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Discussion Starter · #1 ·
Let me first state my current understanding (flawed as it may be) of how the old tried and true twin coil gauge works and see if my intended use "flushes". Aside from the various "chokes" and "shunts" for particular applications (voltage and amperes for example) essentially most analogue gauges have two coils ........one wired to bat voltage (modified as needed)to ground and one to the repective variable resistive device (pressure or temp sensor) .......as the the sensor provides a better ground ........the needle moves towards Full Scale Deflection (xxma).
Here's my question: First assume that I have NO LIGHT in the gauge ......if I used one temp sensor for the gauge's ground ........and my other sensor as designed ...........(reading two different temps or pressures) ......wouldn't the gauges indicate the "difference" between the two (temps or pressures). Now I'm not asking for the value of such an arrangement........just its viability.

Give me your thoughts........

GeeAea

 

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/wwwthreads_images/icons/tongue.gif Lessee....as the ground to the first coil goes to zero, the needle moves toward the other side of the gauge....farther as the second coil gets a better ground from temp rise....seems like it would work.... Needle sits straight up with zero difference....deflects right or left as the temps moved away from each other. What's the application? Are you doing my temp-gauge-for-each-bank trick. We used to use the water connection in the back of the big Chrysler Firepower Hemis for that very thing, with mechanical gauges. You still need a gauge for absolute temp though with your idea. Difference doesn't tell the whole story./wwwthreads_images/icons/crazy.gif

CJDave
 

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Discussion Starter · #3 ·
Just the readout freak in me Dave .......I was thinking of pressure and temp drops across coolers and such .....I think if you use the right kinda switch (some type of rotory)......you could get the same gauge to give you all three readings ...in .....out...and diff (so ya didn't have to do the math in your head). But I think the gauge would sit to the left as usual .....just like it does when it's off (maybe they're sprung that way) and then the coils would have a tug o war ....off of zero.....I dunno....I guess it really wouldn't show much with a 100-320 degree temp/90 degree sweep ......heck if you had a 25 degree differential .. would it even show ?????? I don't think that the digital gauges would work in this manner.......????
My "big truck" drivin' was somewhat limited (5speed direct Macks mostly POSs) ......but a buddy I ran into once and awhile had a Kenworth with ......lessee here..... 480 CAT??? ......lotsa temps ...left head, right head, diff, trans.....etc......very entertaining dash it was. It had to be since he was workin' very hard to pay for such a fine machine.......as I recall.

GeeAea

 

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/wwwthreads_images/icons/tongue.gif back in my Olds diesel days, I was real nervous about overheating, so I deemed it imPOTNENT to have two guages....one for each bank. Heck, the guages should ALL have two dials, top and bottom, like my old MG had. The oil and temp was combined in one gauge, and when the needles were a vertical line, all was well./wwwthreads_images/icons/crazy.gif Yeah, I love full dash arrays...especially since I have no sense of smell whatsoever(yeah, I'm applying for "victim" status), and need to KNOW if the engine is hot...I can't smell the rubber stinkin'./wwwthreads_images/icons/crazy.gifSniff...sob...POOR little handicapped me; I want special rights and priviledges....ALGORE...where are you? I need care and cash...sperm to worm....I'm NEEDY! Save me!/wwwthreads_images/icons/crazy.gif

CJDave
 
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Good try and your kinda close on how it works, but no cigar. If I can offer a better description without having to draw a picture....The gauge is actually based on the old tried and true current loop design...i.e., the guage is actually a current transmitter and the sensor is variable resistor with one side grounded (like the engine block). 12VDC is connected to one side of the meter coil and the other side goes out to the + side of the sensor. Typically, most sensors have a negative resistance factor which means the resistance goes down as the temp or pressure goes up. Less resistance means more currant flow, thus the needle moves up. If the circuit breaks anywhere, the needle reads nothing. This actually makes it very simple to troubleshoot.

