<VV> Thermister Swap

RAMBLERPAT at aol.com RAMBLERPAT at aol.com
Mon Sep 1 19:56:10 EDT 2014



 
  
____________________________________
 From: jb30343 at windstream.net
To: RAMBLERPAT at aol.com
Sent: 9/1/2014  7:37:11 P.M. Eastern Daylight Time
Subj: Re: <VV> Thermister  Swap


The key to accurate readings is getting the right voltage to  the gauge.  
Capacitors won't help.  You have to remember that  the original part and 
the Rochester part are thermistor assemblies.   That is, the thermistor 
itself is a tiny electronic component buried  inside the housing.  I 
suspect that the choice of the actual  thermistor used by GM was partly 
random.  That is, they looked for an  off the shelf component that had a 
reasonable resistance curve in the  necessary range, would handle the 
expected maximum temps, and could be  sourced at a good price.  I would 
guess that they designed the gauge  after they spec'ed the thermistor.  
You can buy bare thermistors from  any electronics supplier for pennies.  
Unfortunately, none of them  will have a resistance curve anything like 
the original.   --J.B.

RAMBLERPAT at aol.com wrote:
> J. B.,
>   Sounds interesting, I will study the situation. I was mindful of  
> keeping the stock appearance of course, I forgot to add that earlier.  
> So, in so many words, the key to utilizing the Rochester unit appears  
> to find the correct capacitor in order to use the stock gauge. Like I  
> mentioned earlier, I initially was searching for an affordable AC  
> thermister used or otherwise, and then I stumbled upon this. My need  
> for a head temp reading is still my priority at this point, however,  
> I would like to address the gap for available and affordable 
>  thermisters for both future and present Spyder and 140 Corvair owners.  
> It is too bad AC or someone else never continued production, but I  can 
> see a small market for them as a possible reason. However, they  
> seem rather simple in design, and they use a small amount of  materials.
> Patrick
> In a message dated 9/1/2014 2:51:49 P.M.  Eastern Daylight Time, 
> jb30343 at windstream.net  writes:
>
>     Patrick,  Talk with your  brother.  This stuff is not rocket
>      surgery.  I
>     have no formal training in  electronics but I have managed to
>     design  many
>     simple circuits to deal with various needs  where off the shelf
>     solutions
>   were either unavailable or expensive.  You will need to do a  little
>     algebra but nothing scary.  Parts like  op amps, resistors, small
>     capacitors and small  transistors are insanely inexpensive.  You
>     can  burn
>     a lot of them up without any real impact on  your wallet.  There
>     are some
>   good cookbooks and websites that will give you basic  schematics
>     that you
>     can  adapt to fit your needs.  You can do this.
>
>   The Rochester part you're looking at is just a thermistor in a  proper
>     housing.  The thermistor is just a  resistor that changes value with
>     temperature.   The problem you're trying to solve is that the
>      values and
>     the curve of the new part is way  different than that of the old part.
>     Here's a  little of what I remember thinking 15 years ago.  I do
>   not have
>     a great memory.  The  circuit in the Corvair is just car's nominal 12
>      volts fed through the thermistor and the gauge.  My thought was  to
>     essentially put an op amp circuit in place of  the gauge and use that
>     circuit to feed the stock  gauge.  An op amp is just a tiny integrated
>      circuit amplifier.  Using two or more, working together and
>   against each
>     other it should be  possible to bash the output into something closely
>      resembling the stock thermistor.  If I got any farther than that,  I
>     don't remember the details.
>
>   The Arduino in an off the shelf microprocessor that is designed  for
>     hobbyists and experimenters.  It's like a  digital erector set.  To
>     use
>   it in an application like this, you would put an Analog to  Digital
>     (A to
>     D)  converter in place of the stock gauge.  The converted  digital
>     signal
>     gets fed  into the processor.  You write a program for the
>   processor that
>     would basically be a  Look Up Table (LUT)  the processor then tells
>      the
>     Digital to Analog (D to A) convertor what  voltage to feed the stock
>     gauge.  The Arduino  route would be a lot bulkier and more
>     expensive  than
>     a "roll your own" solution made out of a few  Op Amps.  It would
>     also be
>   more accurate.  The really intriguing part is that you would  then
>     have a
>     computer  riding around in your Corvair which could be used for all
>   sorts
>     of other applications.   --J.B.
