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Old 06-07-2013, 10:21 PM   #1
WDE46
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Expansion Tank Specs

Okay, so I've had this new CoolXpert (OEM) ET sitting in my closet for a while. I got really bored tonight and decided to take some measurements. Posting them here for anyone who is curious in the future.


Measured Values:

Capacity @ max indicated: 1.12 L
Air space @ max coolant & starting temp: 0.39 L

BMW TIS:
Total coolant capacity @ Starting temp (incl heater): ~8.4 L

Operating assumptions:
Starting temperature ~22 C @ 1 atm
Operating temperature ~ 95 C @ ? atm

Coolant expansion volume from 22 C to 95 C = 3.7 % = 8.4 * 0.037 = 0.3108 L

Air space @ max temp and fill = 0.39 - 0.311 = 0.079 L

Basic PV = mRT balance (pretty good for air, at least within 5%)

P1V1 = P2V2

P1 = 1 atm
V1 = 0.39 L
V2 = 0.079 L
P2 = P1*V1/V2 = 4.9 atm = 72 psi

EDIT: This doesn't take into account the air pressure increase due to temperature. (295 K -> 368 K) That is a 25% increase in temperature, giving a result of 90 psi, assuming the air reaches the same temperature as the coolant.

In conclusion I must say that the pressure relieving feature of our ET cap is absolutely critical. It is not just for emergency pressure relief. It is a very active and functional component of the cooling system. It keeps the system at around 2 atm, or 28-30 psi. Also, I can now conclude that the lower 1.5 bar caps that are available probably do work to relieve stress on the ET, increasing its life.

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Old 06-07-2013, 11:23 PM   #2
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If I understand correctly, there is very little airspace left at max fill at operating temperature: 0.079 L - less than 1 Deciliter.
Even when slightly overfilled, there won't be room for expansion,
I wonder if that contributes to ET tank failures?
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Old 06-07-2013, 11:52 PM   #3
WDE46
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Expansion Tank Specs

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Originally Posted by lszlszx View Post
If I understand correctly, there is very little airspace left at max fill at operating temperature: 0.079 L - less than 1 Deciliter.
Even when slightly overfilled, there won't be room for expansion,
I wonder if that contributes to ET tank failures?
Correct. And that's been my theory for a while. After max full, the space narrows and the volume decreases very rapidly, so overfilling is very bad.


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Old 06-08-2013, 07:49 AM   #4
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That is a wonderful theory, except for the fact that my ET ruptured when my coolant level was low. I am sure of this, because I checked the level before I left the house, and I bought a gallon of distilled water while I was out that night. On the way home, it ruptured.
Of course, mine was the original ET on an 01 model, and it was the old design. BMW redesigned the tank. Most of the failures you read about are probably the old design. I don't doubt that there are still a lot of cars out there with the original tank design on them. That probably contributes to a lot of the reports of ruptured ET's. So, if you are driving around with the new version of the ET, it is easy to sit back and proclaim that people are overfilling their ET's, and causing the failures.
IMO, the majority of people seem to neglect their cars, and wait until they get a low coolant light before even bothering to check their fluid levels. I really have a hard time believing that overfilling is the primary cause of failures.
Here is an old post with an analysis of the original ET tank design, that was much more prone to failure. This is most likely the reason why these cars have a reputation for ET failures: http://forums.bimmerforums.com/forum...d.php?t=877529
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Old 06-08-2013, 08:05 AM   #5
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The other issue is BMW states to check/verify ET level at 68F as I recall.

So if you are dealing with the typical Winter time low coolant light that comes on, then fill the ET to the Max level, you have already short changed yourself quite a bit of air volume in the ET.

I typically fill to the lower level or at best case the 1/2 way make between the high & low level.

2 Bar is too much and last I understood liquids do not compress with any ease.

If someone was willing to cap off a new ET and fill it with 2 bar worth of compressed air, I would love to see the video!
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Old 06-08-2013, 09:43 AM   #6
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Expansion Tank Specs

Re-read my conclusion guys. I don't think overfilling is always the primary cause. I do believe it is an easy way to kill your tank because the cap doesn't vent fluid as easily as air. What I am really suggesting is that tank failures occur because of bad caps that no longer vent at 2 bar. Since I have shown that the cooling system would operate at ~5 bar with no relief cap, the real operating pressure of the system is determined by the cap that you have on.

JFOJ, good insight on the winter temperature fill. That would be a cause for concern. They really didn't give enough room for error with this design is what it's coming down to.

I may try to get measurements of the difference b/w min and max fill levels, but I was having a hard time being that accurate with how I had to plug up the holes.

