Re: oil level reading - simple logic

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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
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"C. E. White" wrote:

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

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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
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"C. E. White" wrote:

    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

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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
"C. E. White" wrote:

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On 06/09/2010 03:47 PM, jim wrote:

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.

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jim beam wrote:

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.
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snip>

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.
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On 06/09/2010 07:58 PM, ACAR wrote:

:(
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ACAR wrote:

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
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On 06/10/2010 04:53 AM, jim wrote:

sure.
oh, wait, many of those engines aren't allowed to cool down, so it must be ok to never check the oil!!!

/which/ engines? [unsurprisingly] you're being totally non-specific.

so all that bullshit about thermal expansion, calibration, drain-back valves, etc. is completely irrelevant!!! oh, wait, you don't need to worry about facts if you're just an asshole.

but according to you, "over-the-road" truck drivers will have to stop and wait. or not - retard asshole.

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On 06/09/2010 07:07 PM, jim wrote:

cite one.

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

depends which end of the scale you are, doesn't it.
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I am of the opinion that a dip stick is just a rough estimate of the oil in the pan. I think the idea is to make sure that when the engine is running there is enough oil left to cover the oil pick-up under all expected operating conditions. For most cars I am sure the dip stick is very conservative. I know if I was designing one, the fill mark would be well above the level at which there was any possibility of oil starvation. Given all the variables, treating the dip stick like a calibrated oil level meter is not reasonable. I can think of the following factors that will affect the accuracy:
Items that effect the initial accuracy versus the specifications (Manufacturing tolerances that may affect the initial accuray relative to the specifications but are consistent from one measurement to the next): - manufacturing tolerances for the oil pan (no better than +/-0.005" on each dimension) - manufacturing tolerances for the actual dip stick (no better than +/-0.005") - manufaturing tolerances on the position of the dip stip tube in the pan/block (probably no better than +/-0.010") - tolerances on how the engine is positioned in the chassis (i.e., angle at which it is installed realtive to the chassis - hopefully the dip stick is positioned to minimize the effect of this) - manufacturing tolerances on block/pan interface (probably pretty tight maybe only +/- 0.002")
Item that effect individual readings (Enviromental factors that effect the consistency of readings): - outside temperature - oil temperature - grade the vehicle is parked on - vehicle load (i.e., stuff in the trunk or passenger compartment that might effect the angle the vehicle is sitting) - how long the vehicle has been sitting with the engine off before the oil is checked - how qucikly the oil "runs" back down the dip stick - how difficult it is to read the dip stick (some are horrible) - the reliability / efficiency of the anti-drainback valve in the oil filter
If you are trying to estimate engine oil consumption you can eliminate the original manufacturing tolerances by properly filling the crankcase and scribing a "full" line on the dipstick (or alternately by carefully adding oil until the level matches the original full mark). But that only eliminates the original manufacturing tolerances from the estimation. To get a true view of oil consumption, you would need to be careful to always park the car in the same location and in the same orientation. You also need to wait for a long time to make sure the oil has all drained back into the pan. I also think you need to try and check the oil with as close to the same ambient temepratures (after allowing the oil to cool to ambient). If you do all that, then you could add "make up" oil until the dip stick reading was back to the full line. But even after doing all this, what have you proved? You still don't know how much of the liquid in the crankcase is "oil" and how much is other "stuff" (unburned hydrocarbons, water, soot, dust, etc.). To minimize the possibility that contamination is masking the true oil consumption, you would also need to make sure that the engine was operated long enough and worked hard enough that the oil got hot enough to eliminate any water or unburned hydrocarbons.
Personally, I think if you did everything "right" the measuranment accuracy would still be worse than +/-5% (say a quarter of a quart).
Ed
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On 06/10/2010 07:32 AM, C. E. White wrote:

