CHT's and system voltage

[RangerJim]

While recently reviewing a Light Plane Maintenance publication (Basic and Advanced Light Plane Systems Maintenance, Second Edition, Belvoir Publications, Inc) I was struck by the following statement on page 59.

 

"In a thermistor type CHT system (as used in virtually all post Eisenhower era aircraft) the instrument movement actually consists of a voltmeter or ohmmeter, calibrated in degrees Fahrenheit instead of volts or ohms.  The readout will vary with line voltage (and hence charge rate).  When the charge rate is high-as following a hard start- system voltage may be as much as 10 percent higher than normal until the battery has come back up.  Ten percent of the CHT scale is about 30 degrees.  Under high charge rate conditions CHT may thus read 30 degrees high.  Conversely, under low voltage conditions CHT will read falsely low."

 

This naturally has me wondering just how much credence to put in our factory CHT gauge readings.  Are the low 400 degree readings often seen on climb out really an area of concern or an artifact of the system voltages until the battery has fully recovered from the start?  Do those who install modern CHT measuring equipment observe a decrease in CHT readouts solely as a result of better measuring technology or is today's superior equipment similarly affected by system voltages?

 

Are we getting sold another owt on "those tightly cowled hot running Mooneys?  Perhaps our technically astute colleagues can weigh in here.  I am always amazed at the depth and breadth of knowledge and experience on this forum and learn something new almost every time I check in.

 

Thanks,

Ranger Jim

68 C

[ArtVandelay]

I have a JPI that logs the voltage, I check a couple of flights, I know it spikes just after charging, but by the time I take off, it's very stable. But it is something to keep in mind, especially after a long hot start procedure.

[carusoam]

Probably depends on the type of digital equipment that has crept into your plane over the years...

1) modern voltage controllers on generator and alternator systems help improve this situation.

2) modern temperature acquisition equipment won't be as sensitive to input voltage.

3) EGT and CHTs are thermocouple based. TCs are not connected to ship's power.

4) oilT is probably still a thermistor with a reading that may be sensitive to input voltage.

Fortunately, the old ship's gauges are so difficult to read with any accuracy the error related to the voltage will be hard to discern.

This is only an opinion. I am not an instrument expert...

Best regards,

-a-

[carusoam]

TJ,

In the same time frame as the voltage spike...

Does the voltage spike lead to any noticeable change in CHT, EGT or oilT?

Best regards,

-a-

[ArtVandelay]

TJ,

In the same time frame as the voltage spike...

Does the voltage spike lead to any noticeable change in CHT, EGT or oilT?

Best regards,

-a-

Oil temp and CHTs, no...EGT 5-15 degrees, maybe caused by voltage change or maybe by a little more drag from the alternator. We're talking just 0.2V change...1.5% change causing a 1% change in EGTs.

 

Easy way to figure it out would be to find out if EGT, CHT, OT sensors have different part numbers for 14v/28v planes, if not....then they don't depend on the voltage.

[carusoam]

The TCs generate their own voltage and relay that back to the box mounted in the IP. Higher TC milivoltage equals higher temperature. The ship's voltage is mostly independent here.

The thermistor has the voltage applied to it and, the box in the IP is measuring the resulting current. This one, I would expect the system voltage would have some direct effect on it.

Modern instrumentation could have the ability eliminate this issue....

Best regards,

-a-

[DonMuncy]

Bear in mind that the original CHT (at least on my K model) is a thermistor which reads varying resistance proportional to temperature, while JPI and EI use thermocouple which read varying voltage generated by the temperature changes.

[Piloto]

Check the engine ground strap to the airframe. Remove probe from cylinder and clean with contact cleaner the threaded hole and reinstall probe. The alternator current negative side flows through the engine to the airframe. This causes a small voltage drop on the engine itself that can affect the EGT and CHT readings. To overcome this most new EGT/CHT probes have two wires so their readings are not affected by engine current flow because the negative side is directly connected to the indicator instead of going through the engine.

 

José