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CHT probes

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dbaleshta(at)tru.ca
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Posted: Sun Aug 30, 2009 7:43 pm Post subject: CHT probes

I don't think I've seen this anywhere on the list, but is there a difference between a type K thermocouple ring style (sits under the sparkplug between the sparkplug and the head) vs the straight style that screws into the head?  And, I believe Bob (or someone) mentioned that standard aircraft wire can connect to the thermocouple wires if  they are not long enough, provided the lengths are identical, is that correct?



thanks

Doug

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ainut(at)hiwaay.net
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Posted: Sun Aug 30, 2009 9:54 pm Post subject: CHT probes

For TC's, Greg, it is not recommended.  The unit depends upon an exact 

measurement between two certain wires (dependent upon TC type.)



David M...



Doug Baleshta wrote:

 	  Quote: 	
		   



 I don't think I've seen this anywhere on the list, but is there a difference between a type K thermocouple ring style (sits under the sparkplug between the sparkplug and the head) vs the straight style that screws into the head?  And, I believe Bob (or someone) mentioned that standard aircraft wire can connect to the thermocouple wires if  they are not long enough, provided the lengths are identical, is that correct?



 thanks

 Doug



   

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harley(at)AgelessWings.co
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Posted: Mon Aug 31, 2009 4:19 am Post subject: CHT probes

Doug Baleshta wrote:  	  Quote: 	
		      	  Quote: 	
		  --> AeroElectric-List message posted by: "Doug Baleshta" <dbaleshta(at)tru.ca> (dbaleshta(at)tru.ca)



I don't think I've seen this anywhere on the list, but is there a difference between a type K thermocouple ring style (sits under the sparkplug between the sparkplug and the head) vs the straight style that screws into the head?  And, I believe Bob (or someone) mentioned that standard aircraft wire can connect to the thermocouple wires if  they are not long enough, provided the lengths are identical, is that correct?



thanks

Doug 	
 	
 Morning, Doug...

 

 I'll let someone else answer the difference between mounting a TC under the plug and in the head...obviously, the temperatures will read differently, but as far as I know, once you get used to what your readings are, either method is acceptable.

  

 As for connecting thermocouples, for the most accurate reading, the wires AND connectors along the entire length, right up to and including the instrument reading the signal itself, should all be of the same two dissimilar materials. This includes any terminal or plug connections as well.  These are available at any supply house that deals in thermocouples and are not expensive. Omega Engineering is a great source of information and components and is online.  It was one source of TC and RTD supplies in my other life when I was involved in automating a pharmaceutical company's manufacturing. 

  

 A TC is simply two wires of dissimilar material connected at one end.  The different types of TC (J,K,T, etc.) have different sets of materials in them for different ranges of temperature.  

 

 Thermocouples don't work by having a voltage or signal supplied to them,  but are generators of a very small voltage themselves, which changes slightly as the temperature changes.  Monitoring this voltage change is what the readout instrument does, then massages the reading and converts it to numbers that relate to a temperature scale that you have selected. 

 

 Connecting ANY two dissimilar metals, not just the ones used in TCs, will produce voltages. So, connecting the specific wire in a thermocouple (iron, constantin, etc.) to another type of wire or connector (copper, steel, brass, aluminum, etc.)along the route to the instrument will (not "might") generate additional voltages that may add to or degrade the signal you are trying to read.   

 For accurate readings, all the wire and connectors should be the same material as used in the TC itself. So, since the two wires in a TC are DIFFERENT materials, even when using TC extension wire or thermocouple couplers or plugs, be sure to connect the wires to the terminal or extension wire that is the same material (they are color coded) or you could seriously degrade or even cancel your signal.  Remember, even solder (which is lead or tin) will generate it's own voltages in contract with the TC wire, and will degrade the signal between the two wires, so use compatible mechanical connections wherever possible. Many TC materials don't solder well anyway. 

 

 For a clearer description, read this: www.omega.com/thermocouples.html and this:

 www.omega.com/techref/themointro.html

 

 Harley Dixon    [quote][b]

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