oil level reading - simple logic

imagine this: you're a car manufacturer, and you sell cars all over the
world. some you sell to taxi fleets, some to farmers, some to people in
places really freakin' cold, some to people in places ass-boilingly hot,
some to moms & pops who live in cities where there's hardly a single
level driveway for miles. like san francisco.

now, since you're trying to be practical about this, and you want
something that's easy to do and which will account for all these
variations, how are you going to calibrate an oil dipstick? are you
going to calibrate it to be read at -30°C ambient? how about +50°C
ambient? after being stood overnight? like all taxi fleets are going
to take a vehicle off the road for 8 hours just to read the oil level?
how about a 30° slope, sideways? /all/ driveways are like that, right?
and what if consumers insist on using crappy oil filters with leaky
drain-back valves? are you stumped for a universal solution to this???

well, these genius companies with their legions of smarty-pants
engineers have come up with a deceptively simple solution: calibrate the
dip stick for a reading that can be made at the gas station!!! utterly
brilliant!!!!

has anyone ever noticed how all the world's gas stations are carefully
designed so their forecourts are level? has anyone ever noticed how it
only takes a few minutes to fill the gas tank? has anyone ever
considered that regardless of whether you're in fairbanks, alaska or las
vegas, nevada, oil operating temps are pretty much the same? that even
leaky oil filter drain-back valves don't leak that fast? well, consider
it now ladies and usenet freaks - your vehicle's manufacturer got all
figured out. smart huh? and the gas station likely provides you with
FREE handy wipes so you can attend to this mundane task cleanly. and
best of all, each and every reading is CONSISTENT!!!!!!!!!!!!

see - it doesn't take a rocket surgeon to think this stuff through.
it's notable how the whiners, complainers and brave macho usenet bullies
never stop their keyboard masturbation long enough to do it though.

 

someone raises a hood at a self serve gas station!

Imagine this...'cause nobody does it.

 

"nobody does it"??? is that why gas stations sell oil and give you free
wipes to do this stuff with?

 

Yeah, but how many in this dumbed down society are actually capable of
reading (or finding) the dipstick?

<sigh>

JT

 

From what I have seen, no one raises the hood at a gas station. I do -- but
I do out of habit as and old motorcycle/sports cars enthusiast dating back
to the 50s. Heck the 64 XKE I had back in the 60s would use around a quart
every couple hundred miles or so-- and it held 10 qts as I recall. So, I
carried my own oil. And, today everything is hidden under a wad of plastic
shielding.

 

people still check under the hood at gas stations. they don't do it as
often because cars are better and most people are in the habit of
checking MUCH less frequently, but it's the place to do it.

 

Agree it is the right place.

 

This comment got me thinking. What does happen to the dip stick
reading solely as a reult of a major temperature change. As a rough
model, I assumed a square steel oil pan, with a horizontal surface
area of roughly 6 inches by 8 inches. At 20 degree C (68 F), the depth
of oil in the pan would be approximately 7.45 inches. At -40 degrees C
(-40 F) the depth of oil in the pan would be 7.18 inches. At +40
degree C (104 F) the depth of oil in the pan would be approximately
7.54 inches. I did figure in the change in the horizontal area of the
pan becasue of the temperature change, but it is trival compared to
the change in the volume of the oil becasue of the termperature
change. Of course the relation ship between the dip stick and pan
would also change with temperature, but only slightly. This can be a
complicated calucaltion since it could depend on the angle of the dip
stick, any material differences etc. In the most trivial case
(everything steel and the dip stick perpendicular to the bottom of the
pan), the posiiton of the bottom of the dip stick relative to the
bottom of the pan would be essentially unchanged (assuming it is
nominally almost touching the bottom of the pan).

So it seems to me that you cannot ignore the effect of temperature on
the measurement of oil level if you are trying to use this method to
estimate oil consumption. If you live in a very cold place and check
the oil when the engine and oil are dead cold, and then check it again
after you have gotten the oil to operating temeprature, but allowed
the oil to drain back to the pan, then the level could be over a half
an inch higher even though you have not added nor lost any actual oil.
I thought this was interesting. Even in a mild climate, the difference
between cold oil and hot oil could be significant (over 0.25 inches).
No wonder the "normal" range on a dipstick is so wide. I never thought
about this when checking the oil in my farm tractors (I have on that
holds 20 quarts of oil - and the oil gets very hot). Now I understand
why sometimes the oil level seems to go down over night even though
the tractor has not moved since I checked it at the end of the
previous days work. I always thought it should go up becasue of the
slow drain back of oil to the sump. Live and learn.

Ed

 

Your numbers are not even close to right. You better check them and try
again. I didn't look up the coefficients or run the numbers but off the
top of my head I would give rough guess that .02" is the maximum you might
see due to thermal expansion/contraction for 7" of steel at those temps.
Twice that if you are talking aluminum.

