MGL Thermocouples

I've got a new MGL EGT gauge with 4 new MGL thermocouples on a test engine here. At room temp, all 4 read identically. One probe appears to be about 150F cold at operating temps as I moved it to another cylinder and then it became the "cold" one.

All connections are identical and solid.

Anyone seen similar issues with these probes (PN 11-07842 ACS)?

MGL TC's

Yes Ross, I have struggled with theses problems from the start. One Cht reads perfect at ambient and reads correct in boiling water, but is WAY off when the engine is operating.
Therefore, My current project is 100% Garmin with short TC'S so troubleshooting and /replacement is MUCH easir.

Hmmm doesn't speak too highly for MGL...

No problems with MGL gear

All my sensors came directly from MGL and I haven't had issues with anything. I have their first EFIS; Enigma. Probably 10 years old now but works well. I'm a satisfied consumer.

Thermocoule Wiring Problem

If I were diagnosing this problem I would first look for three things:

1) Do I have any metal in the thermocouple path that is not of the same alloy as that side of the thermocouple? That is, is there any possibility that the thermocouple wires are connected to a different thermocouple alloy material or something like copper wire. This includes connectors and terminal strips.

2) Are the terminals / connections where the thermocouple wire enters the instrument at the same temperature? This is necessary to mke sure that the cold junction copmensation part of the instrument is removing or compensation for the EMF that is generated where the thermocouple alloy wires transition to the (typically) copper material that is used in the instrument.

Take a look here to help understand how thermocouple measurements are made:

https://en.wikipedia.org/wiki/Thermocouple


I have seen installations that had similar indications to what you are seeing when a Type K thermocouple was connected to a terminal strip / connector in an heated compartment. And then Type K thermocoupld wire was connected to the same terminal strip / connector and went on to the instrument. In the shop everything was fine and the measurements were accurate. Once the equipment was put into service and the compartment started to heat up the measurements went to heck. What happened?

The terminal strip was plated copper so we have chromel to copper thermocouples than the a copper to chromel thermocouple. On the other leg we have alumel to copper and copper to alumel thermocouples. Everything is OK as long as those unintentional thermocouples are at the same temperature. This is because all of the EMF that is introduced in the circuit is nulled out by the other unintentional thermocouple on that leg of the circuit. In addition because both pairs of terminal are at the same temperature (isothermal) any errors introduced by the terminal strip connection are nulled out. Once the compartment was heated we had differential (small) temperature differences across the terminal strip /connector and we started to see these unintentional thermocouples that were no longer nulling each other out and showing up as erroneous measurements. An example of this is using a pair of DSub connectors with crimp pins in the thermocouple path. The fix is to purchase special pin that are of the same alloys as the thermocouple being used to replace the tin or gold plated pins in the connector. Omega is one source that we use for these.


3) Noise or unintentional grounding.


Hope that this helps.


- larosta

Thermocouples connected directly to the supplied MGL harness. All the same way. I've been connecting thermocouples for 30+ years on dynos and airplanes, never had any issues until this brand new one.

This instrument is on a test stand, all junctions are sitting in the prop blast.

Moved the bad one to a different channel, problem moved to the new channel.

Seems I'm not the only one to experience a problem out of the box. Not so impressed considering the price they ask for these probes.

There are some poorly Manufactured EGT Probes Out There

Yep, I have run across another very expensive ($195 ea.) EGT TC probe out there that failed within 25 hours. Broken wire in the transition from the probe to the extension lead, no abuse or external signs of damage. Shipped it back to the manufacturer in Canada at our cost for evaluation. The manufacturer basically said that it was my problem. Replaced it with a much lower cost probe from Omega and never looked back or looked at that manufactures products again.

An exposed junction type K probe is a Type K probe. If it mounts the way you want it to it is as good and anything else.


Nuff said.

- larosta

Just FWIW on CHT thermocouple

Dissimilar metals: All MGL TC's are terminated inside the firewall with Omega flat blade connectors at exactly the same position for temp consistency. DB solder joints are potentially different, but ambient and boiling water readings are all within one degree F.

Swapping problem CHT with another at the Omega connector : The problem follows the "suspect" thermocople.

Some time ago, a VAF comment suggested a stray wire filiment in an Omega connector could creat a second TC that would show cockpit ambient when engine is running, BUT, a boiling water test responds appropriately.

So, my best guess is there is a thermal expansion issue at the thermo couple/ cylinder head that is OK at low temps and not at higher temps. The short TC's sold by Garmin are attractive so you could substute and troubleshoot without dealing with a long wire that is potted in a harness passing through the firewall.