EGT conversion from offset value (PMDG 737-700)

[Brahms]

In my homecockpit I use a Raspberri PI to duplicate the upper and lower DU in the OMDG 737-700, The PI receives the necessary parameters from an Arduino interface, which communicates with the simulator through FSUIPC (version 7.5.4). Among the parameters that I need to feed to the PI, is the EGT, I am reading this value from offset 0x08BE for engine 1. The flt64 values accross all power settings are checked with FSinterrogate. These offset values do not correspond to the EGT indications on the PMDG DU screen. I would like to know how I can convert the offset values to match the PMDG DU indications. Or are more accurate EGT values available at other offsets? 

[John Dowson]

20 hours ago, Brahms said:

I am reading this value from offset 0x08BE for engine 1. The flt64 values accross all power settings are checked with FSinterrogate.

That offset holds a 2-byte integer, not a FLT64, and the description of this offset is:

Engine 1 EGT, 16384 = 860 C. [Note that for Props this value is not actually correct. You will get the correct value from 3B70. The value here has been derived by FSUIPC to be compatible with FS2004, FS2002 et cetera]

As you are not using a Prop aircraft, that offset should hold the correct value, but maybe also try offset 0x3B70:

General engine 1 EGT in degrees Rankine, as a double (FLOAT64). Convert to Fahrenheit by Rankine – 459.67. FS default gauges show Centigrade.

As that is in Rankins, first convert to Fahrenheit and then to Celsius. 

John

[Brahms]

John, Tx for the explanation. However, I still see a problem. I have read both offstes 0x08BE and 0x3B70. They produce approx the same values after conversion. I am still not able to use the conversion formula to reconstruct the readings on the DU of the PMNDG737-700. Furthermore, when N1 goes over approx 30%, with increasing power, the EGT displayed on the DU rises slowly at first, then faster at higher powersettings. The offset values from both offsets decrease in the same way a bit before they become constant, only to rise a little bit at full power. It seems to me that no (lineair) calulcation can generate higher EGT values from decreasing or constant offset values. So I am still at a loss. Would you mind looking into this problem once more? Thanks a lot.

[John Dowson]

FSUIPC is just receiving and storing the value of the associated simvar (GENERAL ENG EXHAUST GAS TEMPERATURE), after conversion. If this is not correct, then you need to raise this with PMDG support. I cannot really help with this, sorry.

However, maybe this is not the same value that is displayed on the DU. There is also ENG EXHAUST GAS TEMPERATURE (see https://docs.flightsimulator.com/flighting/html/Programming_Tools/SimVars/Aircraft_SimVars/Aircraft_Engine_Variables.htm) - maybe this holds the correct value. I do not know the difference between these two simvars, and the documentation does not say much.

You can add this to a spare/free FSUIPC7 offset and see if this matches. Also, you can add GENERAL ENG EXHAUST GAS TEMPERATURE again to a different offset, and also log this to see if the unadulterated/unconverted value matches. See the Advanced User guide on how to add simvars to free offsets.

[AirPanther]

Are you referring to the APU EGT? If so, have you checked 0x648C (Length 4 [FLT32])? Have you checked the "Offset Mapping for PMDG 737" document from the PMDG SDK? Maybe PMDG is doing a more complex/accurate calculation and storing it in a custom area?

Robert

[John Dowson]

Thanks @AirPanther. However, note that @Brahms is using FSUIPC7 so the offset for APU_EGTNeedle is 0x64E8 (still a FLT32, 4-bytes) - the PMDG offsets in FSUIPC7 are not exactly the same as in earlier versions.
I did check the 'Offset Mapping for PMDG 737-700' pdf document but missed this somehow...

I will move this topic to the FSUIPC7 sub-forum where it belongs.

Cheers,

John