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Old 06-08-2013, 10:19 AM   #7
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I understood your conclusion. My reply was mainly directed at the overfilling being the primary cause for failures. I believe anyone who is picky enough to check their levels is probably not going to overfill it. So, I do not think that overfilling is the main cause for failures, but it could account for a small percentage of the failures. But, it is very rare to read about a second ET tank failure. I think the majority are either the old design, or old plastic that has been through too many heat cycles, which is the same thing that affects most of the other cooling system parts.
But, you could also make a case for ET caps that are no longer venting properly too. I change mine out after 3 years or so.
It also seems like there is very little room for error for an ET tank cap that may not vent at exactly 2 bars. Since it is basically a mechanical design, there is going to be some room for error for exactly what pressure the cap vents. If you get one that does not vent at exactly 2 bars of pressure, then it is going to add additional stress to the tank, causing it to expand and contract as the pressure builds up before it finally vents. I would think that a cap that vents slightly higher than 2 bars it is going to put excess stress on the ET that it was not designed for. I would be interested to see a test of multiple caps, to see exactly what pressure they vent at. There are bound to be some that are over spec, even when new.
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Old 06-08-2013, 10:23 AM   #8
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Expansion Tank Specs

Agreed. I am interested to see how they age. If their relief pressure increases with age, then we have a problem.


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Old 06-09-2013, 09:08 AM   #9
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Oh man, you guys are blowing my mind. I hadn't really given any thought to what exactly the coolant bottle was doing on our cars. I guess I had just assumed that there was an unpressurized portion that the excess fluid went into and out of as on a traditional "desert kit" arrangement.

The shape of the bottle isn't overly complicated. It wouldn't be hard to model in solid works or something similar. Once an accurate model was made, the software could calculate various volumes without any hassle.

Speculation around the cap relief pressure seems plausible as the valving may become more susceptible to stiction with age. Maybe I can fashion a pressure tester out of my old tank.


I'm replacing my bottle this afternoon. I don't think it has actually failed yet, but I did see some drips of coolant under the car this winter. (Easy to spot in the snow). After the bottle comes out, the last part of the original system is the hoses, so i'm guessing the leak is at one of the o-ring seals. Mines an 03 with 125k on what I assume is the original bottle. I'm on an accelerated coolant refresh schedule because I'm not using the oem mixture.
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Old 06-09-2013, 09:15 AM   #10
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Fill the tank with sand or sugar to measure the volume. Assume you could use blue tape to cover any openings for this measurement? You could use water, but probably harder to seal the openings with tape for this purpose?

Spare cap with a hole for a pressure pipe adapter and pressure gauge and you are mostly set to get some data!
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Old 06-09-2013, 10:48 AM   #11
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Quote:
Originally Posted by WDE46 View Post
Okay, so I've had this new CoolXpert (OEM) ET sitting in my closet for a while. I got really bored tonight and decided to take some measurements. Posting them here for anyone who is curious in the future.


Measured Values:

Capacity @ max indicated: 1.12 L
Air space @ max coolant & starting temp: 0.39 L

BMW TIS:
Total coolant capacity @ Starting temp (incl heater): ~8.4 L

Operating assumptions:
Starting temperature ~22 C @ 1 atm
Operating temperature ~ 95 C @ ? atm

Coolant expansion volume from 22 C to 95 C = 3.7 % = 8.4 * 0.037 = 0.3108 L

Air space @ max temp and fill = 0.39 - 0.311 = 0.079 L

Basic PV = mRT balance (pretty good for air, at least within 5%)

P1V1 = P2V2

P1 = 1 atm
V1 = 0.39 L
V2 = 0.079 L
P2 = P1*V1/V2 = 4.9 atm = 72 psi

In conclusion I must say that the pressure relieving feature of our ET cap is absolutely critical. It is not just for emergency pressure relief. It is a very active and functional component of the cooling system. Also, I can now conclude that the lower 1.5 bar caps that are available probably do work to relieve stress on the ET, increasing its life.

Fun stuff.
Those are scary numbers
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Old 06-09-2013, 03:15 PM   #12
WDE46
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Expansion Tank Specs

Quote:
Originally Posted by jfoj View Post
Fill the tank with sand or sugar to measure the volume. Assume you could use blue tape to cover any openings for this measurement? You could use water, but probably harder to seal the openings with tape for this purpose?

Spare cap with a hole for a pressure pipe adapter and pressure gauge and you are mostly set to get some data!
I measured volumes with water. I filled the tank to the desired level and then weighed the water. I assumed ~1000 kg/m^3 to get to volume. I sealed the holes with saturated paper towels. It worked well enough.

