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How to Select and Use the Right Temperature Sensor
Every sensor has its own specification on a particular Temperature range. Now,
it is on the user, who will decide that which type of sensor is best suited for
his/her application.
The selection of sensor depends on a variety of specifications viz: Application,
Tolerance, Accuracy and out of the most temperature ranges.
Now the thing is to differentiate between different type of sensor which will
include temperature range, tolerance, accuracy, interchangeability and relative
strengths and weakness for each type of sensor.
REVIEW OF RTD AND THERMOCOUPLE BASICS
RTD'S contain a sensing element which is an electrical resistor that changes
resistance with temperature. This change in resistance is well understood and
is repeatable. The sensing element in an RTD usually contains either a coil of
wire, or a grid of conductive film which has a conductor pattern cut into it.
Extension wires are attached to the sensing element so it's electrical resistance
can be measured from some distance away. The sensing element is then
packaged so it can be placed into a position in the process where it will reach
the same temperature that exists in the process.
Thermocouples, on the other hand, contain two electrical conductors made of
different materials which are connected at one end. The end of the conductors
which will be exposed to the process temperature is called the measurement
junction. The point at which the thermocouple conductors end (usually where
the conductors connect to the measurement device) is called the reference
junction
When the measurement and reference junctions of a thermocouple are at
different temperatures, a millivolt potential is formed within the conductors.
Knowing the type of thermocouple used, the magnitude of the millivolt potential
within the thermocouple, and the temperature of the reference junction allows
the user to determine the temperature at the measurement junction.
The millivolt potential that is created in the thermocouple conductors differs
depending on the materials used. Some materials make better thermocouples than other because the millivolt potentials created by these materials are more
repeatable and well established. These thermocouples have been given specific
type designations such as Type E, J, K, N, T, B, R and S.
TEMPERATURE LIMITATIONS FOR RTD'S AND
THERMOCOUPLES
The materials used in RTD's and thermocouples have temperature limitations
which can be an important consideration in their use.
1. RTD's
As stated earlier, and RTD consists of a sensing element, wires to connect the
sensing element to the measurement instrument and some kind of support to
position the sensing element in the process. Each of these materials sets limits
on the temperature that the RTD can be exposed to.
Table 1 : Sensing Element Materials And Temperature Limits
Material Usable Temperature Range
Platinum -260°C To 650°C
Nickel -100°C To 300°C
Copper -70°C To 150°C
Nickel/Iron 0°C To 200°F
The sensing element in an RTD usually contains a platinum wire or film, a
ceramic housing and ceramic cement or glass to seal the sensing element and
support the element wire. Typically, platinum sensing elements are able to be
exposed to temperatures up to approximately 650°C. Other materials such as
Nickel, Copper and Nickel/Iron alloy can also be used, however, their useful
temperature ranges are quite a bit lower than for platinum. The wires which
connect the sensing element to the readout or control instrumentation are
usually made of materials such as nickel, nickel alloys, tined copper, silver
plated copper or nickel plated copper. The wire insulation used also directly
influences the temperature the RTD can be exposed to. Table contains the
commonly used wire and insulation materials and their maximum usage
temperatures.
2. THERMOCOUPLES
Thermocouple materials are available in Types E, J, K, N, T, R, S and B. These
thermocouple types can be separated into two categories: Base Metal and
Noble Metal thermocouples.
Type E, J. K, N and T thermocouples are known as Base Metal Thermocouples
because they are made of common materials such as copper, nickel, aluminum,
iron, chromium and silicon. Each thermocouple type has preferred usage
conditions, for example the use of bare Type J thermocouples (Iron/Constantan)
are typically limited to a maximum temperature of 540°C and are not
recommended for use in oxidizing or sulfurous atmospheres due to deterioration
of the Iron conductor. Bare Type T thermocouples (Copper/Constantan) are not
used above 370°C due to deterioration of the copper conductor. Temperature
ranges for these thermocouple types are included in Table 3.
Type R, S and B thermocouples are known as Noble Metal Thermocouples
because they are made of Platinum and Rhodium. These thermocouples are
used in applications that exceed the capabilities of Base Metal Thermocouples.
Type R and S thermocouples are rated for use at temperatures between 540°C
and 1480°C, with Type B rated for use from 540°C to 1700°C. When long term
exposure at temperatures above 13700°C is expected, it is prudent to specify
Type B thermocouples for improved thermocouple life. Type R & S
thermocouples can experience significant grain growth if held near their upper
use limit for long periods of time.
