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Oh, I thought you'd already worked out the polynonmial values.
http://www.mosaic-industries.com/embedded-systems/microcontroller-projects/temperature-measurement/thermocouple/calibration-table#computing-cold-junction-voltages
probably way overkill.
Input from a type K (EGT) thermocouple would expect a maximum of 55mV (1370C +).
So you may need a voltage divider, depending on the max expected outputs from the pressure sensors.
Then the trick will be to come up with a formula to counteract the chip's calculation.
For the MAX6675 used in the EM3, it appears fairly simple:
The MAX6675 includes signal-conditioning hardware to
convert the thermocouple’s signal into a voltage compatible
with the input channels of the ADC. The T+ and T- inputs
connect to internal circuitry that reduces the introduction
of noise errors from the thermocouple wires.
Before converting the thermoelectric voltages into
equivalent temperature values, it is necessary to compensate
for the difference between the thermocouple
cold-junction side (MAX6675 ambient temperature) and
a 0°C virtual reference. For a type-K thermocouple, the
voltage changes by 41μV/°C, which approximates the
thermocouple characteristic with the following linear
equation:
VOUT = (41μV / °C) ✕ (TR - TAMB)
Where:
VOUT is the thermocouple output voltage (μV).
TR is the temperature of the remote thermocouple junction
(°C).
TAMB is the ambient temperature (°C).
Fortunately one of the channels in my EM3 is the temp of the PCB where the chip is located (TAMB).
Don't know what chips are used in the Dynon Skyview.
Finn
On 8/7/2020 7:14 PM, Stephen Izett stephen.izett@gmail.com wrote:
Hi Finn.
The pressure transducers I have can measure from memory a differential pressure of about 40” of water.
They have a millivolt output and in the process of cooking up a set of amps and thinking about the fact that Ive used up all my Engine Monitor inputs I pondered all those unused EGT and CHT inputs with amplifiers for millivolt input.
I thought about the thermocouple compensation, impedance matching and common mode currents from the egt amps (don't really have a handle on these) and decided I probably wouldn't damage them!
So I just thought Id try it out.
I wired one straight to an egt input and the Dynon 5v supplied, and applied some lung pressure and the dynon display showed some life.
I reworked a new Polynomial and she seems to provide a linear output over my lung burst pressure which from memory is a little over 1 psi.
I’m not after certifiable numbers but have built a manometer to hopefully roughly calibrate the output by playing with the Polynomial.
So lots of ignorance as far as what’s happening with the electrons at the interface but the results look promising and could be so much more helpful and easier than rigging a manometer in the cockpit.
Happy to have my party spoilt by someone with knowledge of the compensation circuit used in the EGT amps.
Cheers
Steve
On 7 Aug 2020, at 11:01 pm, Finn Lassen finn.lassen@verizon.net <flyrotary@lancaironline.net> wrote:
On 8/7/2020 10:00 AM, Stephen Izett stephen.izett@gmail.com wrote:
I worked out that they interface quite nicely without any other circuitry with the Dynon Skyview's surplus EGT inputs via a new Polynomial in the sensor config settings.
I'm thinking of using my unused?? EGT inputs for current (Hall) sensors.
How did you compensate for the EGT compensation?
Finn
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