To the other guy: the tanks internal shape is actually quite complex. It's outer surface really shows nothing about what's going on in the inside. There are several different passages that bypass the expansion chamber, and that makes the expansion volume very oddly shaped.


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Old 06-11-2013, 07:21 AM   #13
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I measured volumes with water. I filled the tank to the desired level and then weighed the water. I assumed ~1000 kg/m^3 to get to volume. I sealed the holes with saturated paper towels. It worked well enough.
Keep your day job!!!

You are a FANATIC.

So here is what you need to really do.

Drain your entire cooling system of 50/50 distilled water & BMW antifreeze.

Get one of the turkey cooker pots to hold all the coolant.

Use this to test between 0F and 210F to see exactly how much the mixture expands and contracts. Also verify volume at 68F.

Hopefully you have a deep freezer that can hold your turkey cooker pot?

Make sure you have enough propane for the turkey cooker to get the mix up to close to boiling.

Measure the volume change of the entire coolant system over temperature.

When you are all finished gather data, you can refill your cooling system.

Then you can see exactly what we are up against!!

BTW, did I say to keep your day job?
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Old 06-11-2013, 08:19 AM   #14
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Quote:
Originally Posted by WDE46 View Post
I measured volumes with water. I filled the tank to the desired level and then weighed the water. I assumed ~1000 kg/m^3 to get to volume. I sealed the holes with saturated paper towels. It worked well enough.

To the other guy: the tanks internal shape is actually quite complex. It's outer surface really shows nothing about what's going on in the inside. There are several different passages that bypass the expansion chamber, and that makes the expansion volume very oddly shaped.


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A graduated cylinder of sufficient size would avoid the need of weighing the water.

If you only have the scale, you can tare the scale with the empty tank on it and then fill it and weigh it again. Might give you a slightly more accurate weight.
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Old 06-11-2013, 09:14 AM   #15
WDE46
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Quote:
Originally Posted by jfoj View Post
Keep your day job!!!

You are a FANATIC.

So here is what you need to really do.

Drain your entire cooling system of 50/50 distilled water & BMW antifreeze.

Get one of the turkey cooker pots to hold all the coolant.

Use this to test between 0F and 210F to see exactly how much the mixture expands and contracts. Also verify volume at 68F.

Hopefully you have a deep freezer that can hold your turkey cooker pot?

Make sure you have enough propane for the turkey cooker to get the mix up to close to boiling.

Measure the volume change of the entire coolant system over temperature.

When you are all finished gather data, you can refill your cooling system.

Then you can see exactly what we are up against!!

BTW, did I say to keep your day job?
Well, see water properties are well founded. Unfortunately I can't find much data on antifreeze properties at various temps and pressures. If I could, I could easily calculate the exact volumes at a given T&P. Unfortunately the best I can do is assume the mixture behaves almost exactly like pure water so I have to pull out my water tables. It's not a bad assumption. 3.7% is going to be quite close.

Quote:
Originally Posted by neutron325 View Post
A graduated cylinder of sufficient size would avoid the need of weighing the water.

If you only have the scale, you can tare the scale with the empty tank on it and then fill it and weigh it again. Might give you a slightly more accurate weight.
Obtaining a graduated cylinder that large would be difficult for me.

What I did was fill the tank with water, and then empty it into a container that was tared on the scale. Obviously, I would tare b/w each measurement. Weighing the water was the most accurate method by far available to me. As far as assuming 1000 kg/m^3, that is accurate to 0.3%, so no real error there.
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Old 06-11-2013, 04:31 PM   #16
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Just do not forget, ALL the coolant/water mixture will expand, not just the coolant/water in the expansion tank!

I like the turkey fryer idea anyway!
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Old 06-11-2013, 06:29 PM   #17
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Expansion Tank Specs

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Just do not forget, ALL the coolant/water mixture will expand, not just the coolant/water in the expansion tank!

I like the turkey fryer idea anyway!
Yes I took that into account. Look at my calculations. I estimated 3.7% expansion for 8.4 L of water.


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Old 08-26-2013, 09:02 PM   #18
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There's so much science in this thread. I ****ing love me some science.
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Old 09-04-2013, 05:21 PM   #19
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Some more stuff M54-related cooling stuff for you guys to play with:

http://www.bmwclub.lv/files/03_E85_M54_Engine.pdf
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Old 10-04-2013, 02:39 PM   #20
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useful docs

Quote:
Originally Posted by Mango View Post
Some more stuff M54-related cooling stuff for you guys to play with:

http://www.bmwclub.lv/files/03_E85_M54_Engine.pdf
Thanks, Mango. That PDF is very informative. Please share if you have any other e46-related training materials like that.
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