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| TECHNICAL ARTICLE - RTDS VERSUS NTC THERMISTOR |
| 08/03/06 |
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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 -50oC to +250oC 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.
Overview
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, world-wide. 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.
Comparisons
Dimensions – both available in similar very small size
Temperature range
RTD -250oC to +850oC depending on construction.
NTC thermistor -50oC to +250oC
Cost - Thermistor generally cheaper but RTDs becoming competitive
Sensitivity
RTDs 0.385 Ohms per oC for Pt100, 3.85 Ohms per oC for Pt1000
NTC thermistor around -5% per oC but international standards do not apply. Typically equates to many tens of Ohms.
Stability
RTD excellent, short and long term.
NTC Thermistor good short term; long term application dependent.
Thermal response – Both types can offer a low thermal mass but generally the NTC thermistor will be more responsive
Accuracy
RTD ±0.1 to 0.5oC for class B but much better for Class A or 1/10 DIN tolerance.
NTC thermistor ±0.1 to 0.3oC over limited range.
Interface
RTD ideally uses 3 or 4 wire interconnection. Pt1000 can be used as a two wire device for short lead lengths.
NTC thermistor is a high resistance device and 2 wire 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 non-linear. 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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