How to test diesel fuel: sampling, field checks, and lab tests
Start with a sample from the bottom of the tank in a clear jar, and water-finding paste on a gauge stick. Good diesel is "clear and bright," with no haze, particles, or water layer. When fuel has to work, or the jar shows something you can't explain, send a sample to a fuel lab for water and sediment, particulates, stability, cold flow, and microbes.
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What you're testing for
| Problem | Quick check | More |
|---|---|---|
| Free water | Water paste on the tank bottom; water layer in the jar | Water in your diesel tank |
| Microbes | Slime, dark mats, or a hazy layer where fuel meets water; field test kit | Diesel bugs and biocides |
| Sediment and rust | Particles in the jar; plugged filters | Lab particulate test |
| Aging (oxidation) | Darkening, gums, sludge | How long diesel lasts |
| Cold-weather wax | Cloudiness that clears when the sample warms | Diesel gelling |
Step 1: Take a good sample
Sample from the bottom. Water, microbes, and sediment all settle there. Pull from about 3 to 6 inches off the bottom, plus any water layer (Bell Performance). The industry practice for doing this, including sampling free water, is ASTM D4057 (ASTM D4057).
- Use a bottom sampler lowered through a tank opening, or drain a sample from a low-point valve if your tank has one.
- At a dispenser, also take a nozzle sample. Industry guidance for diesel dispensing sites calls for a weekly nozzle sample in a clear jar (CRC Report 672).
- Use a clean, clear glass or plastic jar, and label it with the date, tank, and where in the tank it came from. Sample jars: clear fuel sample bottles on Amazon.
- Dispose of samples properly unless you're keeping them (CRC Report 672).
Field note: On the tanks I service, I pull a bottom sample once a year. Water and sediment are what I find most.
Step 2: Checks you can do yourself
The clear-jar check. Let the jar settle, then hold it up to the light. Good diesel is "clear and bright" with no haze or particles (CRC Report 672). ASTM D4176 standardizes this check: one procedure is a quick pass/fail for contamination, the other rates the haze, and both work as field tests at storage temperature (ASTM D4176). If the haze clears when the sample warms up, it's probably wax, not water.
Water paste on the tank bottom. Industry guidance is to stick the tank bottom with water-finding paste weekly, and to compare the reading with the automatic tank gauge if you have one (CRC Report 672). Check before and after deliveries, too (CRC Report 672). Water paste: water-finding paste on Amazon.
Microbe test kits. Two ASTM methods can be run in the field:
- Immunoassay (ASTM D8070): screens fuel or tank water for antigens from bacteria and fungi, in the field or a lab (ASTM D8070).
- ATP (ASTM D7687): measures the energy molecule in living cells and gives a number you can track over time; it works as a lab or portable test (ASTM D7687).
Culture tests are cheaper but take 48 to 72 hours or more (Bell Performance). ASTM's guide to microbial contamination, D6469, covers the symptoms and how to detect and control it (ASTM D6469). Test water you drain from the tank, too (CRC Report 672). Kits: fuel microbe test kits on Amazon.
Your filters are a test, too. Filters that clog often, or a dispenser that slows down after a delivery, point to contamination; flow should stay above 5 gallons per minute. When you change a filter, look at the old one for rust and slime (CRC Report 672).
Field note: In the field, I mostly go by the jar and the paste. A lab is for when those turn up something they can't explain.
Step 3: Lab tests worth asking for
A fuel lab runs standard ASTM methods. These are the ones that answer the usual storage questions.
| Test | ASTM method | What it tells you |
|---|---|---|
| Water and sediment | D2709 | The percentage of free water and sediment suspended in the fuel, by centrifuge. The diesel spec allows 0.05% (Gilbert) |
| Particulate contamination | D6217 | The weight of particles caught by filtering the fuel. Not validated for biodiesel blends; ASTM points to D7321 for those |
| Oxidation stability | D2274 | How well the fuel resists aging under accelerated conditions at 95°C. For biodiesel blends, ASTM points to D7462 |
| Cold filter plugging point | D6371 | The temperature at which the fuel plugs a test filter, with or without flow-improver additives |
| Microbial ATP | D7687 | How much living microbial growth is in the fuel or tank water |
| Haze and free water | D4176 | A standardized version of the clear-jar check |
The specification diesel is sold against is ASTM D975, which covers seven grades of diesel (ASTM D975). It has no storage-stability test for #1 or #2 diesel (NREL), which is why stored fuel gets tested separately. Tell the lab whether your fuel has biodiesel in it, since some methods differ for blends.
How often to test
| Situation | How often |
|---|---|
| Water paste on the tank bottom | Weekly at dispensing sites, and before and after deliveries (CRC Report 672) |
| Nozzle sample in a clear jar | Weekly at dispensing sites (CRC Report 672) |
| Microbial test | Quarterly as a baseline for stored fuel, more often after a problem (Bell Performance) |
| Standby generator fuel | At least once a year by ASTM methods, under NFPA 110 (Bell Performance on NFPA 110) |
| Farm and yard tanks | A clear-jar bottom sample and water paste monthly and after deliveries is a reasonable minimum |
What to do with the results
- Water: get it off the bottom as soon as you find it. "No detectable water is desirable" (CRC Report 672). See water in your diesel tank.
- Microbes: remove the water first, then treat with an EPA-registered biocide. See diesel bugs and biocides.
- Sediment or failed stability: the usual fix is to recondition or replace the fuel, clean the tank, and change filters (Massachusetts fire code excerpt).
- Cold-flow problems: see diesel gelling and winter additives.
FAQ
Is a sample from the top of the tank good enough?
No. Water, microbes, and sediment collect at the bottom, so a top sample can look clean while the bottom isn't (Bell Performance).
Do biodiesel blends need different tests?
Some lab tests do. ASTM's accelerated stability test and its particulate filtration test haven't been validated for biodiesel blends, and ASTM points to other methods for them (ASTM D2274; ASTM D6217). Most diesel can contain up to 5% biodiesel, so tell the lab what you have.
Do I need a lab, or can I test it myself?
Start yourself: a clear-jar bottom sample and water paste catch most problems. Send a sample to a lab when the fuel has to work, like a standby generator, when the jar shows haze or sediment you can't explain, or before you spend money on fuel you might not need to replace.
Sources
- CRC Report 672: Preventive maintenance guide for diesel storage and dispensing systems (2016)
- ASTM D4057: Manual sampling of petroleum and petroleum products
- ASTM D4176: Free water and particulate contamination, visual inspection
- ASTM D2709: Water and sediment in middle distillate fuels by centrifuge
- ASTM D6217: Particulate contamination by laboratory filtration
- ASTM D2274: Oxidation stability of distillate fuel oil
- ASTM D6371: Cold filter plugging point
- ASTM D6469: Guide for microbial contamination in fuels
- ASTM D7687: Cellular ATP in fuel and fuel-associated water
- ASTM D8070: Microbial screening by lateral flow immunoassay
- ASTM D975: Standard specification for diesel fuel
- Bell Performance: Fuel tests that predict failure (ASTM D6469 and microbial testing)
- Gilbert, National Tanks Conference (2018)
- NREL: Biodiesel handling and use guide
- Bell Performance: What NFPA 110 requires for stored generator fuel
- Massachusetts: Testing of diesel fuel (527 CMR excerpt)
This guide is general information. Follow your equipment manufacturer's fuel requirements and your fuel lab's sampling instructions.