Pitot Tube power affects gauges

[wiseng]

Has anyone noticed that when you turn the pitot heater on it affects the quantity indicated by about 20%..   In my 78 J the Fuel  level oil  and fuel pressure gauges indicate ~ 20% lower .  The oil temp and CHT indicate ~ 20% higher.   Any ideas?

[sleepingsquirrel]

Sounds as if you should leave it on until you are low on fuel then turn it off to extend your range.  

 

[mooniac15u]

Is it a fluctuation that then returns to normal or a sustained change?

[N601RX]

These gauges are pretty crude. They are just a coil that produces a magnetic field to move the needle in series with a resistive sensor.  There is no internal voltage reference they just use the bus voltage so they are susceptible to voltage changes.  The heater pulls about 10 amps.  There may be some higher than normal resistance in your bus connections that are allowing more voltage fluctions than normal when the heater is turned on. 

[mooniac15u]

Since each gauge grounds through its own sensor, if all the gauges are fluctuating then the main airframe ground back at the battery is a good place to start.  Even a small amount of corrosion where the cable attaches to the airframe can cause interesting behaviour in those gauges. 

[wiseng]

When the heat is turned on the gauges react the same In a linear fashion  There is not any fluctuations it is consistent.   What gets me is some increase and others decrease.  

[mooniac15u]

1 hour ago, wiseng said:

What gets me is some increase and others decrease.  

That's just because the the probes function differently for the temperature gauges. Increasing temperature causes increased resistance. In the other probes lower values cause lower resistance. So, if you increase resistance in all the circuits the temperature gauges will read higher and all the others will read lower.