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This is what happens when you lock me up, CHT Meter Improver
Posted by: jetcat ()
Date: July 27, 2020 02:06PM

Finally finished my "CHT Meter Improver". I really like the Westach dual CHT gauge in my Corsa dash but it's NOT accurate. Plus, when I installed their probe style thermocouples into the head, I no longer had the snap switch to sound the buzzer and idiot light if there was an over temperature. The other downside of the meter, it's not cold junction temperature compensated. So if it's freezing outside, the meter displays 40 degrees hotter. If it's 115 degrees, it displays 40 degrees cooler. I wouldn't drive the car in either condition but it bothers me it doesn't compensate.


So my plan was to get my old Microchip In-Circuit Emulator out of the mothballs and design a circuit that would linearize the meter, linearize the J type thermocouple, and use an internal temperature circuit to measure the ambient temperature ie: engine bay temperature to offset the thermocouple readings. Also wanted to have a way to linearize the meter itself (they differ from meter to meter quite a bit - I have 3 meters) and have a settable overtemperature alarm that would signal the buzzer and idiot light. After talking to one of my old friends that was a sound engineer, he directed me to a new system that makes programming and circuit design easy. In a word "Arduino". Not important unless you're into making something like this. I already knew how to program in C and had lots of algorithms saved over the years that would help me out. I had to learn the new nomenclature. It's no longer a motherboard and daughter boards, they're now called shields???? Programs are no longer called programs, they're called sketches. Other than that, I was able to find a processor (SAMD21 running at 48 Mhz, 32-bit accumulator, 256k flash memory) for $4.90....... Unbelievable overkill but for under 5 bucks! Next, I found a "shield" for the thermocouple that uses a special chip that I can hook up any type of thermocouple to ( K, J, N, R, S, T, E, and B ). It linearizes, temperature compensates and spits out a reading that is exactly the temperature of the head in ambient conditions from -4 to +185F. I added a 4 digit display shield, relay shield, and an isolated dc to dc converter. Hooked up the wires and a bit of code, voila, it works!


So here's how it works:

There are two completely independent and isolated circuits. One for each side of the engine. Total redundancy.....

Each circuit has a push-button switch, trim pot, and a 4 digit display.

When you power up, the meter does a self-test with the needle going to 700F for 2 seconds and then to the setpoint for the alarm temperature defaulted to 500F for 1 second then to the actual temperature of the head.

Next, when you press the switch for more than 1 second, it puts it into calibrate meter mode. It starts at 100F and you adjust the trim pot so the meter needle is exactly at 100F. Then each time you press the switch it advances to the next 100F increment and you adjust as needed until you're finished with 700F. It then saves the new points in flash ram and for now on, the program will do a linear interpolation between each 100F mark. That makes the meter extremely accurate.

When you press the switch for less than 1 second you enter the alarm temperature mode. You can adjust the pot for any temperature from 100F to 700F. It has a built-in -25F hysteresis so it doesn't chatter the relay when the alarm temperature has been reached. The two relays are wired or'd so either side can trigger the buzzer.

In normal operation, the 4 digit display cycles between 4 different temperatures every 5 seconds. Each temperature is indicated by one of the 4 decimal points on the display. The temperatures displayed are, current thermocouple temperature, maximum thermocouple temperature reached, current ambient temperature (engine bay temp), and maximum engine bay temperature reached.

Quite a project for such an incidental problem but the settable alarm I think is a really good feature of the design. You could just install the two thermocouples without the meter and use the alarm system for the buzzer and idiot light. A much better system than the snap switch.

I know, too much time on my hands.......

Bob
Paso Robles, CA

Central Coast Corsa
1966 Turbo Corsa convertible
1966 Corsa 140 coupe but now with 110hp heads.



Edited 2 time(s). Last edit at 07/27/2020 02:11PM by jetcat.

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Re: This is what happens when you lock me up, CHT Meter Improver
Posted by: Phil Dally ()
Date: July 27, 2020 02:12PM

See...I told you that you're smart Bob. thumbs up

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Re: This is what happens when you lock me up, CHT Meter Improver
Posted by: MattNall ()
Date: July 27, 2020 02:14PM

Being "Smart" gets you in trouble!! IE: I have a job for you!

MODERATOR
Sea Mountain, between Charleston Harbor and Coos Bay! SW Oregon Coast
Click HERE for My Website...Click HERE for My TechPages!
..............................110-PG.................................................Webered-Turbo

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Re: This is what happens when you lock me up, CHT Meter Improver
Posted by: jetcat ()
Date: July 27, 2020 02:21PM

MattNall Wrote:
-------------------------------------------------------
> Being "Smart" gets you in trouble!! IE: I have a job for you!

I know, I've been thinking about the dwell meter....

Bob
Paso Robles, CA

Central Coast Corsa
1966 Turbo Corsa convertible
1966 Corsa 140 coupe but now with 110hp heads.

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Re: This is what happens when you lock me up, CHT Meter Improver
Posted by: azdave ()
Date: July 27, 2020 02:21PM

Nice project!

