<VV> CHT data (long - possibly boring)

Larry Forman Larry@Forman.net
Sun, 24 Oct 2004 09:57:25 -0700


Hi Roger,
MANY thanks for this information.  Yes, I knew a LITTLE about this, but not 
the significant details.  Maybe I did not pay enough attention in that lab 
class.  LOL.  I'll need to look into this further relative to accuracy for 
our use.  I also run VDO gauges and have a little switch up front to select 
right and left senders.  I did not use thermocouple wire to run to the 
gauge, but I now have access to this wire.  I mounted mine at the stock GM 
locations and used heavy brass washers and a bolt to secure the 14mm 
rings.  I appears to work well, but I have not used any laser temperature 
meter to compare, which should be quick and easy.

Larry

At 10:03 AM 10/24/2004 -0500, Roger Gault wrote:
>Hi, Larry,
>Yes, the VDO gauges are thermocouples.  Iron-Constantan thermocouples, I'm
>almost certain.  And yes, TCs do put out a voltage relatively proportional
>to the temperature at the junction.  But the TCs have a nasty family secret.
>They bring one or two of their relatives along with them.  There are always
>at least two in the circuit.
>VDO doesn't tell us what they assume as a reference temperature, but when I
>measured mine using 200F water and 95F air (Texas summer) I got about 140 on
>the gauge.  So my gauge is calibrated for a 65F connector.  I suspect VDO
>targeted 70F.
>
>By the way, that's why, according to your gauge, your engine doesn't seem
>run any hotter in the afternoon than it does in the morning, even though
>it's hotter in the afternoon.  What you're really measuring is the
>temperature rise of the engine over ambient air, which is pretty much
>constant for a given load condition.  (Yes, there is a density difference,
>so it might cool slightly more efficiently when the air is cold - give me a
>break. <grin>).
>
>I looked for a better explanation, but I couldn't find the article I was
>looking for - web based information glut.  The following one might help.
>http://www.allaboutcircuits.com/vol_1/chpt_9/5.html
>
>Roger Gault