Since Thermocouples do not have sensing elements, they do not have many of
the temperature limiting materials that RTD's do. Thermocouples are normally
constructed using bare conductors which are then insulated in a compacted
ceramic compacted ceramic powder or formed ceramic insulators, This
construction allows thermocouples to be used at much higher temperatures
than RTD's.
TOLERANCE, ACCURACY AND INTERCHANGEABILITY
Tolerance and Accuracy are the most misunderstood terms in temperature
measurement.
The term tolerance refers to a specific requirement, which is usually plus, or
minus some amount. Accuracy on the other hand refers to an infinite number of
tolerances over a specified range.
For example, RTD's contain a sensing element, which is manufactured to have a specific electrical resistance at a specific temperature. The most common
example of this requirement is what's known as the DIN standard. To meet the
requirements of the DIN standard, an RTD must have a resistance of 100 Ohms
±0.12 % (or 0.12 Ohms) at 0°C to be considered a Grade B sensor (a Grade A
sensor is 100 Ohms ±0.06%. The tolerance of ±0.12 Ohms applies only to the
resistance at 32°C and cannot be applied to any other temperature. Many
suppliers will provide an interchangeability table for RTD's, which provide the
user with a table of tolerances at specific temperatures.
Thermocouples on the other hand are specified differently than RTD's because
they are manufactured differently. Unlike the sensing element found in RTD's,
the mV potential generated in a thermocouple is a function of the material
composition and the metallurgical structure of the conductors. Therefore,
thermocouples are not assigned a value at a specific temperature, but given
limits of error, which cover an entire temperature range.
These limits assigned to thermocouples are known as standard or special limits
of error.
Table 3 contains the standard and special limits of error specifications for each
standard thermocouple type. It must be noted that the limits of error values
listed in Table 3 are for new thermocouples, prior to use. Once thermocouples
are exposed to process conditions, changes in the thermocouple conductors
may result in increased errors.
Users are encouraged to perform tests periodically to determine the condition of
thermocouples used in high reliability or close accuracy applications.
COMPARISON OF RESPONSE TIME OF DIFFERENT SENSORS
Strengths & Weaknesses
Each type of temperature sensor has particular strengths and weaknesses.
RTD Strengths:
RTD's are commonly used in applications where repeatability and accuracy are
important considerations. Properly constructed Platinum RTD's have very
repeatable resistance vs. temperature characteristics over time. If a process
will be run at a specific temperature, the specific resistance of the RTD at that
temperature can be determined in the laboratory and it will not vary significantly
over time. RTD's also allow for easier interchangeability since their original
variation is much lower than that of thermocouples. For example, a Type K
thermocouple used at 400°C has a standard limit of error of ±4°C. A 100-Ohm
DIN, Grade B platinum RTD has an interchangeability of ±2.2°C at this same
temperature. RTD's also can be used with standard instrumentation cable for
connection to display or control equipment where thermocouples must have
matching thermocouple wire to obtain an accurate measurement.
RTD Weaknesses:
In the same configuration, you can expect to pay from 2 to 4 times more for an
RTD than for a base metal thermocouple. RTD's are more expensive than
thermocouples because there is more construction required to make the RTD
including manufacture of the sensing element, the hooking up of extension
wires and assembly of the sensor. RTD's do not do as well as thermocouples in
high vibration and mechanical shock environments due to the construction of
the sensing element. RTD's are also limited in temperature to approximately
650°C where thermocouples can be used as high as 1700°C.
Thermocouple Strengths:
Thermocouples can be used to temperatures as high as 1700°C, generally cost
less than RTD's and they can be made smaller in size (down to approximately
.020'' dia) to allow for faster response to temperature. Thermocouples are also
more durable than RTD's and can therefore be used in high vibration and shock
applications.
Thermocouple Weaknesses:
Thermocouples are less stable than RTD's when exposed to moderate or high
temperature conditions. In critical applications, thermocouples should be
removed and tested under controlled conditions in order to verify performance.
Thermocouple extension wire must be used in hooking up thermocouple sensors
to thermocouple instrument or control equipment. Use of instrumentation wire
(plated copper) will result in errors when ambient temperatures change.
SUMMARY OF SELECTION OF TEMPERATURE SENSOR
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