Resistance Temperature
Detectors (RTD’S)
ADVANCED THERMAL PRODUCTS, Inc.
Advanced Thermal Products, Inc. • P.O. Box 249 • 328 State Street • St. Marys, PA 15857 • Phone: 814-834-1541
Fax: 814-834-1556 • E-mail: sales@atpsensor.com • Web Site: www.atpsensor.com
Resistance Temperature
Detectors (RTD’S)
ADVANCED THERMAL PRODUCTS, Inc.
Introduction:
Because of their linearity, stability and accuracy over a
wide temperature range, RTDs (resistance temperature
detectors) have been the gold standard of temperature
measurement devices for many years. Compared to
thermocouples and thermistors, RTDs have also been
known as the most expensive sensing elements. But
recent advances in thin-film technology have produced
RTD elements that now offer the design engineer a
product that can be competitively priced in OEM quan-
tities.
Resistance Temperature Detectors
operate through
the principle of electrical resistance changes in pure
metal elements. Platinum is the most widely specified
RTD element type although nickel, copper, and Balco
(nickel-iron) alloys are also used. Platinum is popular
due to its wide temperature range, accuracy, stability,
as well as the degree of standardization among manu-
facturers. RTDs are characterized by a linear positive
change in resistance with respect to temperature. They
exhibit the most linear signal with respect to tempera-
ture of any electronic sensing device.
There are two common constructions for RTD ele-
ments.
Wire-wound
devices are manufactured by
winding a small diameter of wire into a coil on a suit-
able winding bobbin. A number of methods have been
used to protect the wire element from shock and vibra-
tion. One common technique is to use a ceramic bob-
bin with a glass or epoxy seal over the coil and welded
connections. An alternative to the wire-wound RTD is
the
thin-film
element. It consists of a very thin layer of
the base metal, which is deposited onto a ceramic sub-
strate and then laser trimmed to the desired resistance
value. Thin-film elements can attain higher resistances
with less metal and, thus tend to be less costly than the
equivalent wire-wound element.
Most RTD elements are too fragile to be used in their
raw form. Theyʼre typically connected to extension lead
wires and housed in a protective sheath. The housing
immobilizes the element while protecting it from me-
chanical strain and environmental conditions. ATP spe-
cializes in packaging thin-film RTD sensors in a wide
variety of surface, air, and immersion housings that can
operate from -100°C to +600°C.
The table below compares some of the different types
of base metals that are used in the construction of RTD
elements.
Element
Material
Platinum
Thin-film
Nickel
Thin-film
Balco
Temperature
Range
-200˚C to 800˚C
Benefits
Best range
Best stability
Good linearity
Low cost
Best sensitivity
Low cost
High sensitivity
α
(%/˚C)
0.375 to
0.385
0.618
0.518 to
0.527
-100˚C to 260˚C
-100˚C to 204˚C
Note:
α
is normally used to distinguish between RvT
curves of the same element or those of different material.
RTD Terminology
RTDs are generally characterized by their base resis-
tance at 0°C. Typical base resistance values available
for platinum thin-film RTDs include 100Ω, 500Ω and
1000Ω. For other element types, typical base values
include 120Ω for nickel, and 1000Ω and 2000Ω for
Balco.
The resistance versus temperature relationship for a
platinum thin-film RTD follows the following equation
over its operating temperature range:
t(ITS-90)
(°C)
Equation
<0°C
R
t
= R
0
[ 1 + At + Bt
2
+ Ct
3
(t – 100)]
>0°C R
t
= R
0
[ 1 + At + Bt
2
]
Where:
R
t
= resistance at temperature t
R
0
= base resistance at 0°C
A, B and C are constants of the equation
t = temperature in accordance with ITS90
The most popular thin-film RTD is manufactured using
platinum and has an
α
of 0.385%/°C and is specified
per DIN EN 60751. The A, B and C constants for this
material are as follows:
A = 3.9083 x 10
-3
B = -5.775 x 10
-7
C = -4.183 x 10
-12
Advanced Thermal Products, Inc. • P.O. Box 249 • 328 State Street • St. Marys, PA 15857 • Phone: 814-834-1541
Fax: 814-834-1556 • E-mail: sales@atpsensor.com • Web Site: www.atpsensor.com
Resistance Temperature
Detectors (RTD’S)
ADVANCED THERMAL PRODUCTS, Inc.
Different constants are available for other grades of
platinum. Please contact the factory for information on
these other platinum grades.
