Temp Gauge Calibration Marks

[Robert C.]

This may sound stupid, and has an obvious answer, but here goes; On the 63-65 temp gauges the 1st mark on the gauge is at 100deg.. then there is an intermediate mark. the next designated mark is labeled 180degs. then there's another intermedeate mark. then the final designated mark is 240degs. The question is; What are the intermediate mark temperatures? Does anyone know for sure? No guesses please..........Thank you, Bob


Texas Chapter NCRS

[Fred Oliva]

Re: Temp Gauge Calibration Marks

The temp gauge has non-linear graduations to compensate for the non-linear characteristics of the temperature sender.

What I mean by this is that at 100 degrees, the change in resistance of the sender is alot more than when the temperature is at, say 180 degrees. Because of this, the graduations on the gauge are non-linear in the opposite direction to compensate for the sender. This is what leads to the confusion about what non-numbered graduations really represent.

One way to find out is to use a an infrared thermometer to measure the temp of the manifold around the sender (or the sender body itself) when the gauge is reading at one of those intermediate graduations. This assumes that the gauge reads accurately throughout the entire temperature range.

Another way to use a linear approximation approach, ie, assume that the gauge is linear from 100 to 180 degrees, and linear (with a different slope) from 180 to 240 degrees. Then the problem is easy to solve. Just take the difference between the 2 numbered graduations, divide by 2 & add to the lower number. Thus, the first non-numbered graduation would be 140 degrees, and the second is 210 degrees.

This will give a decent approximation of what the values really are.

Fred O

[Jack H.]

Re: Temp Gauge Calibration Marks

Intermediate marks are for 'intermediate' temperatures. While this sounds like a 'flip' answer, Bob, it's about as good as it gets. Here's why.

Temp sender's temp/resistance curve saw several ECR (Engineering Change Revisions) over the years to 'tweak' and adjust for gauge technology (always trying to be downward compatible). Early temp gauges were 'calibrated' by setting gauge in a test jig providing controlled 'nominal' supply voltage and a specific resistance to ground. Then, the needle was pulled off and pushed back on to point to a given temp with a given mechanical tolerance (loose!).

Later gauges were calibrated in a similar fashion (fixed supply voltage and fixed resistance to ground). Instead of pulling and moving the pointer, a wire wound resistor was hand selected and installed between posts on the back of the gauge (you can see it once the gauge is out and in your hands) and this eliminated assembler handling effects on the gauge's movement.

Looking at the GM source drawings on temp sender and gauges is fascinating. It's pretty obvious (to me) about the only thing of concern was the gauge be accurate at its OVERHEAT limits and that it was inherently inaccurate in the range of nominal operation (around thermostat pop point 170-190F). This can be seen in the accept/reject (temp vs. resistance) loci of the temp sender's response curves. Basically, the senders have a WIDE range of acceptable resistance at low temperature with the variance narrowing with increasing temp until it pretty tightly plateaus to a 40 ohm asymptote at 240F and above.

Toss in the variables of voltage regulator triming/adjust on a given car (since gauge is simple ammeter reading absolute current from supply voltage to ground through the temp sender's variable resistance, absolute supply voltage DOES affect readings) and you've got a system that wasn't precision from the 'git go'.... Hence, when you look at the fine print disclaimers in major catalogs (Paragon, C-Central) you see notes to the effect 'these are AC senders and will vary +/- 30F.... I suspect the reason temp guages of this era are NOT labeled at intermediate gradule marks is designers KNEW the system was inherently inaccurate given the compound tolerance stack up involved.

[Bill Lloyd, NCRS#33481]

Re: Temp Gauge Calibration Marks

Jack,

Given what you have said about the Thermo-response of the gauge. Can I install the appropriate varistor on the back of the Temp gauge to enable me to calibrate it for my temp sender/gauge setup. This varistor range should be close to the installed wound type in the back. I have a 72'and the sending unit to gauge cal is so so.

[Jack H.]

Huh???

Varistor is a contraction of the words 'variable' and 'resistor' meaning an electrical component that dynamically changes its impedance with respect to some other input(s) (light, heat, radiation, Etc.). What is the 'varistor' going to respond to that you suggest installing on the back of the gauge that's sitting in the dark behind your dash in a reasonably static environment?

On the other hand, you may have choosen the wrong word and you really mean you want to 'trim' your ammeter with a fixed value, selected resistor. Using a 'shunt' resistor is EXACTLY what the factory did on later model mid-year cars per my post. The Catch-22 here is the gauge (an ammeter) is a linear device (face is graduated in a NON-LINEAR read out) and the temp sender has a non-linear response (temp vs. resistance) and the two devices are intended to agree with each other.

Now, by GM's accept/reject curves set for the temp sender, a non-ergodic transfer function was created -- variance is NOT constant across the dynamic range of the system. By using a linear trim device (resistor) you can only push the existing temp/resistance curve of your existing temp sender up/down within the fixed silhouette of the gauge. Plus, those negative resistors (if you need to go the 'wrong' way) are kind of hard to find....

