--- title: "Hydrometer Analysis | Intro to Civil Engineering" description: "Hydrometer analysis measures fine soil particle size in Intro to Civil Engineering, using sedimentation data to classify silt and clay for design." canonical: "https://fiveable.me/introduction-civil-engineering/key-terms/hydrometer-analysis" type: "key-term" subject: "Intro to Civil Engineering" unit: "Unit 6" --- # Hydrometer Analysis | Intro to Civil Engineering ## Definition Hydrometer analysis is a lab test that estimates the particle size distribution of fine-grained soils by tracking how soil particles settle in water. In Intro to Civil Engineering, it is used for silts and clays that are too small for sieve analysis. ## What It Is Hydrometer analysis is the soil lab test you use when the particles are so fine that a sieve cannot separate them well. In Intro to Civil Engineering, it gives you a way to estimate the size distribution of silt and clay particles by watching how a soil-water mixture settles over time. The setup is simple: you disperse a dried soil sample in water, usually with a dispersing agent so the particles do not clump together. Then you shake or mix the suspension and let it stand in a cylinder. A hydrometer is placed in the liquid to measure the density of the suspension at different times as particles settle out. The science behind the test comes from Stokes' Law. Bigger or denser particles fall faster through water, while smaller particles stay suspended longer. That means the hydrometer reading at each time tells you how much fine material is still floating in the water, which can be converted into the percentage of particles smaller than a certain diameter. This is why the test is tied to fine-grained soils, especially particles smaller than 0.075 mm. Sand and gravel are usually handled with sieve analysis, but silt and clay need sedimentation methods because they settle much more slowly and behave differently in water. The final result is a grain size distribution curve for the fine fraction of the soil. In class, you may combine hydrometer analysis with sieve analysis to get the full picture, then use that data to classify the soil in systems like USCS or AASHTO. If the sample flocculates, has temperature effects, or is not fully dispersed, the curve can be off, so the lab technique matters as much as the math. ## Why It Matters Hydrometer analysis matters because civil engineers need to know how much of a soil is made up of fines, not just whether the soil is "sand" or "clay." Fine particles control water movement, compressibility, and how a soil behaves when it is loaded. In a foundation or roadway context, a soil with a lot of clay-sized material can hold water, drain slowly, and respond differently under stress than a sandy soil. That changes design choices for subgrades, embankments, and earthwork. The hydrometer test gives you data that feeds into soil classification and later engineering decisions. It also connects directly to lab interpretation. You are not just reading a number from the hydrometer. You are turning a settling process into particle size information, then using that information alongside sieve results and plasticity data to describe the soil more accurately. That makes hydrometer analysis a bridge between basic soil mechanics and real project decisions. A boring log, lab report, or geotechnical problem often depends on whether the fine fraction is minor or dominant, and this test gives you the evidence for that call. ## Connections ### Stokes' Law Hydrometer analysis depends on Stokes' Law because the test uses particle settling velocity to estimate size. The law explains why larger particles drop faster than smaller ones in a fluid. If you know the fluid properties and the settling time, you can connect the hydrometer reading to an approximate particle diameter. ### Grain Size Distribution The hydrometer test produces the fine-grained part of a grain size distribution curve. In Intro to Civil Engineering, that curve shows how much of the sample falls into different particle-size ranges. When you combine hydrometer results with sieve analysis, you get a more complete picture of the soil sample. ### [Atterberg Limits](/introduction-civil-engineering/key-terms/atterberg-limits) Atterberg Limits and hydrometer analysis often appear together in soil classification work. Hydrometer analysis tells you how much clay and silt is present, while Atterberg Limits tell you how that fine soil behaves with changing water content. Together, they help you describe both particle size and plasticity. ### [coefficient of uniformity (cu)](/introduction-civil-engineering/key-terms/coefficient-of-uniformity-cu) Hydrometer analysis helps fill in the fine end of the size curve that is used to calculate coefficients like cu. That means the test supports judgments about how broadly graded a soil is. A soil with a wide spread of particle sizes behaves differently from one made mostly of similar-sized grains. ## On the AP Exam A quiz question or lab practical may give you a hydrometer reading table and ask you to identify whether the soil is mostly silt, clay, or a mixture. You may also be asked to explain why the test is