The corresponding equation for one style of a thin-film
nickel elements with an
α
of 0.618%/°C is
R
t
= R
0
[1 + At + Bt
2
+ Ct
3
+Dt
4
+ Et
5
+ Ft
6
]
Where:
A = 5.485 x 10
-3
C=0
E=0
B = 6.65 x 10
-6
D = 2.805 x 10
-11
F = -2 x 10
-17
RTDs and self-heating
For Balco, individual manufacturers have developed
proprietary curves for their elements based on actual
measurements at defined points along the resistance
versus temperature curve. A number of methods are
used to define the curve outside those points. Please
contact the factory to obtain information on the types
of curves available as well as resistance versus tempera-
ture information.
Temperature Coefficient of Resistance (α)
The temperature coefficient of resistance,
α,
for RTDs
is normally defined as the average resistance change per
°C over the range 0˚C to 100°C divided by R at 0°C.
The temperature coefficient is expressed in ohms/ohms/
°C or more typically %/°C. Note that this definition
differs from the definition of
α
for an NTC thermis-
tor. The
α
for an NTC thermistor will vary widely over
its temperature range, from as high as 8%/°C to less
than 2%/°C. The
α
for an RTD does not have nearly
as large a change over its temperature range. For a stan-
dard platinum thin-film RTD, the
α
is 0.385%/°C while
other grades of platinum have
α
values of 0.3911%/°C
and 0.3926%/°C.
α
for other common RTD elements
include:
Thin-film Nickel = 0.618%/°c
Balco = 0.518 to 0.527%/°C
In one sense
α
defines the sensitivity of the RTD ele-
ment as it defines the average temperature change of a
1Ω RTD. However,
α
is normally used to distinguish
between resistance/temperature curves of the same ele-
ment or those of different materials.
An RTD is a passive device and requires a measuring
current to produce a useful signal. Because of I
2
R heat-
ing, this current can raise the temperature of the RTD
sensing element above that of the ambient temperature
unless the extra heat can be dissipated. The amount of
self-heat that will be generated is dependent upon the
measuring current as well as the ability of the sensor
assembly to dissipate that heat. The ability of the sen-
sor to dissipate heat is defined by its dissipation factor,
δ,
which has units of mW/°C. The definition for
δ
is the
amount of power that it takes to the raise the body tem-
perature of the sensing element 1°C. The ability of the
sensor to dissipate power is a function of the size and
construction of the sensing element as well as the ma-
terials that surround it in the assembly and the environ-
ment that the sensor is used in. The higher the
α,
the
less amount of self-heating that will occur. The amount
of self-heating is more for higher resistance elements
used in constant current circuits, as well as in construc-
tions where the sensing element cannot shed heat to its
outside environment. Also, self-heating is more in air
than in a liquid and in still air rather than moving air.
Effects of leadwire resistance
Because the RTD is a resistive device, any resistance
elsewhere in the circuit will cause errors in the readings
for the sensor. The most common source of additional
resistance is in the leadwires attached to the sensor, es-
pecially with assemblies that have long extension leads
of heavy AWG# wire. The amount of error introduced
into the system will depend upon the length and AWG#
of the wire as well as the base resistance value of the
RTD.
Leadwire error can be significant, especially with long
runs of small diameter leads or low resistance elements.
Fortunately, the use of a 3-wire or 4-wire system will
reduce errors to negligible levels in most applications.
The need for a 3-wire or 4-wire system will be depen-
dent upon the resistance value of the sensing element,
the length and AWG of the leadwires as well as the
amount of accuracy required. Please contact the fac-
tory to discuss your specific application.
Advanced Thermal Products, Inc. • P.O. Box 249 • 328 State Street • St. Marys, PA 15857 • Phone: 814-834-1541
Fax: 814-834-1556 • E-mail: sales@atpsensor.com • Web Site: www.atpsensor.com
Resistance Temperature
Detectors (RTD’S)
ADVANCED THERMAL PRODUCTS, Inc.
Features:
•
•
•
•
•
•
•
•
Excellent long-term stability
Platinum and nickel elements
-60°C to 250°C operation for Nickel
-200°C to 600°C operation for Platinum
Values from 100Ω to 1,000Ω
Small size, fast response time
Resistant to vibration and thermal shock
Available in standard DIN class accuracies
Description:
Resistance temperature detectors (RTDs) are charac-
terized by a linear change in resistance with respect to
temperature. RTDs exhibit the most linear signal with
respect to temperature of any sensing device. RTDs
are specified primarily where accuracy and stability are
critical to the application. RTDs operate through the
principal of electrical resistance changes in pure met-
al elements. ATP offers elements manufactured with
platinum, the most common element, as well as nickel.
The RTD element consists of a thin film of platinum
or nickel which is deposited onto a ceramic substrate
and laser trimmed to the desired resistance. Thin-film
elements attain higher resistances with less metal and,
thus, tend to be less costly then the equivalent wire-
wound element.
