Splicing sensor wires?

I'm wondering if it's possible to splice sensor outputs (CHT, EGT, OAT, etc) so that the data can be read by 2 different devices simultaneously without affecting the data accuracy in any way? I've never done it so have no idea whether this is feasible or a really bad idea.

Nope.

CHT and EGT probes are thermocouples, meaning that they generate a tiny voltage/tiny current when heated. If you simply connect two different measuring devices in parallel to one probe, then the device with the lowest input impedance will take the lion's share of the teeny-tiny current generated by the probe and both devices will display grossly inaccurate temperatures.

OAT probes are generally thermistors (thermal resistors) which change their resistance (ohms) in relation to their temperature. By connecting two measuring devices in parallel to one probe, you will again be throwing a calibrated circuit way out of calibration and get wrong measurements.

Hi Todd,

I believe this is nearer the "really bad idea" end of the spectrum. CHT and EGT sensors are thermocouples built from dissimilar metals. They have a 'hot junction' at the sensor end and a 'cold junction' at the readout gauge end. The cables are a key part of the physics.

The dissimilar metals used for CHT sensor and cables are usually copper and constantan. For EGT sensors/cables they are usually Chromel and Alumel.

These metals are chosen because a temperature differential between the hot junction and the cold junction causes a current to flow around the circuit made by the two cables. The gauge measures the flow of electricity which is indicative of the temperature differential.

With older style gauges I believe that even changing the length of the cables could mess up the reading. Things may be different now that it is often a computer at the cold junction end that can be recalibrated. Even so, I do not think you would get good results if you tried to introduce other devices into the thermocouple circuit.

Hope that makes sense.

Steve

Auburntsts said:

I'm wondering if it's possible to splice sensor outputs (CHT, EGT, OAT, etc) so that the data can be read by 2 different devices simultaneously without affecting the data accuracy in any way? I've never done it so have no idea whether this is feasible or a really bad idea.

Click to expand...

Actually you can, providing the gauge is a powered gauge!
By this I mean the gauge has a 12 volt feed and a ground that powers the gauge (not the lighting). This type of gauge reads the presents and amount of voltage from the thermocouple and will function fine with as many as you wish. This type of gauge also isn't sensitive to the length of the sensor wires etc. You can't use a gauge that is powered by the thermocouple with multiple heads. Regards, Allan

Hi Allan,
Thanks for bringing me up to date,

Are you saying that the multiple powered heads on a single sensor would work out of the box without any calibration (i.e. the heads do not interfere at all with each other) ?

If these heads are multiple discreet and redundant glass-panel engine monitors at minimum I would have expected some interference with each other that would need calibrating out of the system.
It was my assumption that the interference would not be correctable.

Regards,
Steve

Which devices?

I have engine data on both my MVP-50 and on my GRT EFIS. The MVP-50 makes this possible. All the input go into the the MVP-50 which then sends Serial Data to the EFIS.

So I don't send the raw output to 2 devices but I do have 2 output devices.

Hmmm. The devices in question are a G3X EFIS and a GRT EIS. Why the EIS? Because I already have a VP-200 which needs engine data that the G3X will not export. To get the engine data to the VP-200 means having a stand-alone EIS. But I'd like to have the G3X display engine data as well. I had previously asked about the G3X exporting data to the VP but I never thought about asking anyone if the G3X could import the EIS serial data and display it in lieu of being directly hooked to the sensors. My guess is it can't but it's worth asking the question.

Todd,
From Dynon's perspective:

You could share EGT / CHT data between two EMS units. These will not be affected by running to two units. The comment in this thread that as long as the gauge is a powered thermocouple gauge it can be shared is generally correct.

Any sensor that is resistive cannot be shared. So resistive fuel levels, OAT, oil pressure, oil temp, and fuel pressure can't be shared. The EMS sends current out to these sensors, and if you hook up tow EMS units, you get double the current and both units read wrong.

You may also have the issue that the G3X uses quite different sensors than a lot of the other experimental EFIS vendors do, so getting those working with the GRT may cause issues as well.

How about a fuel flow sensor with pos /neg and a signal wire ? I also have an oil pressure sender that has three wires . I was hoping to run my original fuel flow meter and the EFIS one to get the EFIS calibrated .

Marc

Fuel flow can generally be shared, but even that depends a bit on the internal electronics of each engine monitor.

I can't answer about the oil pressure sender, that would again depend on how the internal stuff in each engine monitor works.