5.1 The porometer was developed in 1988 at North Carolina State University-Raleigh. It was originally designed as a research tool to measure structure for horticultural substrates. The substrates were blends of materials using one or more components, such as peat moss and pine bark. Prior to the creation of the porometer, soil retention curves were the primary method used to evaluate horticultural substrate.

5.2 Over the course of testing many substrates, it became clear that differences during the creation of the structure impacted the test results. The main cause for variation in test results between multiple laboratories could be attributed to unstable structure creation. It is important for the substrate to have a structure that does not change and is stable.

5.3 This method can be used in quality control, in development of new substrate mixes, and for comparison testing.

Note 1: The quality of the result produced by this standard is dependent on the competence of the personnel performing it and the suitability of the equipment and facility used. Agencies that meet the criteria of Practice D3740 are generally considered capable of competent and objective testing/sampling/inspection. Reliable results depend on many factors; Practice D3740 provides a means of evaluating some, but not all, of those factors.

1.1 This test method is designed to determine the properties of the structure for substrates and components used in horticulture as a growth media. The total porosity, container capacity, and air space are three properties that are determined on a percent volume basis using this method. Testing is typically performed in a laboratory in a controlled environment.

1.2 This method uses the Fisons International British Standard Procedures AFNOR Nomes (FIBSPAN) device (also known as the 1 to 5 method) to determine the bulk density so the pack mass can be calculated. The test specimens are then created based on this pack mass. Using the FIBSPAN device is more precise and provides more repeatable results.

1.3 The types of materials tested using this standard are generally organic substrates that are blends of one or more components of peat moss, pine bark, perlite, vermiculite, sand, coconut fiber, yard wastes, and composts. Other types of materials, such as biochar and wood fibers, can also be tested using this method.

1.4 The porometer was developed to provide certain parameters similar to soil moisture retention curves, but in a much shorter time frame. Soil moisture retention curves typically take about three weeks to complete. Using the technique in this standard allows for a range of uses, such as the creation of different structures to test the effects of compaction, water content and hydrophobicity.

1.5 There are two known limitations with porometers: creation of the substrate structure and shrink/swell of the substrate during testing.

1.5.1 The test results are affected by how and when the structure of the substrate/component is created. The structure of the substrate/component is created at the point of use and does not exist in a bag or bale of substrate/component. The structure is developed during specimen preparation and by precise packing of the specimen cylinders using a specific bulk density and mass wetness targets. This test method provides the steps needed to control specimen preparation and filling of the cylinder to reduce variations from occurring that result in developing a different structure with subsequently different test results.

1.5.2 Under certain conditions, the substrate/component may either swell over the top of the specimen mold during saturation or shrink below the top of the cylinder during drainage, or both. This situation occurs in some components such as peats and coconut fiber when they are not fully saturated during preparation. A shrink or swell of ±0.12 in. (3 mm) from the top of the mold can alter the structure significantly and cause erroneous readings. Therefore, paying careful attention to specimen preparation with respect to the mass wetness and bulk density helps to prevent such occurrences. If bulk density and mass wetness are tightly controlled, the same test material can be tested again months later with nearly identical values.

1.5.3 While uncommon, there are some types of substrates/components that will exhibit a shrink/swell more than ±0.12 in. (3 mm) even when the bulk density and mass wetness are correct. Materials that have a low bulk density tend to shrink more than 0.12 in. (3 mm).

1.6 Units—The values stated in inch-pound units are to be regarded as standard. The values given in parentheses are mathematical conversions to SI units that are provided for information only and are not considered standard. Reporting of test results in units other than inch-pound shall not be regarded as nonconformance with this standard.

1.6.1 The gravitational system of inch-pound units is used when dealing with inch-pound units. In the system, the pound (lbf) represents a unit of force (weight), while the units for mass is slugs. The slug unit is not given, unless dynamic (F = ma) calculations are involved.

1.6.2 The slug unit of mass is typically not used in commercial practice; that is, density, balances, and so on. Therefore, the standard unit for mass in this standard is either kilogram (kg) or gram (g), or both. Also, the equivalent inch-pound unit (slug) is not given/presented in parentheses.

1.6.3 The SI units presented for apparatus are substitutions of the inch-pound units, other similar SI units should be acceptable providing they meet the technical requirements established by the inch-pound apparatus.

1.6.4 The terms density and unit weight are often used interchangeably. Density is mass per unit volume, whereas unit weight is force per unit volume. In this standard, density is given only in SI units. After the density has been determined, the unit weight is calculated in SI or inch pound units, or both.

1.7 All observed and calculated values shall conform to the guidelines for significant digits and rounding established in Practice D6026.

1.7.1 The procedures used to specify how data are collected/recorded or calculated in the standard are regarded as the industry standard. In addition, they are representative of the significant digits that generally should be retained. The procedures used do not consider material variation, purpose for obtaining the data, special purpose studies, or any considerations for the user’s objectives; and it is common practice to increase or reduce significant digits of reported data to be commensurate with these considerations. It is beyond the scope of this standard to consider significant digits used in analysis methods for engineering design.

1.8 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.

1.9 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.