used instead of sieves for very fine particles. In a lab report, you might compute the percent finer at each time, plot the grain size curve, and comment on whether the sample was well dispersed. If a problem asks about soil classification, hydrometer results often help you justify the fine fraction that goes into USCS or AASHTO decisions. ## Hydrometer Analysis vs sieve analysis Hydrometer analysis and sieve analysis both measure particle size distribution, but they work in different size ranges and use different mechanisms. Sieve analysis separates larger particles mechanically through mesh openings, while hydrometer analysis estimates fine particle sizes from how they settle in water. If the soil has lots of silt or clay, the hydrometer test is the one that gives useful data. ## Key Takeaways - Hydrometer analysis is the lab test used to estimate the size distribution of fine-grained soils, especially silt and clay. - The method works by measuring how fast particles settle in water, which connects the test to Stokes' Law. - A dispersing agent is used so particles do not clump together and give a false reading. - The test fills in the fine end of a grain size distribution curve that sieve analysis cannot measure well. - In civil engineering, the results help with soil classification and with judging how a soil may behave under loads and moisture changes. ## FAQs ### What is hydrometer analysis in Intro to Civil Engineering? It is a soil lab test used to estimate the particle size distribution of fine-grained soils. Instead of passing soil through sieves, it measures how soil particles settle in water over time. That makes it useful for silts and clays that are too small for normal sieve analysis. ### Why is hydrometer analysis used for clay and silt? Clay and silt particles are so small that sieves do not separate them well. They settle slowly in water, so a hydrometer can track their concentration in suspension over time. That gives you a way to estimate the fine fraction of the soil sample. ### How is hydrometer analysis different from sieve analysis? Sieve analysis sorts larger particles by size using mesh screens, while hydrometer analysis estimates very fine particle sizes from sedimentation in water. The two tests are often combined because one covers the coarse fraction and the other covers the fine fraction. If a soil has mostly sand and gravel, sieves matter more; if it has lots of fines, the hydrometer test matters more. ### What do you do with hydrometer analysis results? You use the readings to build a grain size distribution curve for the fine part of the soil. That curve helps with soil classification and gives clues about drainage, compressibility, and overall behavior. In class, it often shows up in lab reports, plotting problems, or classification questions. ## Related Study Guides - [6.1 Soil Classification and Properties](/introduction-civil-engineering/unit-6/soil-classification-properties/study-guide/jcVOir1wUsnPxLJj) ## About This Document Canonical Fiveable pages are available as Markdown at the same path plus `.md`. - [llms.txt](https://fiveable.me/llms.txt): index of Fiveable's sections and URL patterns - [llms-full.txt](https://fiveable.me/llms-full.txt): complete subject and unit listing - [MCP server](https://fiveable.me/mcp): call Fiveable as tools instead of fetching pages (`https://fiveable.me/api/mcp`) - [MCP server for AP teachers](https://fiveable.me/mcp/teachers): a teacher's classes, assignments and AP-rubric grading (`https://fiveable.me/api/mcp/teacher`) ## Structured Data ```json {"@context":"https://schema.org","@graph":[{"@type":"LearningResource","@id":"https://fiveable.me/introduction-civil-engineering/key-terms/hydrometer-analysis#resource","name":"Hydrometer Analysis | Intro to Civil Engineering","url":"https://fiveable.me/introduction-civil-engineering/key-terms/hydrometer-analysis","learningResourceType":"Concept explainer","educationalLevel":"AP® / High School","about":{"@id":"https://fiveable.me/introduction-civil-engineering/key-terms/hydrometer-analysis#term"},"audience":{"@type":"EducationalAudience","educationalRole":"student"},"dateModified":"2026-07-03T02:22:55.594Z","isPartOf":{"@type":"Collection","name":"Intro to Civil Engineering Key Terms","url":"https://fiveable.me/introduction-civil-engineering/key-terms"},"author":{"@type":"Organization","@id":"https://fiveable.me/editorial#content-team","name":"Fiveable Content Team","url":"https://fiveable.me/editorial","parentOrganization":{"@id":"https://fiveable.me/#organization"},"member":[{"@type":"Person","name":"Amanda DoAmaral","sameAs":["https://www.linkedin.com/in/adoamaral"]},{"@type":"Person","name":"Becky Bahr","sameAs":["https://www.linkedin.com/in/becky-bahr-b52743132"]}]},"publisher":{"@type":"Organization","name":"Fiveable","url":"https://fiveable.me"}},{"@type":"DefinedTerm","@id":"https://fiveable.me/introduction-civil-engineering/key-terms/hydrometer-analysis#term","name":"Hydrometer Analysis","description":"Hydrometer analysis is a lab test that estimates the particle size distribution of fine-grained soils by tracking how soil particles settle in water. 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