Now, here is where your idea kinda sputters. First, unless you can find and un-grounded sensor, there is no way you can hook them in series. If you hook 12vdc to the + of the sensor without some resistance between it and ground u will also burn it out. This leaves parallel as the only connection method. If you connect each sensor to +12 via a resistor (WAG 100 ohms) and then put a meter across the + side of each sensor, it will show a positive or negative deflection depending on which sensor is reading higher. This also assumes you are using a differential meter, i.e., 0 in the middle and a positive and negative scale on the sides. As for accuracy, it would take a voltmeter, adjustable resistors and some playing around to get the resistance values right and figure out what the meter is actually reading.

Or you could just add another temp guage.

Hope this helps....

John......southern CA
84CJ7,3"lift,32"BFG
 

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Discussion Starter · #6 ·
Everybody!!!!!!!! Dave has no sense of smell !!! Let us not have this injustice continue any longer than necessary.........those of alfactory impedements have suffered long enough!!!! How can we as "normal" human beings not be sensitive to such a crippling afliction? I'm calling everyone in congress right now! I want legislation introduced to ease the plight of these unfortunate individuals like Dave. I think that all things should have "alfactory" aides attached to them.....the thanksgiving turkey (and who would denigh anyone the joy of smelling a thanksgiving turkey?) would have a sign attached, with the bi-lingual words "wholesome turkey aroma" printed on it. Perhaps as a stop gap measure funding could be provided for "Nasal Surogates" (specailly trained "smellers") that will give verbal prompts to those who can not smell for themselves......I can see a future with special sections in resturants..... How else will these people be "mainstreamed" into our society?
Won't you please get on the phone .....right now and call your congressm...congressperson?........and with your tax dollars.......please give generously!

GeeAea (what I'm not telling you is that I just opened a chain of "alfactory treatment centers" and am chairma...chairperson of the Alfactory Impairment & Research Foundation and hope to get special funding for junket retreats to resorts)

 

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Discussion Starter · #8 ·
DepD.......let me step back a second and think of what you're saying ..........I agree in the case of a volt or ammeter.....run a given current through a resistor and it will move your analog meter needle only so much toward FSD ........for a given voltage range ....that is, xma through xohm "multiplier" = x volts (or actually Xvolts applied through Xohm resistor = Xma = so much of FSD).....for amps you merely "shunt" the excess current around the meter ......so the parallel path of the current flow does not exceed your spec for FSD.......in the case of 60 amp meter which has a FSD of .006A (6ma) ......you would want to "shunt" 59.994 amps around the meter so your internal resistance of the meter has an actual ratio of 1000:1(if I got my math right here)......that is, the shunt should have 1/10000 the resistance of the meter .......hence it will have 10000 x the current flow. This I buy........100%
But ........your other meters .......at least on those I'm aware of ......would not work accurately if this was the arrangement....suppose you have the same temp guage that I have .........but your altenator is only putting out 14.7volts........while mine is putting out 15.8v ..........at the same temp .....you've got a different reading since the resistance of the sensor is identical for both - (same resistance - less voltage applied - less deflection)......hence, to my understanding to prevent this from occurring .....the manufacturer puts two coils in the meter .......one is wired to BAT voltage to ground (the left coil) .....the other variable resistor (ResistiveThermalDevice -sensor) to ground (the right coil) ........in this way no matter what the bat voltage is .............the reading is correct. That is why I thought that the gauge being grounded through a temp sensor would work .......but .....now that I think about it (and with your input on the matter)......would actually provide less "back pull" on the needle and would in fact add positive deflection.

This is why I have a hard time with your second paragraph .........if what I say is true (and I can be wrong ......but that was what I learned in "electrics" back in the day.....) then I would still get an accurate reading with your arrangement. That is, if I installed a non-grounding variable Resistive Thermal Device (RTD) in series with the meter I would not only reduce the voltage to the varible (RTD'd) coil that goes to the "original" sensor ......I'd also reduce the voltage to the fixed (or more accurately stated "constant") coil's voltage ........and therefore reduce it's "pull back" ........hence I believe I'd get the same reading..........as this would just simulate a "low bat" condition. This also puts your "resistive voltage divider" into question since anything except a "difference in amplitude" type meter would have a fixed spec for FSD .......which cannot compensate for varied supply voltage.