big picture, you're absolutely correct - this is neither rocket science nor precision brain surgery - the dip stick has two marks on it - "full" and "add more". between those two, you're fine. but if you're driving an oil burner, and you're dipping cold and not paying attention, you could get an expensive surprise.
as for the point about manufacturers being conservative and leaving a safety margin, that is not something you can rely on. i've seen vehicles where the oil warning light comes on if the oil is near the bottom and you corner at speed. clearly, that's /way/ too close for comfort. otoh, a girlfriend's lexus had the light come on, and that needed /three/ quarts before it got to the /bottom/ of the stick - pretty conservative, but clearly not infallible. bottom line - you can't rely on anything you can't see on the stick - so check the freakin' oil. hot. on the gas station forecourt. just like it says in the book.
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I agree, but was curios about the exact working in my 2007 Ford Fusion Owners Guide(pictures / engine variations deleted) -
Checking the engine oil
Refer to the scheduled maintenance information for the appropriate intervals for checking the engine oil. 1. Make sure the vehicle is on level ground. 2. Turn the engine off and wait a few minutes for the oil to drain into the oil pan. 3. Set the parking brake and ensure the gearshift is securely latched in P (Park) (automatic transmissions) or 1 (First) (manual transmissions). 4. Open the hood. Protect yourself from engine heat. 5. Locate and carefully remove the engine oil level indicator (dipstick). 6. Wipe the indicator clean. Insert the indicator fully, then remove it again. .If the oil level is within this range, the oil level is acceptable. DO NOT ADD OIL. .If the oil level is below this mark, engine oil must be added to raise the level within the normal operating range. . If required, add engine oil to the engine. Refer to Adding engine oil in this chapter. .Do not overfill the engine with oil. Oil levels above this mark may cause engine damage. If the engine is overfilled, some oil must be removed from the engine by an authorized dealer. 7. Put the indicator back in and ensure it is fully seated.
The line that reads - "Refer to the scheduled maintenance information for the appropriate intervals for checking the engine oil" caught my eye, and I followed up by looking at the maintenance guide. The maintenance guide only specifies checking the oil monthly. They do not mention doing so at every fill-up. This surprised me.
Ed
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On 06/10/2010 08:45 AM, C. E. White wrote:

nothing about frod surprises me. under nasser, that company was spectacularly ruthless in its cost cutting, and would weasel around anything it could. [don't get me started on the exploder fiasco.] that language summarizes as "don't do anything, but if it goes wrong, you're not covered because you should have done something". nasser legacy language.
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<snip for brevity>

<snip for brevity>
All this is fine for making sure the engine has enough oil in it so as not to seize-up on the roads. The admonition not to add oil during the outlined procedure is intended to prevent drivers from accidentally overfilling the crankcase. Depending on the temperature of the oil when checked, a significant amount of oil can be hung-up in the engine, giving a falsely- low reading on the stick.
But if your engine uses a significant amount of oil, and you want to determine exactly how much, then you need a much more exacting and rigorous method. Like mine.
--
Tegger

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On 06/10/2010 09:48 AM, Tegger wrote:

no dude, that is a fundamental misconception on your part. the dipstick is calibrated for the level to be read when hot and just switched off, not cold and subject to factors i've outline repeatedly above and that make for massive inconsistency.
example: on some automatic transmissions, they are calibrated to be dipped with the motor running. if you dip it with the motor off, the level you will see on the stick is about 3 inches "too high". you can't possibly argue that departure from the owners manual and approved dipping method is appropriate in that situation, so why are you trying to do so here?

only if it's being read correctly in the first place. it needs to be read hot, as per the calibration and owners manual, not cold and subject to at least three major variables that lead to inconsistency and subsequently false readings.
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"C. E. White" wrote:

From personal experience I can assure you that you are being very generous. the tolerance for the volume of the pan that holds the oil is considerably looser than you imagine. The volume of the pan and what the end user sees on the dipstick is not a major priority for the manufacturer. Manufacturers are far more concerned about how the oil pan design is going to perform on the assembly line than how it performs after the car is sold.

Yes all true, But you can eliminate considerable error by taking only one dipstick measurement over the entire oil change period. Keep track of how much make up oil you add and how much it is low before draining the oil and you have the total consumption.
    Taking multiple random incremental readings on the dipstick will only degrade the measurement accuracy and increase the uncertainty of how much oil is actually being consumed all together.
-jim

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jim beam wrote:

Ha Ha Ha you are a joke.     The purpose of checking oil as the manufacturer recommends is to keep the oil from getting dangerously low.
    If however you want to do what the OP is attempting: to accurately determine the rate of consumption it is not going to produce accurate results if you base it on a bunch of dipstick readings at the gas pump every few hundred miles. Even the OP wasn't that stupid.
-jim
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