Sensors, Ohm's law, and gauges.

[McRat]

testNobody ever accused me of being a fast learner!

Seems there are 4 basic kinds of sensors that affect me:

Thermocouple pyrometers. These generate voltage based on how hot they get, but only generate millivolts, or thousanths of a volt. You need an amplifier that is normally built into the gauge to read it.

12v supplied, 1-5v output. This means the voltage goes from 1 to 5 depending on what is happening to the sensor. Most of these have circuits in them to control the supply voltage so it doesn't affect the readings. Your car will make 12.5 to 14.7v depending on what it's doing, and most these sensors can go 10-24v or more before accuracy is harmed.

5v supplied, 0.5-4.5v output. These are sensors in our trucks. They need a fairly stable voltage to work according to the datasheet that comes with the sensor. Our ECM provides this stable voltage at ~5.08v

Now the last kind is 4-20 milliamp. They do not have a power lead, and are basically variable resistors. It appears the the LMM Pyro sensor is one of these. Ohm's Law is I = V / R , or amps equals volts divided by resistance. So at room temp, it's 20 milliamps, and at high temp it's 4 milliamps.

It is this last kind that is driving me nuts. The LMM pyro appears to be one of these. I can use a "voltage divider" circuit on it, and feed it 12v, and get 0-5v out of it, but the range of voltage sucks. I want the maximum "sweep" I can get between .5 and 4.5v, but I can only get 2.5v.

There must be some kind of circuit they use with these get the full sweep, but I can't find it.

 

[Cobra#3747]

I know you started to figure out what the temp vs resistance was at one time, but here is the GM break down


Temperature vs Resistance Values (Approximate)

°C
°F
OHMS


1000
1832
849

900
1652
795

800
1472
738

700
1292
680

600
1112
618

500
932
554

450
842
521

400
752
488

350
662
455

300
572
420

250
482
385

200
392
349

150
302
313

100
212
276

50
122
238

25
77
220

0
32
200

-20
-4
185

-40
-40
170

 

[McRat]

Man, that jive REALLY well with what I'm seeing! Cool! Thanks.

 

[Mike]

FWIW

Pyrometers work because of electron flow from two different metals. If amperage is used from source ( probe ) more heat is required to compensate for wattage harnessed from this probe. For example, copper and iron. These two metals touching together and heated will create electron flow. ( Ever seen what happens with copper and galvanized pipe at the water heater [ corrosion { and an occassional zap in the shower} ] ). Depending on the type of metals used for the specific probe, electron flow is dependent on just that. If the stock gm probe is used as an example, the gm specs can be used as reference. Milli amps can only be generated if small amounts of metals are used to generate this voltage flow.

 

[McRat]

Not the LMM one. It's weird. It's sensor is ceramic, and it's 0 ohms at absolute zero. Trippy.

 

[Mike]

Not the LMM one. It's weird. It's sensor is ceramic, and it's 0 ohms at absolute zero. Trippy.

What ceramic conducts electricity to a point of flowing all possible current at absolute zero? Interesting, that means that this ceramic becomes more dense as temperature comes down. Must not be a self powered probe. Is this a two wire probe? I've never seen an llm probe. Just wondering.

 

[Cornell]

I took a wiring class, remember none of it.

Sorry Pat

 

[McRat]

What ceramic conducts electricity to a point of flowing all possible current at absolute zero? Interesting, that means that this ceramic becomes more dense as temperature comes down. Must not be a self powered probe. Is this a two wire probe? I've never seen an llm probe. Just wondering.

Yes, two wire. Like the chart above, about 220 ohms at room temp. Ohms go up with temp.

This is apparently very new technology.

 

[C.A.P]

The angle of the dangle is directly proportional to the heat of the meat which in turn is related to the sag of the bag !


Oh Ohms Law ! Well
http://en.wikipedia.org/wiki/Ohm's_law

Dont ask me to use it ! There is not enough beer in Texas !

 

[Mike]

Yes, two wire. Like the chart above, about 220 ohms at room temp. Ohms go up with temp.

This is apparently very new technology.

So, I take it you get more resistance as heat goes up and less resistance as heat is removed. Have you tried heating it with a cig. lighter or something to see if it generates voltage on it's own? I may have missed this earlier as I did the above, but bare with me.

 

[McRat]

Nope. It doesn't make voltage. According to the parts catalogues, this item does not, and cannot exist.

 

[McRat]

General information.

On an LMM there are two EGT sensors, one "pre DPF" 12598443 and one "post DPF" 12598444.

I ordered at first through GMPartsDirect a long time ago, then a month later they emailed me they were "dealer only, exchange" items. Then I tried to order them from my dealer and was told they are not available except exchange. Finally I got the 12598443 "pre DPF" today, there are 79 of them in the US according to my dealer. The "post DPF" there are 0 nationwide according to them.

These are 17" long and 12" of it is wire. They terminate in a female Delphi 2-prong weather tite connector. The males for these are readily available from Casper Electronics (.com) as PN 406029, using pin 521003, grommet 409022, and lock 427057.

It is a "bottom sealing" unit, which means the hole prep is first an 8-10mm (5/16 to 3/8 ) through hole for the sensor tip, then a tap drill dia of 12.5 (1/2") that is between 10-15 mm deep. If you use a normal 135 degree drill, it will seal on the 135 cone. You will tap M14x1.5. This hole prep and the long length of the probe (50mm - 2" submersion) excludes this tip from being a plug and play item on the Duramax. This will however work most excellent for recording EGT's in the PPE riser block that comes with the 42 and up chargers.

To use this on a stock truck, I'd suggest a custom bung be welded to the passenger side riser pipe, or even better, go post turbo and weld bung on intermediate pipe.

Sketch is "bung" for probe. Make out of 3/4" stainless bar stock.
Other pics are probe.

 

[Mike]

Now I remember reading some of this on another thread. It's coming back.

When your done testing, I think a probe break-in is in order. Just to discover what that probe is made of. I'll be keeping my eyes out for sure.

 

[McRat]

People might be throwing these away, so keep your eyes open.

Next adventure is going to be logging the EGT's with one of these through the turbocharger vane position sensor.

 

[Mike]

Are these therm-ister's of some type?

 

[McRat]

Thermistors aren't supposed to go past 600°C and if they do, they can't do low temperatures. Not these.

 

[Mike]

Thermistors aren't supposed to go past 600

 

[McRat]

I see, but you said it is new technology. Just throwing out questions is all i'm doing.

No problem. I have no clue how these work, or how to get some from sources other than GM.

These work so similiar to normal $25 Summit oil-pressure gauges, that you could actually make a "normal" pyro out of them by plugging them in instead of the supplied sending unit. All you have to do is rescale it with a felt pen.

Dirt cheap, yet high tech Pyro?

 

[2wd_Sled_Puller]

Pat why couldnt u measure it in umps?

 

[2wd_Sled_Puller]

Nvm after thinking about it i know why now