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So the answer to that question seems to be yes/maybe and no.

Yes, DEF can crystallize in the tank if it is low and left sitting. Will this cause you problems? Well if you listen to some of the hyperbole on the topic one would be left to freak out. I have learned that if something doesn't smell right on social media, you might need to check it out further. So that is what I have done on this topic.

First, it goes without saying that if you have your DEF tank low and crystals form, and then you add DEF to that tank, you now have added more urea to the new DEF from the old crystals dissolving, so you could be out of spec high with urea. That can be a problem and you may throw a code. So that is one problem that is very possible. I personally haven't heard or read this happening, but basic chemistry says it could happen.

Second, I have repeatedly read that once DEF has turned to crystalline form, it won't re-dissolve when contacted with liquid DEF. This has always set my BS alarm off, as it is outside what the norm would be with other crystalline substances that start out in a solution. So, I decided to do an experiment to see if I was wrong or if it truly is BS.

Below are some pics I took during the experiment. The narrative for the picture is under the picture.

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These are two pans that I had in the garage. They were cleaned first with isopropyl alcohol and then washed with Dawn detergent and well rinsed and dried. I then added 6 ounces of Blue DEF to each pan. This was a new box of DEF with a date that indicated it was manufactured on July 2 2021. The pans were allowed to dry. This were the pans after 12 hours of drying, there is still a little moisture in the bottom of the pan after 12 hours of drying.

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Same pans after drying for 24 hours. All moisture is gone.



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This is where I took the DEF crystals from for the test.


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This the Blue DEF I used.


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Here are the crystals in jars for mixing. I put 4 grams of crystals in each jar. The jars were clean and dry.

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I put tap water in the sample on the left first. The tap water was ran slowly and carefully down the side of the jar so as to minimize any mixing. The crystals dissolved instantly and completely as soon as they contacted the water without any agitation. I put 5 ounces of tap water in the jar. On the right side, I poured 5 ounces of DEF carefully down the side of the jar. The crystals didn't dissolve completely immediately, but without agitation about 75% of the crystals dissolved immediately. The wetness in the middle is some spillage I had when pouring. You can see some of the crystals in the picture above on the bottom of the jar.

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I slowly stirred the jar with the DEF 3 strokes CCW and 3 strokes CW. After stirring the crystals are gone.

Also, after I was done with the experiment, I cleaned the pans above outside with a garden hose. I allowed the water from the hose just to trickle into the pans and the crystals dissolved quickly.

So, for me this puts to bed the myth that once the urea crystalizes in the tank it won't go back into solution. No need to worry about that hogwash. But I do believe that is probably best to keep your tank full or nearly full to minimize the overall solution from becoming too strong.
 

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@Pinkmist ,
Thanks for the experiment. Definitely puts the issue at ease.
 

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Daayyuummm!! I love a good back yard excitement to finally put a little rest on something! Awesome pictures you have all the way though!
(yes it’s supposed to be excitement not experiment) 😂😂
 

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Full def tanks, using good quality def, gives these systems the best fighting chance to work correctly and for the longest time, period. This has been the direction given since the beginning

Now, the longevity and operation of the dosing nozzles and nOX sensors is a different story but I digress.

Great experiment, btw. 👍🏻
 

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Liquid DEF is some chemistry straight from a Harry Potter movie. Potions Class | Harry Potter and the Half-Blood Prince - YouTube

The 'spec' for the DEF is in the whole percentage points.

Its hard to say how many %. Because the sensor itself is frankly not that accurate. The more you deviate from the magical 33% w/w the more likely the sensor will catch you.

Some trucks are using dedicated sensors now which may be more picky. But all our trucks use the crappy old ultrasonic system.
 
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