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Oil temp

kevin O

Well Known Member
I have found a 3o degree difference with the oil temp on my efis versus using a temp gun to measure the heat at oil cooler. This was after shutdown with hot engine. There is no difference with cold engine.
The oil temp sender is a vdo 40405. Efis is a AFS 5600.
Has anybody else seen this. I keep being told that a vdo oil temp sender doesnt fail.
Would appreciate any advice and or experience anybody has.
 
Was that a 3° or 30° difference?

If the former, that's not worth worrying about. If the later, then yes, swap sensors, prove up with boiling water, etc. to debug.

Everything fails given a long enough timeline...
 
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It is not difficult to pull the sender out and test it for accuracy. That would be the most accurate way of finding its accuracy. With the engine shut, pull the temp prob out and put it in a can of water or oil and heat it up with a verified thermometer. Observe the temp on your EIFS and against what the thermometer is reporting.
It is not unusual to have 5-7 degree inaccuracy but 30F is not good.

Also, often the inaccuracy is not linear so this way you can find the trend.
 
I have found a 3o degree difference with the oil temp on my efis versus using a temp gun to measure the heat at oil cooler. This was after shutdown with hot engine. There is no difference with cold engine.
The oil temp sender is a vdo 40405. Efis is a AFS 5600.
Has anybody else seen this. I keep being told that a vdo oil temp sender doesnt fail.
Would appreciate any advice and or experience anybody has.

How do you measure the heat at the oil cooler with an IR gun? Shooting it at the oil cooler? I'd expect the oil cooler to register substantially cooler than the sensor mounted on the engine.

Also, remember that IR thermometers are sensitive to the surface being measured. You could have 3 different surfaces, all at the same actual temperature, read substantially different.

Where I'm going is that an IR thermometer shooting the temperature of the oil cooler may not be a valid measurement.
 
oil temp

You do not say how and where you are measuring temperature at the cooler. At normal oil temperatures the oil temperature at the probe will be cooler than the oil in the cooler. The probe is basically measuring oil coming out of the cooler, which on a 70 degree plus day should be the coolest location in the engine. I question how one would check cooler temperature except to somehow put a temporary probe inside the hose exiting the cooler.
Not trying to insult anyone but you MAY have a solution in search of a problem.
A slightly different take on this issue, Piper was famous for using ram air on the temperature probe location to lower the oil temperature. No way of knowing in that case what the actual oil temperature was. Long time ago I asked the Lycoming rep at Oshkosh about hot running engines. His response was the engine will go to tbo with the oil temperature at the red line. I don't advocate that but the Lycoming will survive some extremely hot oil temperatures.
 
This isnt an apples to apples comparison, unfortunately, so i wouldnt expect the temps to be the same. I wouldnt even worry about it. The temp of the oil cooler isnt the same as the temp of the oil in it.
 
Why would the outside of the oil cooler be expected to be the same temperature as the oil inside of it?
Why would the outside wall of a house be different that the temperature inside the house?

An extreme example, but the concept of heat transfer is the same, just to a lesser degree.
 
Which one is hotter (heat gun or oil temp sender)? Have you tested the gun against a known other temp source? Is the cooler attached to the cylinder baffle? Is the hot oil from the engine going in to the bottom of the cooler, or the top? - and where is your gun pointed? I wouldn’t expect these two temp indications to be the same, and would expect to see a greater variation based on the above questions. That cooler, attached to the hot cylinder head, through the baffle (excellent conductor of heat) may not be indicative of an accurate measurement of the oil inside it. Bottom line - measure the accuracy of your oil temp probe. If it is accurate within a few degrees, then who cares what the surface temp of your cooler is? The oil temperature at the oil temp probe is what your bearings will see, and Lycoming says that’s all you need to know. There are places inside the engine that see oil temps maybe 40-60* hotter than that oil cooler exit temp. This is normal.
 
Why would the outside wall of a house be different that the temperature inside the house?

An extreme example, but the concept of heat transfer is the same, just to a lesser degree.

Huh? The inside of my house is presently 69F. The outside wall of my house is 30F. I would expect the same to be true of an oil cooler. I could certainly be wrong but even tho the oil cooler is designed to transfer heat, measuring the outside of an oil cooler and expecting it to read the same as an oil temp probe in the actual oil—whether in the oil cooler but especially elsewhere—does not seem very logical.
 
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An oil cooler is designed to transfer heat. A house wall is designed to not transfer heat.
 
The house and the oil cooler..

An oil cooler is designed to transfer heat. A house wall is designed to not transfer heat.

Both transfer heat from high to low.

The inside of the house is at one temperature. The outside of the house is at another temperature. In between you have materials that resist that heat transfer.

In the case of the house, the materials that resist the transfer of heat are the sheet rock, any air space, the insulation, the exterior siding, AND the air at the outside surface (boundary layer). The rate of heat transfer from the house to the cold outside is greatly slowed by the design of the wall assembly.

The oil cooler also transfers heat from the hot oil to the surrounding air. However it too has materials that provide resistance to the heat transfer process.

In this case the materials are designed to MINIMIZE the the resistance to heat transfer. However, resistance still occurs. There is the resistance of the copper tube and then there is the resistance of the aluminum fins. In addition the surface, or boundary layer of air, also has a resistance to heat transfer. The net result applies to all heat exchangers. As far as I know, there has never been a 100% efficient heat exchanger (and I've been in the heat transfer industry for 40 years).

The higher the delta T between the inside and outside air, the greater the rate of heat transfer.

So, the surface temperature of the oil cooler will never be the same as the temperature of the liquid passing thru it.

The are three other aspects to the process for a heat exchanger as well.

One includes the size of the tubes and fins of the heat exchanger. The larger the tubes, and the bigger the fins, the greater the efficiency of the heat exchanger.

Secondly, the velocity of the oil through the heat exchanger has a significant affect on the heat transfer rate. The slower the flow, the greater the heat transfer (assuming laminar flow does not occur at the lower flow rate).

Lastly, a heat exchanger has air flowing over it, either by the forced air in flight, or by simple convection while ground running. In this case the ground running is convective. As the rising column of air passed over the fins it further cools the surface area of the heat exchanger.

So no, the IR reading of your heat exchanger will not be the same as the temperature of the oil. How much of a difference depends on the design of the heat exchanger, temperature of the outside air and the amount of air passing thru the fins.
 
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