I do agree with your assertion that this would be an waste of time compared to just adding another guage. I like Daves idea of a centered gauge with plus or minus readings...........but......yes.....add another gauge.

I would invite further input on this.........I would think CJTaz has some thoughts on this

GeeAea

 
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Gary,
Based on what I wrote, your right. I forgot to add that in order for the gauges to work correctly, it requires a constant voltage source. Most systems use a cheap zener diode in the gauge to drop the voltage way down below the battery voltage in order to prevent the problem you noted regarding the effect of battery voltage on meter reading. I remember when my oil pressure sending unit was acting up. I measured the current going through the unit and confirmed that it was the sending unit (and how the meter was set up to measure). I fixed it by tightening the nut on the signal post.

Back to your problem. Given the inaccuracy of vehicle temp gauges, may I suggest that we install a PLC (like a AB SLC-500) with temperature measurements cards (RTD cards are expensive). We could then wire up several points on the engine for temperature. In fact, we could wire air flow, air temp and even the O2 sensor. Program a touchscreen display such as a PanelView or PanelMate and you could monitor everything in one glance. Whats really nice is we could add more information with simple programming and we wouldn't have to cut any holes in the dash. If we get really adventurous, we could wire into the diagnostic connector and see problems as they happen..............umm......what was the original question?

John......southern CA
84CJ7,3"lift,32"BFG
 

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/wwwthreads_images/icons/tongue.gif Uh.....what ever happened to a good ol' bourdon tube gauge, fed by a tube...from a bulb. Now THAT I understand./wwwthreads_images/icons/crazy.gif

CJDave
 
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myself i like lots o gauges. i also use the racers trick of turning them so that when all is good the needles point straight up. saves having to look at them to figure out what is going on. i am also not a fan of electrical gauges. not accurate enough. but for what you are doing, there are some two prong senders(isolated ground) that would work for what you are trying to do. i do like the idea of using thermo probes and a programable display.

dan

I do not intend to offend,I offend with my intent
 

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My advice (like I know gauge theory/wwwthreads_images/icons/wink.gif), is to approach the question from a KISS angle and just install two gauges.

later on......

jimbo

'74 CJ5,232 I6,T15,Dana 30&44
 
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Man, I'm glad you got off those resistorstats, rotortooters and ERKs a started talking about something I can understand. Hey, DAVE, Hey, I'm sorry. I didn't know you didn't smell. I thought thought everyone that drove Jeeps had to smell. Must be that California air. Just about everyone I know here in Houston smells. My kind of folks!!!

Doug '97 TJ
My Web Site
 
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/wwwthreads_images/icons/frown.gifPLC??PLC?? Programable logic controller??right?/wwwthreads_images/icons/frown.gif
/wwwthreads_images/icons/crazy.gifI know what it is but I just can't spell it./wwwthreads_images/icons/crazy.gif
/wwwthreads_images/icons/blush.gifJust where you going to mount the darn thing./wwwthreads_images/icons/blush.gif
/wwwthreads_images/icons/tongue.gifCould we get that thing to give us inside and outside
air temp too./wwwthreads_images/icons/tongue.gif
 
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Your right....of course we could also use a Micrologix or SBC in an embedded controller application. Want to measure outside temp?...no problem...I can even give you the temp of the gas tank. Its like everything else, how many temps u want to measure?...how much money you got?

John......southern CA
84CJ7,3"lift,32"BFG
 

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Discussion Starter · #18 ·
I'll use the somewhat obsolete Honeywell 3000 UDC ...coupled to a GEnuius block .......daisy chain the RTDs and then just toggle the display ........lessee......I should be able to pole at least 15 output devices .......for pressures ...the Foxboro DPT ......should work ok.....4-20ma output......span it for the appropriate usable range .......



 

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Discussion Starter · #20 ·
Yes.......they're still alive and well ......I guess that they produce good enough hardware ......and can't be replaced off shore ......"for a few dollars less". I guess that it would be different if they made consumer goods.

 
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