>
>     RAMBLERPAT at aol.com  wrote:
>     > J. B., My oldest brother is the  electrical engineer in the
>     family and
>   > he is retired in a different state, on the other hand I am a  social
>     > worker with little electrical trouble  shooting knowledge. I was
>     > thinking that the  Rochester unit is already DC because it can be
>      used
>     > in light aircraft (if that is what you  were referring to). Not sure
>     > what the voltage  would be in those though. The rest of what you
>      were
>     > saying make as much sense to me as  algebra which is close to
>     none. At
>   > some point I may try to purchase one of these Rochester  units
>     and see
>     > which  direction it leads me, and perhaps learn a few things
>   along the way.
>     >  Thanks!
>     > Patrick
>      > In a message dated 9/1/2014 12:29:15 P.M. Eastern Daylight  Time,
>     > virtualvairs at corvair.org  writes:
>     >
>     >   Sounds like you might want to take on this project,  Patrick.  I
>     >      started
>     >     down a similar road  about 15 years ago.  It's a worthwhile
>      project
>     >     but
>   >     it wasn't anything I wanted to pursue.  Here's more or less
>     where I
>   >     left
>      >     it.  A few things have changed.  There were  no bare
>     thermistors on
>      >     the
>     >      market that were close to following the curve of the one
>   used in the
>     >      Corvair.  I had not considered replacing the gauge nor was  the
>     >     Rochester
>   >     unit you found available when I was  looking.  Replacing the
>     gauge
>   >     might
>      >     be a good option but it will likely mean sacrificing  stock
>     >      appearance.
>     >     I don't think  you'll be able to match the Rochester sensor
>     to  the
>     >     stock
>   >     gauge with just a resistor network but  you should be able to get
>     >      close
>     >     with an op amp or  two.  That's the direction I was going
>     before  I
>     >     decided not to  proceed.  If I were taking on that project
>      today, I
>     >      would
>     >     think about using an  Arduino processor, an A to D converter,
>     a D to  A
>     >     converter and a  LUT.  The Arduino is major overkill for a
>     job  like
>     >     this
>   >     but once it's in the car I'm sure you  could think of numerous
>     >      other jobs
>     >     for it.  If  you have the time, you might enjoy working out a
>      >     solution.
>     >   Have fun.  --J.B.
>      >
>     >
>      >
>     >     Patrick via  VirtualVairs wrote:
>     > > To all, I have been  reading the feedback on the  ROCHESTER 
3080-38
>      > > TEMPERATURE PROBE that I happened upon while trying to save   a
>     >     few dollars
>   > > on the unfortunate shortage of affordable  thermisters
>     >     situation. It  is true
>     > > that the $300 - $400 needed to  purchase  an NOS AC thermister is
>     > >  prohibitive for many Corvair owners, and  the $80 or so
>   >     necessary for a used one is  a
>     > > risk as well. In  addition, I well  aware that this is the way it
>     >   is in the
>     > > old car  business  world and hobby. For theses reasons, and
>   >     others, I was so
>   > > excited when  I discovered the new Rochester temp  probe for $80
>     >     in the   2015
>     > > Spruce Aircraft catalog and I  recognized the possibilities that 
it
>     > >  represents.
>     > >
>      > >      It appears that no one here knew of  this  so the Rochester
>     >     3080-38  is
>     > > untested in a Corvair auto application  as  of today of course.
>     >   However, the
>     > > good news is  that there is VV confirmation that  this unit it is
>   >     essentially
>      > > the same dimensions including the thread size  as the  AC
>     thermister.
>     > >  Perhaps, this is half the battle right there. It is  also  my
>     >     understanding is  that
>     > > this thermister will work for us  if  we match it to a compatible
>     >   gauge,
>     > > after all it is  designed to measure head  temps in an air cooled
>   >     engine. This
>      > > discovery may help some of us save some  dollars and give us  an
>     >     upgrade to
>   > > more efficient and reliable  equipment too. Though  I have not
>     >     researched  any
>     > > head temp gauges yet I  am  hopeful the someone out there will
>     >   try this set up
>     > > and let us  know how it  works, provide us with the costs, and
>   >     let us know if
>      > > it was worth the  efforts.
>     >  >
>     > > Patrick Laus
>   > >
>     > > 64 Spyder  Coupe
>     > > Metro Detroit area
>   > >
>     > >
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