Oil Temp probe wiring; Overthought amateur question of the day

[Freemasm]

Trying to maintain fidelity with the G3X manual.

Wiring for the oil temp probe. G3 manual calls for a shielded, twisted pair.

OK. The contradictions are:
- Other TCs (EGTs, CHTs) utilize relatively expensive, unshielded, non-twisted TC leads.
- Flying leads from the Oil Temp probe are simple 22gage wires (no shielding).

Follow-up question. Shield would obviously have to float on the device end. What size unshielded window would be acceptable?

Anybody here get me smarter on this? Thx.

 

[DRMA]

Trying to maintain fidelity with the G3X manual.

Wiring for the oil temp probe. G3 manual calls for a shielded, twisted pair.

OK. The contradictions are:
- Other TCs (EGTs, CHTs) utilize relatively expensive, unshielded, non-twisted TC leads.
- Flying leads from the Oil Temp probe are simple 22gage wires (no shielding).

Follow-up question. Shield would obviously have to float on the device end. What size unshielded window would be acceptable?

Anybody here get me smarter on this? Thx.

The EGT and CHT sensors are thermocouples, thus the use of TC wire. The oil temp sensor is a resistance sensor, and so uses twisted pair wire. Shielded provides a little more protection from interference/noise, as well as a bit more physical protection, and is wire you will have "in stock" from other wiring in your plane. The unshielded window at the sensor is not critical. Just leave a service loop of a few inches at the sensor to allow for sensor replacement in the future.

Regards,

 

[Freemasm]

The EGT and CHT sensors are thermocouples, thus the use of TC wire. The oil temp sensor is a resistance sensor, and so uses twisted pair wire. Shielded provides a little more protection from interference/noise, as well as a bit more physical protection, and is wire you will have "in stock" from other wiring in your plane. The unshielded window at the sensor is not critical. Just leave a service loop of a few inches at the sensor to allow for sensor replacement in the future.

Regards,

Thank you. Should have known. Is it (not un)common to have them labeled Hi/Lo versus +/-, etc? Never seen the aforementioned in my current industry.

 

[DRMA]

Thank you. Should have known. Is it (not un)common to have them labeled Hi/Lo versus +/-, etc? Never seen the aforementioned in my current industry.

I suspect this labeling is because neither side of the device is grounded. So the Garmin LRU terminals are connected across the device, with the Hi side at a higher potential (voltage) than the Lo side. Just a different way of representing it. But in this case of the oil temp sensor, it doesn't matter which wire from the sensor is connected to which terminal, as all that is being measured is the resistance of the sensor, which varies with temperature.