cabin with diesel engine or to heat diesel in cold
climatic conditions before engine injection.
• PTC thermistors used as current-limiting devices for circuit protection, as
replacements for fuses.
• NTC thermistors used to monitor the temperature of an incubator.
• Thermistors are also commonly used in modern digital thermostats and to
monitor the temperature of battery packs while charging.
• NTC thermistors are used in the Food Handling and Processing
industry, especially for food storage systems and food
preparation. Maintaining the correct temperature is critical to
prevent food borne illness.
• NTC thermistors are used throughout the Consumer Appliance
industry for measuring temperature. Toasters, coffee makers,
refrigerators, freezers, hair dryers, etc. all rely on thermistors for
proper temperature control.
• Hot ends of 3D printers; they monitor the heat produced and
allow the printer’s control circuitry to keep a constant
temperature for melting the plastic filament.
• NTC thermistors are used as resistance thermometers in low-
temperature measurements of the order of 10 K.
• NTC thermistors can be used as inrush-current limiting devices
in power supply circuits.