Ordering Information
ATP Part Number
RP102T22
RP502T22
RP103T22
RN102T25
RN502T25
RN103T25
RTD
Type
Platinum
Platinum
Platinum
Nickel
Nickel
Nickel
R@0˚C
(Ω)
100
500
1,000
100
500
1,000
DIN Class
Resistance Temperature Detectors
Drawing of a Nickel RTD
A, B
A, B
A, B
1/2 DIN,
DIN 43760
1/2 DIN,
DIN 43760
1/2 DIN,
DIN 43760
Drawing of a Platinum RTD
Basic P/N
Examples:
RTD Type
Width in mm
Length in mm
Tolerance Class
RP102T22-A ................ Platinum RTD, 100Ω@0˚C, thin-film element, 2mm x 2mm, DIN Class A
RN502T25-5D.............. Nickel RTD, 500Ω@0˚C, thin-film element, 2mm x 5mm, 1/2 DIN
Advanced Thermal Products, Inc. • P.O. Box 249 • 328 State Street • St. Marys, PA 15857 • Phone: 814-834-1541
Fax: 814-834-1556 • E-mail: sales@atpsensor.com • Web Site: www.atpsensor.com
Resistance Temperature
Detectors (RTD’S)
ADVANCED THERMAL PRODUCTS, Inc.
Resistance tolerance and temperature accuracy
Accuracy classes for platinum RTDs are defined by
IEC 751 and are typically listed as either DIN Class A
or DIN Class B. The following table shows the accura-
cies associated with the two class of elements.
Temperature accuracies according to IEC751 and
DIN EN 60751
Temperature (˚C)
-200
-100
0
100
200
300
400
500
600
350
Class A
Limit
±0.55˚C
±0.35˚C
±0.15˚C
±0.35˚C
±0.55˚C
±0.75˚C
±0.95˚C
±1.15˚C
±1.35˚C
±1.45˚C
Class B
Limit
±1.3˚C
±0.8˚C
±0.3˚C
±0.8˚C
±1.3˚C
±1.8˚C
±2.3˚C
±2.8˚C
±3.3˚C
±3.6˚C
These tolerances can be specified in another way in the
following formulas:
Class A : ∆t = ±(0.15°C + 0.002 | t | )
Class B : ∆t = ±(0.3°C + 0.005 | t | )
Note : | t | is absolute value of temperature in °C
For thin-film nickel RTDs, they are typically broken up
into three classes of temperature accuracies as defined
by DIN 43760. These classes are as follows:
Class
± Temperature accuracy in ˚C
t < 0˚C
1/2 DIN
DIN 43760
2 x DIN
0.2 + 0.014 [T]
0.4 + 0.028 [T]
0.8 + 0.028 [T]
t > 0˚C
0.2 + 0.0035 [T]
0.4 + 0.007 [T]
0.8 + 0.007 [T]
No specific standard exists for defining temperature
accuracies for Balco RTDs. However a typical inter-
changeability would be ±1°C at 0°C for Balco.
The following table lists R
t
/R
0
for the three most common types of RTDs used by ATP. To obtain the value at any tem-
perature multiply the R
t
/R
0
value at that temperature by the base resistance of the part, R
0
. For example for a platinum
RTD with a R
0
= 1000Ω, the nominal resistance value for the part at 230°C would be 1.868 x 1000 = 1868Ω.
Temp (˚C)
-50
-40
-30
-20
-10
0
10
20
30
40
50
60
70
80
90
100
R
t
/R
0
Platinum
0.803
0.843
0.882
0.922
0.961
1.000
1.039
1.078
1.117
1.155
1.194
1.232
1.271
1.309
1.347
1.385
Nickel
0.743
0.791
0.841
0.893
0.945
1.000
1.055
1.112
1.171
1.230
1.291
1.353
1.417
1.482
1.549
1.618
Balco
0.810
0.845
0.882
0.920
0.960
1.000
1.041
1.084
1.127
1.172
1.218
1.264
1.312
1.361
1.411
1.462
Temp (˚C)
110
120
130
140
150
160
170
180
190
200
210
220
230
240
250
260
R
t
/R
0
Platinum
1.423
1.461
1.498
1.536
1.573
1.611
1.648
1.685
1.722
1.759
1.795
1.832
1.868
1.905
1.941
1.977
Nickel
1.688
1.760
1.833
1.909
1.986
2.066
2.148
2.231
2.318
2.407
2.498
2.592
2.689
2.789
2.892
2.998
Balco
1.514
1.567
1.622
1.677
Advanced Thermal Products, Inc. • P.O. Box 249 • 328 State Street • St. Marys, PA 15857 • Phone: 814-834-1541
Fax: 814-834-1556 • E-mail: sales@atpsensor.com • Web Site: www.atpsensor.com
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