Engine/Transmission (1998.5 - 2002) Pre & Post Thermocouple Toggle Switching

[rblomquist]

Hey guys. Just installed my Westach Combo last night and decided to go post turbo with the thermocouple that came with the kit. Now, before you start to beat me up about going post turbo please notice that I'm stock so I figured that going post turbo at first would be benefitial for the following:



- Knowing when to shut down would be more functional at the present time



- Would be better for a novice to practice tapping in the elbow vice the manifold



- I can easily add another thermocouple pre-turbo later for $19 at Geno's (prior to staring any fueling BOMBs)



Now, with that said, I've noticed some folks are using a toggle switch from Radio Shack to switch between thermocouples (I like that concept). I did however see 1 post that mentioned it (the switch) would throw off the accuracy of your readings as well as seeing someone else mention something about installing a amplifier along with the switch.



I'm about as dumb as they come in regard to electronics so if someone could give me the 411 on what the "real deal" is on using these switches i. e. semi-accurate/need an amlifier etc, I would much appreciate it.



p. s. I gathered that one should ROUGHLY not go over 900 deg temp while monitoring post turbo? I realize it is not totally accurate and also not sure if I could get that high stock anyway.

 

[LSMITH]

A switch will not affect the accuracy of the pyro if the switched leads are the positive leads and the negatives are hooked together. You could also put in a double throw double pole switch and isolate the probes completely. I would isolate them with the DP/DT switch, but can't really give you a good reason why.



The amplifiers are needed with electronic pyros, not the mechanical type.



EGT stock can get high, don't be surprised to get it up there.

 

[15w40]

For anyone who does not want a “Nerd” answer to the thermocouple switch question, skip the following.



The type “K” thermocouple, (the one we use to measure EGT), puts out 20. 640 mV (millivolts) at 500*C (932*F). At 600*C (1,112*F) it puts out 24. 902 mV.



If I do the math, a 100*C (212*F) change in exhaust temperature will make about a 4. 26 mV change in the thermocouple output voltage. The thermocouple voltage does not follow a straight line for temperature vs. output voltage but for small temperature changes you can assume it does.



Any time two metals touch a thermocouple is formed. If the two metals are the same, (like copper-to-copper) the thermocouple voltage will be almost 0. If the metals that are touching are different a voltage is created, even at room temperature. The type K thermocouple is made from Chromel/Alumel metals. Connecting the thermocouple wires to a switch will create two more thermocouples at the connection and another two at the interior switch contacts. I have measured 10s of millivolts across switch contacts. If you add that error to the error created in the thermocouple to switch connections, it is not uncommon to see 25 mV of error. This works out to be about 260*C (500*F) and will vary greatly with the temperature of the switch.



A switch designed to switch thermocouples will use the same metals as the thermocouple and have a property called low thermal EMF. They are expensive but if you use a Radio Shack switch, you might as well hold your finger on the exhaust to get a temperature.

 

[LSMITH]

Wow, good answer 15w!!



I stand corrected, buy a switch from a pyro maker.

 

[illflem]

I saw the switch connection creating a thermocouple and high readings first hand when installing a pyro toggle. Had a feeling that this might occur and knew right away where to look. Changing over to a switch that had copper terminals rather than the nickel plated ones on the first switch fixed the problem. Both were solder connected.

 

[Michael Hughes]

15w40,

Based on what you are saying, I should see 500F or higher on my gauge all the time (I am using a Radio Shack 275-663 DPDT Mini Toggle Switch), yet when the engine is cold, my gauge doesn't read anything (Westach combo gauge). I have been running it this way since Mar. 2001. Even if I disconnect the switch from the gauge (nothing connected), the gauge is in the same place. You can look at the wiring diagram .

 

[15w40]

The 500* error in the thermocouple reading is probably exaggerated in my example given above. The problem is it can get as bad as this when the switch is hot, on a hot summer day. The error caused by the parasitic thermocouples created by the switch can be in either direction. It may cause your EGT reading to be low by 500*. Since the EGT gauge has a hard stop a 0*, you may never see any error on the gauge but the ?real? number for EGT is higher.



The Radio Shack switch you chose to switch between the two thermocouples looks to be of reasonable quality. Typically this switch has gold flashed contacts internally which is good to get a low thermal EMF connection. The problem is the gold flash is only measured at about 2 to 5 micro-inches and will be displaced after some number of switch actuations. By using a DPDT switch as you have done, any parasitic thermocouple created on the chromel side of the thermocouple is balanced by a parasitic thermocouple on the alumel side. This will help with the accuracy as long as they match. As the switch wears, the matched parasitics degrade. Do you know what the base material is under the gold flashed contacts? How many mV, and in what direction (+ or -) will the error be if one pole of the DPDT switch wears faster than the other.



If you run stock or close to it, the accuracy may not be important. If you are pushing the envelope with HP, do you want to take the chance of inaccurate EGT readings?

 

[rblomquist]

Originally posted by 15w40

The Radio Shack switch you chose to switch between the two thermocouples looks to be of reasonable quality. Typically this switch has gold flashed contacts internally which is good to get a low thermal EMF connection. The problem is the gold flash is only measured at about 2 to 5 micro-inches and will be displaced after some number of switch actuations.

15w/40 - Can you recommend a source/part number to get a switch worthy of this purpose? Putting in another gauge is not an option for me.

 

[15w40]

To answer the question on which switch to use with thermocouples is a little complicated, but here goes. (Moderator, if this thread is getting too nerdy, I will respond off-line using a PM. )



EGT gauges that are self powered, do not use amplifiers, are the ones that require a special switch. The Westach gauge mentioned above is self powered. I am unable to find an accuracy specification for the Westach gauge but they claim it has been calibrated at 75*F. Where the thermocouple wire from the probe connects to the copper wire from the gauge an error is created. This wire connection will create a negative 1* error for each 1* temperature rise above 75* and a positive 1* error for every degree below 75*. If your under-hood temperature is 200*F, your EGT gauge is reading 125* lower than the actual EGT.

If you go to the Westach web sit at:



www.westach.com



And click on the catalog link, page 37- switches, they show some that would work at about 8X the cost of the Radio Shack switch. I am sure there are others out there making these switches.





EGT gauges that use amplifiers that connect between the thermocouple and gauge do not require a special switch as long as you add the switch between the amplifier and gauge. This requires buying a second amplifier. These amplifiers contain a temperature compensated ice point reference and accuracy is not compromised by under-hood temperature, or at least it should not be.



If I was switching between two thermocouples using a self powered, non-compensated, EGT gauge, I might use the Radio Shack switch and change it out any time my stabilized idle EGT was not at about 350*, or at anytime the two thermocouple readings disagreed at idle. If making lots of horsepower, an amplified EGT gauge would be my first choice, and I would periodically check the calibration of the meter using a known voltage source. Thermocouples should not need to be checked unless they show damage.

 

[Big-D]

Thanks 15W40. Very informative information.

 

[Michael Hughes]

15w40,

Westach has in their instructions to locate the termocouple cold junction to patch cable away from heat or extreme cold to minimize thermal error. I would think placing the junction inside the cab will help reduce the error.



I see temperatures at idle of around 300° for both pre and post turbo (after allowing the turbo to cool).