Plastic viscosity (PV) and yield point (YP) are the two numbers the Bingham plastic model pulls out of a drilling fluid, both read on a Fann 35 viscometer. PV is the slope of the line between the 600 and 300 rpm dial readings; YP is where that line, projected back, crosses zero shear rate. PV = R600 − R300 and YP = R300 − PV. Two readings, two parameters, two completely different stories.
The Bingham plastic model — why two numbers
A drilling fluid is not Newtonian: its viscosity changes with how hard you shear it. The Bingham plastic model is the simplest useful description — a straight line of shear stress against shear rate, with a slope (the plastic viscosity) and an intercept above zero (the yield point). The model is built from just two points on the rheogram, taken at 600 and 300 rpm, because those two speeds bracket the flow regime in the drill string and annulus well enough for daily control.
The measurement
The standard instrument is a Fann 35 in the R1 B1 F1 rotor-bob-spring configuration. The rotor turns the fluid; the bob deflects against a torsion spring; the dial reads that deflection. Per API RP 13B-1 you record each reading once the value is steady, with field water-based mud usually measured at 120 °F. For this geometry the conversions are fixed:
| Rotor speed | Shear rate | Reading used for |
|---|---|---|
| 600 r/min | 1021.8 s−1 | R600 |
| 300 r/min | 510.9 s−1 | R300 |
| 3 r/min | 5.11 s−1 | gels / low-shear |
The formulas
Plastic viscosity (cP) = R600 − R300
Yield point (lbf/100 ft²) = R300 − PV = 2 R300 − R600
Apparent viscosity (cP) = R600 ÷ 2
Worked example. The viscometer reads R600 = 56 and R300 = 35.
| PV | 56 − 35 = 21 cP |
| YP | 35 − 21 = 14 lbf/100 ft² |
| AV | 56 ÷ 2 = 28 cP |
What plastic viscosity tells you — it is a solids gauge
PV is the viscosity extrapolated to infinite shear, so it reflects the mechanical friction of solids sliding past one another in the base fluid. It is driven by the type, size and concentration of solids — the more particles, and the finer they are, the higher the PV. A rising PV with no chemical change almost always means one thing: your solids are accumulating, usually the fine, colloidal fraction the centrifuge owns. That is why PV is the number a solids-control engineer watches. A climbing PV is a removal problem, not a chemistry problem — reaching for thinners treats the symptom while the fines keep grinding finer.
What yield point tells you — it is carrying capacity
YP is the shear stress the fluid needs before it will flow at all — the Bingham intercept at zero shear rate. Physically it comes from the attractive, electrochemical forces between particles, not their concentration. YP governs the fluid’s ability to lift cuttings in the annulus: a higher YP carries cuttings better and raises annular frictional pressure loss. Unlike PV, YP is a chemical lever — you raise it with viscosifiers (bentonite, polymers) and lower it with deflocculants, independent of solids loading.
PV and YP are independent levers
This is the practical payoff of separating them. High PV, normal YP → a solids problem; run the removal train harder. Normal PV, low YP → poor hole cleaning; build YP chemically. High PV and high YP → you are both dirty and gelled; fix the solids first, because thinning a fluid that is overloaded with fines is a losing battle. The YP/PV ratio is itself a useful shape factor — a higher ratio means a more shear-thinning, better-suspending fluid at drilling shear rates.
Where Bingham stops — and what comes next
The Bingham line is built from the two high-shear points, so it over-predicts the yield stress at the low shear rates that actually matter in the annulus and at rest. For low-shear behaviour you read the 6 and 3 rpm points and the gel strengths, or fit the power-law or Herschel-Bulkley models. From the same two readings the conventional power-law gives the flow-behaviour index n = 3.32 log10(R600/R300) and the consistency index k = R300 / 511n. But for daily control, PV and YP remain the two numbers you act on.
Key takeaways
PV and YP come from two viscometer readings and mean opposite things. PV (R600 − R300) is mechanical — your solids gauge; a rising PV is a removal job. YP (R300 − PV) is electrochemical — your carrying capacity; you tune it with chemistry. Read them together, treat them separately, and never thin a fluid whose real problem is fines.