How to choose between temperature sensors
A
Labfacility product story
Edited by the Engineeringtalk editorial team
May 8, 2006
This short article compares and contrasts the benefits and shortcomings of RTDs and NTC thermistors in today's demanding measurement applications.
The demand for improved temperature sensor accuracy is increasing with the development of improved measurement and control systems and with the demands of industry.
For certain applications in the range -50 to +250C including environmental monitoring, HVAC, food processing and distribution, aerospace and automotive, the system designer has two preferred alternatives (although others are available); these are RTDs (resistance thermometer detector) and NTC (negative temperature coefficient) thermistors.
This technical note refers to sensing elements only, not to sheathed assemblies.
RTDs, usually Pt100 or Pt1000 sensing resistors offer ultimate accuracy over a wide temperature range; they are generally rather more expensive than NTC thermistors and have a lower sensitivity.
However RTDs do have almost universal interchangeability, worldwide.
NTC thermistors are high-sensitivity devices with a restricted temperature range and are less expensive than RTDs.
They are more rugged and are therefore preferable in applications subject to high vibration.
In terms of dimensions, both are available in similar very small sises.
RTDs cover a temperature range from -250 to +850C (depending on construction), whereas NTC thermistors cover -50 to +250C.
Thermistors are generally cheaper but RTDs are becoming competitive.
RTDs have sensitivity values of 0.385ohms/C for Pt100 or 3.85ohm/C for Pt1000; NTC thermistors display around -5%/C but international standards do not apply.
This typically equates to many tens of ohms.
In terms of stability, RTDs are excellent on both short and long term aspects.
NTC thermistors are good in the short term; however long term stability is application dependent.
Both types of sensor can offer a low thermal mass but generally the NTC thermistor will be more responsive.
For accuracy, RTDs provide +/-0.1 to 0.5C for class B but much better for Class A or 1/10 DIN tolerance.
NTC thermistors offer +/-0.1 to 0.3C accuracy over a limited range.
At the interface, RTDs ideally use a three- or four-wire interconnection.
A Pt1000 can be used as a two-wire device for short lead lengths.
The NTC thermistor is a high resistance device and two-wire interface is used.
So which is better? This depends on the type of application and on the operating temperature.
Both require a suitably configured instrument input.
The NTC thermistor provides high accuracy and high resolution over a more limited temperature range and is generally cheaper; the transfer characteristic is very nonlinear.
The RTD is universally interchangeable with a near linear transfer characteristic and provides high accuracy over a wide temperature range but may cost more.
Labfacility can offer applications advice on these and other temperature sensors.
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Tel +44 1243 871280
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