<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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