Dave W. / Gilbert Arizona
65 Corsa 140/4
66 Corsa 140/4
66 Corsa 140/4 w/factory A/C
66 Corsa 455 Toro V8
65 Monza Convertible 110/4
66 Monza Convertible 140/4 A/C
65 Monza 4DR 140/PG w/factory A/C
65 Monza 4DR EJ20T/5



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Re: This is what happens when you lock me up, CHT Meter Improver
Posted by: MattNall ()
Date: July 27, 2020 04:36PM

jetcat Wrote:
-------------------------------------------------------
> MattNall Wrote:
> -------------------------------------------------------
> > Being "Smart" gets you in trouble!! IE: I have a job for you!
>
> I know, I've been thinking about the dwell meter....

=====================================
I knew !'d hooked you!!!GGGG

MODERATOR
Sea Mountain, between Charleston Harbor and Coos Bay! SW Oregon Coast
Click HERE for My Website...Click HERE for My TechPages!
..............................110-PG.................................................Webered-Turbo

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Re: This is what happens when you lock me up, CHT Meter Improver
Posted by: Paulsgt ()
Date: July 27, 2020 04:40PM

Can we buy one? Or at least instructions and a parts list?

Enjoy the Corvair!

Paul Sergeant
CORSA Central Division Director / CORSA Treasurer
Lee's Summit, MO
CORSA since 1975
Member – HACOA, Corvair Minnesota, CORSA, Little Indians, POCI


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Re: This is what happens when you lock me up, CHT Meter Improver
Posted by: jetcat ()
Date: July 27, 2020 09:44PM

Paulsgt Wrote:
-------------------------------------------------------
> Can we buy one? Or at least instructions and a parts list?
>
> Enjoy the Corvair!

I could program a processor for you and make a wiring, parts and vendor list. I have enough parts to make three units. Two for my Corvairs and one for a local guy to use on his dyno. Not sure if I want to make any more than that. It took me all day Saturday to build one and used up all my patients fighting my failing close up vision.

Let me get a few miles on it to make sure nothing fails first......

Bob
Paso Robles, CA

Central Coast Corsa
1966 Turbo Corsa convertible
1966 Corsa 140 coupe but now with 110hp heads.

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Re: This is what happens when you lock me up, CHT Meter Improver
Posted by: Ron62 ()
Date: July 28, 2020 08:36AM

I would also be interested in this. No problem building it, just need parts list and basic schematic along with theory of operation

Ron62
1967 500 Coupe 110 PG
Residing in Massachusetts.

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Re: This is what happens when you lock me up, CHT Meter Improver
Posted by: BobV66Vair ()
Date: July 28, 2020 08:54AM

I will buy one from you. I am running the same CHT setup and really need to be accurate.

Bob Vinnacombe
Sandy, Oregon
1965 Corsa 140 stock
1966 Monza Soon to be race car
1968 Monza Parts for now

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Re: This is what happens when you lock me up, CHT Meter Improver
Posted by: cnicol ()
Date: July 28, 2020 09:26AM

Impressive! They say DaVinci invented calculus while on a plague-related lockdown. Your invention doesn't quite have the same impact but a close second, right?smileys with beer

Craig N. Coeur d'Alene ID.
66 Black Monza 4dr, 4.2L V8 49k
61 Seamist Jade Rampside 140 PG
60 Monza coupe (sold, sniff sniff)
66 Sprint Corsa convt - First car! Re-purchased 43 years later
2+2 gnatsuM 5691

+17 Tons of parts

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Re: This is what happens when you lock me up, CHT Meter Improver
Posted by: oldqmguy ()
Date: July 28, 2020 09:55AM

Vairy Impressive!

You don't mention it, but I "assume" the Output of your box drives the Westach Dual Needle Temp gauge???

Dale cool smiley

Dale E. Smiley CPBE
Life Member The Society of Broadcast Engineers
RETIRED Broadcast Engineer
CERTIFIED CORVAIR NUT
CORSA/Circle City Corvairs/Corvair Performance Group
Avon, Indiana
WB9SFF
1967 4-Door Monza PG!

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Re: This is what happens when you lock me up, CHT Meter Improver
Posted by: jetcat ()
Date: July 28, 2020 10:15AM

Yes it drives the Westach meter. They need between 18 to 22mv for full deflection. The meter is around 7.5 ohms. I have
a 1.3k series resistor with the output. The DAC output is just over 3V at maximum. It probably won't drive much more than 1k without putting in a follower amp.
Actually, you could probably drive just about any kind of meter since you can scale the output at 7 different points.

Bob
Paso Robles, CA

Central Coast Corsa
1966 Turbo Corsa convertible
1966 Corsa 140 coupe but now with 110hp heads.

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Re: This is what happens when you lock me up, CHT Meter Improver
Posted by: jetcat ()
Date: August 03, 2020 06:52PM

I had to make a modification to one of the "shields" and now it's working reliably. I'm in the process of making a second one for my coupe. Lots of tedious wiring....

If anyone is interested in the build info, PM me with your email address and I'll send it to you. I can program the microcontrollers for you or send you the code and you can do it. It's a lot of stuff you need to download to get the ICE running but it's all free.....

Bob
Paso Robles, CA

Central Coast Corsa
1966 Turbo Corsa convertible
1966 Corsa 140 coupe but now with 110hp heads.

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Re: This is what happens when you lock me up, CHT Meter Improver
Posted by: Ron62 ()
Date: August 04, 2020 07:21PM

PM sent

Ron62
1967 500 Coupe 110 PG
Residing in Massachusetts.

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