Bottom line: unless the gauge is matched to the sender, you're squeezing on a baloon. If you trim 'er to match at, say, mid-range (180F) you'll be in error at one end of the scale or the other. Where do you want your error to be? That's why GM spec'd the alignment process (my guess) at full scale deflection of the gauge. What was/is important is knowing when you're genuinely pushing against the system thermal overload barrier (shut 'er down NOW, before you do internal engine damage).

[Robert C.]

Re: Double HUH??

Jack," NON ERGODIC TRANSFER FUNCTION"!!!!!!!! I am in awe, Thanks, Bob


Texas Chapter NCRS

[Jeff]

I, too, am in awe...

I have always envied the guys who actually understand the difference between impedance, reluctance and resistance.

BTW, Jack, that was a 'no' answer, right? ;)

jp

[Terry M.]

[Bill Lloyd, NCRS#33481]

Re: Huh???Huh??? and more Huh!!

Jack, et al.,

You are correct - trim pot. is what I meant. Anyway, I would be able to one point calibrate the sending unit to the meter at the overheat point(230F). Even though the response of the sending unit (thermistor)is non-linear, this one point is better than the alternative. I suspect that the 25 cent resistor is an even cheaper proposition. The advantage of a wide range trim pot is the ability to change the setting with new sending units.

A 750 KV line rack in is positively impressive - Will raise your HAIR !

[Fred Oliva]

Re: Huh???Huh??? and more Huh!!

By adding resistance to the system for the purposes of calibration, you can make certain areas of the gauge read more accurately. However, where to add resistance is the key. Depending on which way the sender/gauge pair is off, you may want to add series resistance, or shunt resistance (across the sender itself). Since the most common form of inaccuracy is gauges reading lower than actual temperature, adding shunt resistance across the sender is the most common solution.

As Jack states, tweaking the system by adding resistance brings an area of the gauge into better calibration, however, introduces additional non-linearity into an already non-linear system. The sender/gauge can usually tolorate this provided it was not too far out to begin with.

Once again, as Jack states, it is by far a better approach to match the gauge to the sender than the other way around. This needs to be done on the bench with a good lab type power supply & a heated water bath. One interesting discovery while doing this was that when I varied the voltage from 12 volts to the nominal 13.8 volts specified by GM to calibrate the gauge, the gauge needle didn't really move very much.

I'm anxiously awaiting my Restorer to see if any of these issues are addressed in the article.

Fred O

[Fred Oliva]

Re: Temp Gauge Calibration Marks

If you mean can you adjust the calibration by adjusting the resistance on the back of the gauges, yes you can. The resistor Jack speaks of is only on the back of 65 & later mid-year gauges ( Jack, do shark cars have the shunt resistor?). Pre '65 gauges have no shunt resistor on the back of the gauge.

Since the resistor is a wire wound resistor tht uses resistance wire, you can either add or remove wire from it until the gauge reads where you want it to read. Naturally, you need a setup in order to calibrate it correctly.

See comments later in this thread that address what adding resistance really does to the gauge calibration.

Fred O

[Jack H.]

Ergodicity....

Sounds like a bunch out there in Corvettdom got a grasp of EE concepts. Also appears there's a weakness in theory of statistics....

For those who requested an update, most conventional statistical theorems are based on the concept of ergodicity -- meaning the universe from which the sample population is drawn is time invariant in comparision to the parent population. If a system is non-ergodic (mean, std deviation, Etc. vary from sample interval to sample interval), then conventional statistical analysis lemmas fall apart!

In the case of the this system, designers were 'dinking' with the transfer function of the sender itself. Sooooo, you've got a mixed bag of worms....

Is the sender you're matching to your gauge old, new, real old, intermediate new (get the picture)? In simple Western American English, we call it squeezing on a balloon. That's why Fred Oliva's comment (pull both of the key culprits, sender and gauge and match 'em on a lab bench) is where the real rubber meets the road!

There are so many variables in this 'Corvette overheats' syndrome that it's like pulling rabbits from a hat and I'm downright shocked to see how many reasonable men approach the problem/issue from the standpoint of taking the )(*&darn cockpit gauge reading as Gospel! Can't start to tell you how much well intentioned (but ignorant) $$$$ I've seen thrown at this 'problem' by hit and miss swap out methods.

Heck, I've got one temp sender that was hand selected (from a BUNCH of scrap yard orignals) to run smack dab down the middle of the GM temp/resistance curve. I use it as a lab standard.... Had one guy with a '60 who'd tried EVERYTHING and spent over $2000 on various repairs actually offer me $750 for the sucka when I showed him it made his gauge read properly!!!!

Oh well, knowledge is power, the soothsayers say....

[Wayne M.]

Re: Ergodicity.... Whetstone ?

Jack --- now that we've got that perfectly clear, please define your (frequent) use of the term "Whetstone". All this aging CE remembers from the mandatory EE courses is the term WheAtstone bridge, used to measure resistance with 4 resistances, one of which is unknown, and one of the known being adjustable.