ASTM D7261 Standard Test Method for Determining Water Separation Characteristics of Diesel Fuels by Portable Separometer
1. Scope
1.1 This test method covers a rapid portable means for field and laboratory use to rate the ability of diesel fuels (both neat and those containing additives) to release entrained or emulsified water when passed through fiberglass coalescing material.

1.2 This test method is applicable to diesel fuels such as Specification D975 Grade No. 1-D and Grade No. 2-D of all sulfur levels, Specification D7467 biodiesel blends B6-B20, and MIL-DTL-16884, naval distillate fuel (NATO F-76).

NOTE 1 - This test method is similar to Test Method D3948 which is applicable to aviation turbine fuels.

1.3 The values stated in SI units are to be regarded as standard. No other units of measurement are included in this standard.

1.4 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety, health, and environmental practices and determine the applicability of regulatory limitations prior to use.

1.5 This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.

2. Referenced Documents
2.1 ASTM Standards:
D975 Specification for Diesel Fuel Oils
D1193 Specification for Reagent Water
D3948 Test Method for Determining Water Separation Characteristics ofAviation Turbine Fuels by Portable Separometer
D4057 Practice for Manual Sampling of Petroleum and Petroleum Products
D4176 Test Method for Free Water and Particulate Contamination in Distillate Fuels (Visual Inspection Procedures)
D4177 Practice for Automatic Sampling of Petroleum and Petroleum Products
D4306 Practice for Aviation Fuel Sample Containers for Tests Affected by Trace Contamination
D4860 Test Method for Free Water and Particulate Contamination in Middle Distillate Fuels (Clear and Bright Numerical Rating)
D6300 Practice for Determination of Precision and Bias Data for Use in Test Methods for Petroleum Products and Lubricants
D6426 Test Method for Determining Filterability of Middle Distillate Fuel Oils
D7224 Test Method for Determining Water Separation Characteristics of Kerosine-Type Aviation Turbine Fuels Containing Additives by Portable Separometer
D7467 Specification for Diesel Fuel Oil, Biodiesel Blend (B6 to B20)

2.2 Military Standard:
MIL-DTL-16884 Fuel, Naval Distillate

3. Terminology
3.1 For definitions of terms used in this test method that are not shown below, refer to Test Methods D3948 and D7224.

3.2 Definitions:
3.2.1 reference fluid, n - in MSEP and DSEP, [diesel separability] water separability tests a reference fluid base to which a prescribed quantity of a known surface active agent has been added.
3.2.1.1 Discussion - The known surface active agent is typically bis-2-ethylhexyl sodium sulfosuccinate, commonly referred to as AOT, dissolved in toluene.

3.2.2 surfactant, n - in petroleum fuels, surface active material (or surface active agent) that could disarm (deactivate) filter separator (coalescing) elements so that free water is not removed from the fuel in actual service.
3.2.2.1 Discussion - Technically, surfactants affect the interfacial tension between water and fuel which affects the tendency of water to coalesce into droplets.

3.2.3 strong surfactant, n - in petroleum fuels, surface active material that disarms filter separator elements, allowing water to pass.
3.2.3.1 Discussion - Strong surfactants can be refinery process chemicals left in the fuel or contaminants introduced during transportation of the fuel.

3.2.4 weak surfactant, n - in petroleum fuels, surface active material, typically certain types of additives such as static dissipator additive, that does not adversely affect the performance of filter separator elements in actual service.

3.3 Definitions ofTerms Specific to This Standard:
3.3.1 DSEP rating, n - the diesel separability rating of diesel fuel as measured by this test method.
3.3.1.1 Discussion - DSEP ratings are only valid within the range of 50 to 100, with ratings at the upper end of the range indicating a clean fuel with little or no contamination by surfactants, which is expected to show good water-separating properties when passed through a filter-separator (coalescing type filter) in actual service; see 14.1.

3.3.2 reference fluid base, n - a distillate diesel fuel that has been cleaned in a prescribed manner to remove all surface-active contaminants (agents), and having a minimum DSEP rating of 97.
3.3.2.1 Discussion - The reference fluid base should be a diesel fuel typical of fuels to be tested.

3.4 Abbreviations:
3.4.1 ac - alternating current

3.4.2 AOT - Aerosol OT (see 8.1)

3.4.3 C/S - collect sample

3.4.4 dc - direct current

3.4.5 DSEP - diesel separability

3.4.6 MSEP - Micro-Separometer