How do you set the barometric pressure on your altimeter encoder on your transponder?
Most likely cause is your altimeter is slightly out of calibration -- the tolerance limit for the biennial 91.411 test is +/- 20 feet at sea level (see Part 43, Appendix E, Table I), and "below sea level" is more than 20 feet off from 29 MSL. However, since the operational tolerance is +/- 75 feet, you may still be good to go within the FAA's recommended parameters (depending on just how far below sea level it's reading) -- see AIM section 7-2-3. However, I'm not sure I'd be so happy flying an ILS to mins with an altimeter that read 75 feet high. So, what to do?
Other than the input correction mentioned just above, no -- nobody can legally tinker with an altimeter except a certified repair station with an instrument rating.
Yes, it does so automatically. There will be an altimeter setting readout at the controller's position, and that's the setting that controller's display is using, and the one the controller will read to all aircraft in that sector. Note that this varies from sector to sector, so each controller may read you a different number. Further, with a big enough sector, there may be different settings within that same sector, so the controller may read you one setting while reading another to someone a distance away.
another is that ATIS are usually updated once an hour if pressure is changing rapidly you can see 40' variations between settings.
OK, let's work it out -- "high to low, look out below." If the altimeter reads 140 when you're at 100, you're going to go low, i.e., you'd be in that "high to low" situation where your altimeter setting is too high for conditions, so you'd have to turn it down. Yes, you're right -- it would be -.04 in that situation. Earlier post fixed.
Nonstandard temparature has no effect when you're at the same elevation where the pressure is measured (i.e., on the field). The effect increases with the altitude differential. See Table 7-2-3 in AIM Section 7-2-3. Unless the elevation where you're parked varies dramatically from where the pressure was measured, this won't change what you see when setting the altimeter on the ramp. Even on the south ramp at Pittsburgh almost 100 feet below the field el, it would have to be below -30C before the error became noticeable, and then only if the ASOS were up at that high point (which it's not -- it's much closer to the south ramp's elevation). Where this error becomes an issue is when flying nonprecision approaches with high MDA's in extremely cold conditions, which is why Canada requires pilots to have this chart in the cockpit in certain conditions/locations. In those situations, the effect can potentially drive you into an obstruction if not accounted for.
I can understand how oil on the antenna could absorb enough signal energy so it wasn't getting out enough to be received ("No mode C observed"), but not how it could alter the digital data in the transmission. Perhaps you misunderstood your instructor, or s/he failed to ensure the point was made clearly.
I think what Ron is saying is that non-standard temperature has the same effect on the calibrated altimeter used to report the baro setting as it does on the altimeter in your airplane.
Joe thinks right. Hence, non-standard temperature would not cause the OP's problem stated in post #4. Further, since the pressure source is the same, it would not change the difference between the encoder altitude and the altimeter altitude as queried in the first post.
The effect of nonstandard temperature on altimeter error is proportional to the difference between the altitude of the reference station and the airplane in question. When both are at field elevation temperature induced altitude error is pretty much in the noise. Of course you did say "to a much smaller degree"
I'd like to know how the Universal Gas Law would explain an altimeter set to the pressure setting obtained from another altimeter at the same elevation and temperature (i.e., the one in your plane sitting on the ramp versus the ASOS/AWOS/other source on the same field) reading a different altitude from the reference altimeter other than an internal error of one of the altimeters.
) can reset most altimeters. There's a release in the altimeter that allows the hands to be set separately from the barometric pressure in the kollsman window. It can be done from the front of the altimeter with the altimeter still in the panel. I suppose if it needs more work than that it would need to be removed from the panel. 