Hydrometer Testing per ASTM D7928: Why It Matters and Why Digital Recording Makes a Difference
February 05, 2026
In construction materials testing (CMT) and geotechnical engineering, the smallest soil particles pose the greatest risks. While sieve analysis captures sand and gravel, it’s the hydrometer test that measures silts and clays—the fine-grained materials that control settlement, drainage, and long-term performance.
Understanding the hydrometer test is critical. But just as important is how the results are recorded, calculated, and reported. Modern construction materials testing software helps labs replace spreadsheets and paper with standardized digital workflows.
What Is a Hydrometer Test?
A hydrometer test is a laboratory soil test used to determine the particle-size distribution of fine-grained soils, specifically silts and clays smaller than the No. 200 sieve (0.075 mm). The test measures how soil particles settle in water over time to calculate the percentage of fine material in a sample.
While sieve analysis accurately characterizes coarse-grained fractions, it cannot measure particle sizes smaller than the No. 200 sieve. Hydrometer testing fills this gap by determining the distribution of fine particles based on sedimentation principles.
Hydrometer testing in the United States is performed in accordance with ASTM D7928, the current ASTM standard for sedimentation analysis of fine-grained soils. ASTM D7928 was introduced in 2016, when ASTM withdrew ASTM D422 and separated hydrometer analysis into a dedicated standard.
ASTM D7928 defines the procedures, corrections, and calculations required to obtain accurate, repeatable hydrometer results.
Hydrometer testing is typically performed alongside sieve analysis to support engineering design and soil classification.
Why Hydrometer Testing Is Important in Materials Testing
1. Completes the Soil Evaluation
Sieve analysis alone cannot measure very fine particles. Hydrometer testing:
- Captures silts and clays
- Prevents underestimating fine-grained content
- Reduces the risk of unexpected soil behavior
Without hydrometer data, soils may appear acceptable in the lab but perform poorly in the field.
2. Directly Impacts Soil Classification
Hydrometer results play a major role in soil classification systems used by engineers and agencies. Even small changes in clay content can significantly alter:
- Soil classification
- Design recommendations
- Suitability for construction use
3. Fine Particles Control Engineering Performance
Hydrometer testing is important because fine soil particles control settlement, drainage, strength, and long-term performance. Even a small increase in clay content can significantly affect soil behavior, making accurate hydrometer results critical for engineering design and construction decisions.
4. Required by Industry Standards
Hydrometer testing per ASTM D7928 is commonly required for:
- Geotechnical investigations
- Pavement and subgrade evaluation
- Structural fill approval
- QA/QC documentation
Accurate, well-documented results are essential for engineering review and project approval.
The Challenge with Traditional Hydrometer Reporting
Despite its importance, hydrometer testing is often documented using:
- Handwritten logs
- Manual time readings
- Spreadsheet-based calculations
- Re-entering results into reports
These workflows introduce:
- Calculation and transcription errors
- Inconsistent reporting formats
- Slower QA/QC review
- Poor traceability during audits
For busy labs, this becomes a bottleneck.
Why Recording Hydrometer Results Is Easier with CMT Software
Modern eFieldData CMT software simplifies hydrometer testing from data entry to final report.
1. Standardized Digital Data Entry
Structured digital forms ensure:
- Consistent data entry across technicians
- Fewer transcription errors
- Cleaner, review-ready records
2. Automated ASTM D7928 Calculations
Hydrometer testing involves multiple calculations. With ASTM D7928 compliant reporting, CMT software:
- Applies formulas automatically
- Handles corrections consistently
- Eliminates spreadsheet errors
This improves accuracy while saving significant lab time.
See Digital Hydrometer Reporting in eFieldData
Built for geotechnical and construction materials testing labs.
3. Faster QA/QC and Reporting
Digital workflows allow:
- Immediate review by lab managers
- Faster turnaround times
- Clear audit trails
What once took hours can now be completed in minutes.
4. Long-Term Data Access and Traceability
With cloud-based CMT software:
- Hydrometer results are searchable
- Historical data is preserved
- Documentation is always available for audits or disputes
Note on DOT and AASHTO Standards:
While this article focuses on ASTM D7928, many transportation agencies reference AASHTO T 88 as the equivalent hydrometer method for DOT work.
The underlying test procedure and reported results are largely comparable; however, labs should always follow the standard specified in the project documents.
Frequently Asked Questions About Hydrometer Testing
1. What does a hydrometer test measure?
A hydrometer test measures the percentage of fine soil particles, specifically silts and clays, in a soil sample by analyzing how particles settle in water over time.
2. What is the relationship Between Sieve Analysis and Hydrometer Test
A complete particle size distribution (PSD) curve typically includes:
- Sieve analysis → Gravel & Sand fractions
- Hydrometer analysis → Silt & Clay fractions
Both results are combined to generate a continuous gradation curve, which is critical for:
- Soil classification (USCS / AASHTO)
- Engineering behavior predictions (permeability, compressibility, shear strength)
- Design decisions in earthwork, pavement, and foundation projects
3. Why is the hydrometer test important in soil testing?
The hydrometer test is important because fine particles strongly influence settlement, permeability, shrink-swell behavior, and shear strength, which directly affect pavement and foundation performance.
4. What ASTM standard is used for hydrometer testing?
Hydrometer testing is performed in accordance with ASTM D7928, the current ASTM standard for sedimentation analysis of fine-grained soils.
- Applies specifically to fine-grained soils
- Provides clear calculation procedures
- Separates hydrometer testing from sieve analysis
- Improves repeatability and reporting consistency
- Improves repeatability and reporting consistency
5. What is ASTM D422?
ASTM D422 was the former ASTM standard for particle-size analysis of soils that combined sieve analysis and hydrometer testing into a single method. ASTM officially withdrew D422 in 2016 as part of a standards update.
6. Why was ASTM D422 withdrawn?
ASTM withdrew ASTM D422 in 2016 to improve clarity and consistency. The standard covered multiple testing procedures in one document, which led to inconsistent interpretation. ASTM replaced it with separate, dedicated standards for sieve analysis and hydrometer testing.
Note:
Despite being withdrawn, many laboratories still reference ASTM D422, especially in:
- Older DOT specifications
- Legacy templates
- Manual or spreadsheet-based workflows
7. What is the best practice today for particle-size distribution (PSD) testing?
Best practice today is to use separate ASTM standards for coarse and fine soil fractions and then combine the results:
- ASTM D6913 → Gravel & Sand fractions
- ASTM D7928 → Silt & Clay fractions
- The results are then combined to produce a complete particle-size distribution (PSD) curve
This approach replaced the former all-in-one method under withdrawn ASTM D422 and reflects current ASTM best practice.
8. Is hydrometer testing required for geotechnical projects?
Hydrometer testing is commonly required for geotechnical investigations, pavement design, subgrade evaluation, and QA/QC testing, particularly when fine-grained soils are present.
9. How does CMT software help with hydrometer testing?
CMT software like eFieldData helps by standardizing data entry, automating calculations, reducing errors, and speeding up reporting compared to manual spreadsheets or paper workflows.
10. Why Digital Hydrometer Reporting Matters
The hydrometer test may focus on the finest soil particle, but its impact on project performance, cost, and risk is significant.
Using CMT software to record hydrometer results:
- Improves accuracy
- Reduces rework
- Speeds up reporting
- Strengthens compliance
That’s why more geotechnical labs and CMT firms are moving away from paper and spreadsheets toward digital lab management platforms.
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