The best way to tell whether you've made high quality fuel is to have it tested using a gas chromatograph. This will tell you how much free and total glycerin is in your fuel, and will show you how well you've converted from triglycerides to fatty acid methyl esthers (FAME or biodiesel). The ASTM cutoff for total glycerin (which is the major culprit in bad fuel- ie the stuff that will plug your fuel filter and could do more damage) is 0.24% by mass or less. The cutoff for free glycerin is 0.02% by mass. Free glycerin is just the backbone of the triglyceride molecule, and a lot of that can be washed out of your fuel. The bound glycerin part of the total glycerin is either triglyceride that never reacted, or di or monoglycerides, ie partial products of the reaction. These can not be washed out and may be present in fuel that looks pretty good otherwise. Utah Biodiesel Supply is now offering biodiesel analysis and the free and total glycerin is only $80.
There is a very simple test (nicknamed the "poor man's GC) called the methanol test, 3/27 test, or Warnqvist test (originally posted on Journey to Forever). All you do is put 27 ml of methanol in a tube or small glass jar, mix in 3 ml of your biodiesel, and shake it hard a few times. Then let it sit for a few minutes and look to see whether a puddle of yellowish liquid forms at the bottom. If the processing was done well the biodiesel will mix completely with the methanol and you will not see a puddle at the bottom. If your reaction wasn't complete, the puddle will be there. The test appears to be amazingly in synch with the ASTM cutoff for total glycerin, ie if your fuel has 0.24% total glycerin or less, there will be no precipitate, though the mixture may be cloudy (really great fuel will give a clear mixture). Fuel above 0.24% will invariably yield the puddle of yellow liquid which are tri and di glycerides. If you're having filter plugging problems with certain fuel you're either making or buying, you might want to do a 3/27. You can always buy a small container of methanol at the auto parts store- it's the yellow bottle called "Heet."
3/27 test Pass! (ie no puddle, which would indicate incomplete reaction)
It is definitely possible for homebrewers to make fuel that meets or exceeds the ASTM D6751 standard for glycerin (arguably the most important standard, and the one most homebrewers probably don't meet). On the advice of someone I met at the coops conference, I raised my KOH base amount from the generally recommended 7 g / L to 10 g / L and my total glycerin went from just out of spec 0.29 to in-spec 0.21. The reduction was most dramatic with triglycerides which went down by 67%, and free glycerin (down 51%), whereas monoglycerides went down 20% and diglycerides 18%. See Bob Armantraut's great presentation for more info on interpreting GC results. Then take a look at the GC graph of my in-spec results- you can see that even tho there is a bit of a hump in the di and tri glycleride region, it's still under the ASTM cutoff.
Also check out John Bush's presentation on quality control for homebrewers. These presentations were made at the biodiesel coops conference in Golden, CO in July 2006- the site, which includes other downloadable presentations is b100.org.
Good fuel should be crystal clear- so that you could read through it. It's color may be a little dark if made from dark oil. But again, you can't tell the true quality by how it looks- get it tested.
The fuel should also be able to go to near freezing temperatures before clouding up or gellingThe cloud point is the temperature at which crystallization of the fuel is beginning, and the gel point is when the fuel gels up so it won't pour. Fuel made from soybean oil will start to cloud up at around freezing, but fuel made from Canola can go down to about 26 deg F before clouding (especially if you dry it well!). In the winter, it's a good idea to put a jar with some of your fuel in your car where you can see whether it's starting to cloud up or gel. If it is, then it might be time to start adding some yucky petrodiesel to your tank. However, the fuel will stay a bit warmer in your car's fuel tank, especially if you drive every day. So just because the fuel in your little test jar is clouding doesn't mean you're in trouble - yet... but it does tell you to pay attention to the weather (and the forecast).
The pHlip test is a new test for fuel quality that is available from b100supply.com. In fact, click here for their excellent section on evaluating fuel quality. The kit consists of 9 vials each half full of a red chemical. You add an equal volume of your biodiesel, and flip it upside down 10 times, then let it sit. It's a qualitative test, so if you've got glycerides in your fuel you'll see a fluffy layer in the middle (whereas ASTM fuel will have a mirror smooth interface). It comes with a little guide with good photos of possible results and their diagnosis.
pHLip test pass- ie fuel on top is clear and bright (not turbid hich would indicate soaps), interface in middle is smooth (no crud that would indicate glycerides), red stays red (ie purple would mean pH is still alkaline; yellow would indicate acidity from old fuel).