-jim

 

OK, but how about what happens to the oil!

Did you read the line that said " I did figure in the change in the
horizontal area of the pan becasue of the temperature change, but it
is trival compared to the change in the volume of the oil becasue of
the termperature change."

In other words, I dismissed the exapnsion/contraction of the actual
steel as a factor. It is the change in density of oil with temeprature
that matters! Look up the numbers for that. I'd be interested in
hearing what you find. Here is what I used for the density of the oil
versus temeprature:

Oil Temp Oil Density
Degree C kg^m^3
-40 923
-30 917
-20 911
0 899
20 888
40 876
60 864
80 852
100 840
120 829
140 817
150 810

The 0 to 150 degrees came from a table for "motor oil." I extrapolated
backwards for the 0 to -40. The result are probably not exact for any
particualr motor oil, but the principal should work - unless I made a
error in calculation. Check my theory.....

Assume that at 20 C you have 6 quarts of oil. That is roughly 5.04 kg
of oil and requires a volume of 0.005678 cubic meters. At +100
degrees, the same weight of oil, presents 0.00600 cubic meters. The
area of the pan has changed very litte. Assuming a pan with a 0.15
meter x 0.20 meter size, the area of the bottom of the pan is 0.03
square meters. At 20 C this means the oil must be 0.1892 m deep
(7.45"). At 100 C, this means that the oil must be 0.20 m deep
(7.87"). This is adifference of 0.42". See anything wrong with my
calculation? If you include the expanison of the steel then the
difference drops to 0.40". The effect of the linear thermal
coefficient of expansion for stee is small, but not negligible.
However, since I was just doing a rough estimate, it is not that
important.

Ed

 

Not sure, but the thermal expansion of oil would have to be many times
higher than steel for your calculation to be right. If all the materials
(steel and oil) involved expanded and contracted at the exact same rate
then the level on the dipstick would not change at all. I don't know if
you can completely neglect the shrinkage of the pan (when the pan gets
smaller the oil level goes up). The oil would have to have a coefficient
of expansion of around 50 times as much as steel for your numbers to be
right.

Looks like you are using volumetric expansion rate of about .0011/C°.
I'll look into it more this evening if I get a chance.

-jim

 

Hi Ed,
I found two different references for oil coefficient thermal
expansion of .0004/F and and a .0008/C. But I don't have much confidence
in either source.

So if you start with 5 quarts at 20C then at 100C the volume will
increase by about .28 quarts. But the oil pan volume will also expand and
increase in size by something like .03 quarts so you are looking at maybe
a 1/4 quart higher than full on the dipstick. But that assumes all the
oil is run back to the pan which won't be the case if the engine was just
shut down a couple minutes prior to checking. So the difference you
actually see on the dipstick probably be much the same as after the engine
sits and cools down and the oil is back in the pan.
Of course if you have a big engine with say 12 quarts in it, you
should have something like 2/3 of a quart more in volume at operating
temp.

-jim

 

eh? it takes about 10s to find reliable sources online. ever heard of
"google.com"??? or do you in fact mean "a source that agrees with my
preconceptions"? [rhetorical]

so that's why manufacturers CALIBRATE for it to be read just after
shutdown asshole, just like that say in their owners manuals - they have
no control over where the oil level will end up, but they /do/ over
where it will be within a specified period after shutdown from full
operating temp. read my original post.

oh, wait, you don't "do" reading. my bad.

 

Come on, Jim. If they sell oil it's hidden behind the chips, T-shirts
and soda. At least half the time those wipe dispensers are empty.
Other than guys driving obvious oil-burning beaters, nobody raises a
hood at gas stations.

 

Some engines manufacturers recommend checking the oil before start up. Either
way what you see on the dipstick is pretty close to the same. Either way you
aren't going to miss by enough to harm the engine.

 

that information is found in the owner's manual; which is opened just
about as often as the hood when at a self service gas station; never.

 

cite one.

except that it's not. see o.p.

depends which end of the scale you are, doesn't it.

 

ok, so you get people who never check their oil... doesn't mean it's
not the place to do it though - if someone were to bother.

 

No typically the recommendation to check the oil when the engine is cold
is made by engine manufacturers who build industrial engines, or
construction/mining equipment, or over-the-road trucks. Those people who
operate those engines do read the manual.
The procedure to check at a gas station is common mostly because that
covers the manufacturers ass in a warranty claim. If someone runs out of
oil the engine builder doesn't pay.
Obviously everybody stops to fuel eventually. But if someone drives
from LA to New York without making any prolonged stops and run out of
oil then simply checking the oil the last time the engine was cold may
not be enough

-jim