Product guide

14
RTD ▼
RTD Selection Guide
What is an RTD?
RTD stands for Resistance Temperature Detector. This is the
sensing technology that determines temperature by measuring
the change in electrical resistance across two metal wires. The
resistance value is then measured and interpreted by a RTD
thermometer, and displayed for a user to view. While RTD wire
can be made of any metal, platinum is the metal of choice due
to its excellent repeatability, stability, and resistance to corrosion
and chemicals.
The temperature to resistance curve varies for different RTD
elements. All Oakton RTD probes have an Alpha coefficient of
0.003850 Ω/Ω/°C (DIN IEC 751). Resistance at ice point (0°C) is
100 Ω. This curve is well documented and so ensures cross-
compatibility between Oakton RTD thermometers and probes
and those made by other thermometry suppliers.
Why choose an RTD?
RTDs are more accurate and stable than
other sensors, such as thermocouples, but
they cannot be used to measure extremely
high temperatures. Choose an RTD sensor
if you are willing to pay a little more for
increased accuracy and repeatability.
Conversely, RTDs have a wider temperature
range compared to thermistor probes but
lower overall system accuracy. When your
expected measurements require a balance
between range and accuracy, RTDs are the
best choice.
The table at right shows the general
trade-offs.
Instrument
Recommended
temperature
range
Widest possible
temperature
range
Typical accuracies
Platinum RTD
Probes –297 to 932°F (–182 to 500°C) — ±0.2 to 0.35% of reading
Meters
— — ±0.1% of reading and ±1°F (±1°C)
Thermistor
400-series probes –40 to 302°F (–40 to 150°C)
— ±0.36°F (±0.2°C) from 32 to 167°F (0 to 75°C)
Meters — — ±0.2 to 0.4°F (±0.1 to 0.2°C)
Thermocouple
Type J probes
32 to 1336°F
(0 to 724°C)
–310 to 1832°F
(–190 to 1000°C)
±1.8 to 7.9°F or ±0.4% of reading above 32°F
,
whichever is greater
Type K probes
32 to 2300°F
(0 to 1260°C)
–418 to 2507°F
(–250 to 1375°C)
T
ype T probes
–299 to 700°F
(–183 to 371°C)
–418 to 752°F
(–250 to 400°C)
±0.9 to 3.6°F or ±0.4% of reading above 32°F,
whichever is greater
Type E probes
32 to 1600°F
(0 to 871°C)
32 to 1650°F
(0 to 898°C)
±1.8 to 7.9°F or ±0.4% of reading above 32°F,
whichever is greater
Meters — — ±0.1 to 1% of reading and ±1.8°F (±1°C)
Time constants and
temperature response
Temperature probe response is often
stated as time constant. By definition,
a probe reaches 63% of its final value
within one time constant. Within five
time constants, the probe will reach
99% of final reading. The time constant
or response depends on a number
of factors including junction design,
sheath materials, and type of sensing
element.
System Accuracies—RTD vs Thermistor and Thermocouple
WD-17002-04 NIST-traceable certificate for RTD system (meter + probe)
WD-17000-04 NIST-traceable certificate for RTD meter
WD-17001-04 NIST-traceable certificate for RTD probe
Service includes test data calibrated at four temperature
test points.
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