Marcy Density Cup Guide
What a Marcy density cup (pulp density scale) is, how plant operators use it to read slurry density and infer percent solids, why the solids SG matters, and the common measurement mistakes that make a reading misleading. A concept guide — not an operating procedure or a substitute for lab testwork.
Definition
A Marcy density cup — also called a pulp density scale or pulp density cup — is a simple, rugged field instrument used to read the density of a slurry (pulp) at the point of sampling. It is a container of fixed, known volume (commonly 1000 mL) that hangs from a spring scale with a circular dial. The operator fills the cup until slurry overflows a fixed weir, so exactly one cupful of slurry is held, then reads the dial. Because the volume is fixed, the weight of that one cupful is a direct measure of the slurry density, and the dial is graduated to display it as a slurry specific gravity (pulp SG). Many cups also carry a set of inner dial rings calibrated for specific solids SG values, so that — once you select the ring matching your ore — the same reading can be presented as percent solids by mass directly. It is a spot-reading hand tool, not a continuous instrument.
Why it matters
Slurry density is the single most-used operating number in a wet processing plant: it sets percent solids in flotation, grinding, leaching, and thickening, and it drives pump and pipeline duty. Operators need a way to check it quickly, anywhere on the plant, without sending a sample to the lab. The Marcy cup gives a reading in seconds at the sample point, so operators can confirm a grinding circuit is at target density, check a thickener underflow, or verify a dilution before a downstream stage. It is cheap, needs no power, and survives the plant floor. Its value is speed and convenience for routine control — but because it reads density (not solids directly), the percent-solids it implies is only as good as the assumed solids SG and the quality of the sample, so it complements rather than replaces laboratory measurement.
Formula
Units involved
- •ρ_slurry, ρ_liquid, ρ_solids — densities in kg/m³ (or SG, dimensionless)
- •V_cup — fixed cup volume (typically 1000 mL = 0.001 m³)
- •m_slurry — mass of one cupful (g or kg)
- •Cw — percent solids by mass (the dial’s inner ring, when SG-matched)
Concept diagram
Worked example
A Marcy cup reads a pulp SG of 1.50 (slurry density 1500 kg/m³) on a grinding-circuit sample. The ore solids SG is 2.65 (2650 kg/m³) and the liquor is water (1000 kg/m³). What percent solids by mass does that reading imply?
- 01ρ_slurry = 1500, ρ_solids = 2650, ρ_liquid = 1000 kg/m³
- 02Cw = 2650 × (1500 − 1000) / [1500 × (2650 − 1000)] × 100
- 03Cw = 2650 × 500 / (1500 × 1650) × 100 = 1,325,000 / 2,475,000 × 100
- 04Cw ≈ 53.5%
About 53.5% solids by mass — but only if the solids SG really is 2.65. Read the cup against the dial ring for the correct ore SG, or convert the density with the Slurry Density Calculator.
Common mistakes
- •Using the wrong dial ring (wrong assumed solids SG) — the density reading can be right while the inferred percent solids is wrong.
- •Not filling to the overflow, so the cup holds less than its rated volume and the density reads low.
- •Trapped or entrained air in the slurry, which lightens the cupful and biases the density low.
- •Letting fast-settling coarse solids drop out before the cup is weighed, so the sample is not representative.
- •Taking a single grab from a non-representative point instead of a well-mixed, representative stream.
- •Skipping the water calibration check (a full cup of clean water should read SG 1.00) so a worn or zero-shifted scale goes unnoticed.
- •Treating the Marcy reading as a lab-grade or design-grade number rather than a quick operating check.
When to use the calculator
Use the Slurry Density Calculator to convert a Marcy reading (a slurry density or pulp SG) into percent solids by mass or by volume for any solids SG — it removes the dependence on which dial ring was used. Use the Percent Solids Mass ↔ Volume Calculator when you need the Cw ↔ Cv basis conversion explicitly, and the Slurry Mass Balance Calculator to turn the stream flow and the measured density into solids and liquid mass flows.