RTD or Thermocouple?

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coolguy24 Participant Join Date: Nov 2010 Posts: 3	RTD or Thermocouple? 02/19/2012 10:02 AM
	I read in one particular specification that RTD's shall not be used in service where high frequent vibration is present (example high velocity gas streams). Thermocouples shall be used in such cases.Why is that so?
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tonykuphaldt Power-User Join Date: Sep 2011 Location: Bellingham, WA Posts: 191 Good Answers: 46	#1 Re: RTD or Thermocouple? 02/19/2012 11:34 AM
	RTD sensors use a coil of wire (usually platinum) at the tip to create a measurable resistance. In order for the resistance of this wire coil to be large enough to provide a reasonable resistance range (100 ohms or 1000 ohms), the wire coil must be long and small-gauge (very fine wire). This makes the wire coil fragile. Thermocouples, by contrast, are formed of a single dissimilar-metal junction of any gauge wire. So long as fast response time is not a requirement, you could form of thermocouple out of thick metal bars welded together at the junction and it would work just the same as any thermocouple made of fine wires of the same metal types. Thus, thermocouples may be manufactured more rugged than RTDs. __________________ They call me "lightning" when wielding a hammer, because I never strike twice in the same place
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Ptrend Active Contributor Join Date: Sep 2009 Posts: 22 Good Answers: 2	#2 Re: RTD or Thermocouple? 02/20/2012 11:21 AM
	An additional consideration in chosing rtd or t/c is how accurate a measurement do you want. If the connecting wires are not immersed in the heat source it is possible for the copper connection wires to conduct heat away from a hotter than ambient rtd, thus lowering its reading slightly. (The rtd wires can be very much thinner than the connecting wires). The same effect is possible with t/c's but as the wires (copper-constantan apart) are usually poorer heat conductors there is less effect. I have had years of suffering temperature indications about 1°c apart, at 100°c above ambient, where t/c and rtd probes share the same dry pocket. In a bath they read the same because they are deeply immersed in uniform temperature. If the probe is in the medium being measured there is not so great a difference.
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AboutInstrumentation Member Join Date: Feb 2012 Posts: 5	#3 Re: RTD or Thermocouple? 02/22/2012 7:18 AM
	Visit this blog for answer: http://aboutinstrumentation.blogspot.in/search/label/Temperature
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