Fuel delivery can change what a tank sample represents. New fuel entering a storage tank may disturb settled water, sediment and microbial material. A clear sample taken from a convenient point immediately after delivery is not proof that the tank bottom is clean or that the fuel is suitable for critical equipment. This guide provides a practical decision framework for operators of standby generators, marine vessels, agricultural equipment and bulk diesel storage.
Why the timing of sampling matters
A delivery may introduce new material, mix existing contamination into the fuel phase, or temporarily obscure a stratified water layer. Sampling immediately after a delivery can answer a different question from sampling after the contents have settled. Neither sample is universally better: the appropriate timing depends on whether the objective is to investigate the delivered product, the tank bottom, the dispensing point or the fuel supplied to an engine.
Establish the question before taking a sample
Record the tank identification, nominal capacity, estimated fuel volume, delivery date and quantity, sample point, sampling time, recent circulation or dispensing, and any alarm or filter event. Define the question: Is the new delivery suspected? Is contamination recurring in the tank? Is the generator supply at risk? A sample from the delivery line cannot substitute for a tank-bottom assessment.
Choose sampling locations that represent the risk
Delivery or receipt sample: useful when investigating incoming fuel, subject to the applicable sampling procedure and custody controls. Tank-bottom or low-point sample: useful for detecting free water and settled residues, where a safe designed sample point exists. Dispensing or engine supply sample: useful for understanding material reaching equipment, but may miss contamination below the draw-off point. Multiple depths or locations: may be needed for large, irregular or compartmentalised tanks. Never open confined spaces or improvise access to hazardous fuel systems.
What to record during a delivery-related investigation
| Observation | What it may indicate | Next evidence |
|---|---|---|
| Free water at a low point | Water accumulation or ingress | Measure extent, inspect source, follow approved removal procedure |
| Cloudiness after transfer | Suspended water, fine particles or other changes | Appropriate water and particulate testing |
| Shortened filter life | Mobilised sediment, deposits or other restriction | Retain filter evidence and compare upstream/downstream samples |
| Slime or recurring deposits | Possible microbial activity; not a definitive diagnosis | Representative microbial testing and tank inspection |
| Clear sample at dispenser | Appearance at that point and time only | Check whether tank-bottom and critical supply risks were represented |
Separate water, particles and microbial findings
These are different questions. A microbial screen does not measure water concentration or particulate cleanliness. A clear-looking sample does not establish fuel stability, microbial absence or compliance with a fuel specification. Use a test method appropriate to the decision, with laboratory analysis when the consequence of failure warrants it. See water in diesel, microbial growth and why one clear sample does not prove a clean tank.
Decide whether to isolate, investigate or continue monitoring
Where there is significant free water, repeated filter blockage, suspected off-specification fuel or a critical-power exposure, follow site procedures and equipment manufacturer requirements before releasing fuel for use. For lower-risk findings, record a baseline, assign an owner, set a review interval and compare like-for-like samples. Do not add chemicals as a substitute for removing substantial free water or physical sediment. Treatment, filtration, tank cleaning and monitoring serve different purposes and may be complementary when justified by evidence.
Document the outcome and verify the corrective action
Maintain a record of the sampling objective, method, time relative to delivery, chain of custody where needed, laboratory or field results, photographs, corrective actions and repeat checks. Verify whether the original failure mechanism has been controlled: for example, reduced recurring water, improved filter life or consistent results from equivalent sample points. An improvement in appearance alone is insufficient.
Practical checklist for site operators
- Confirm the purpose of sampling and the equipment at risk.
- Record delivery and circulation history.
- Select safe, representative sample points using approved procedures.
- Document sample identification, timing and handling.
- Separate observations from test-confirmed findings.
- Investigate the source of water or solids rather than treating symptoms alone.
- Define a corrective action and measurable follow-up.
Further technical guidance
Explore the Fuel Right diesel technical resources and the standby generator readiness checklist. Fuel Right Australia provides fuel treatment, microbial testing and water-management product guidance; product selection should follow assessment of the actual fuel and tank condition. About Fuel Right Australia.