To diagnose what solids are doing to the mud, you need two numbers from a rotational viscometer — plastic viscosity (PV) and yield point (YP). They split the mud's flow resistance into two distinct causes, making the solids engineer's diagnosis sharper than anything a Marsh funnel can deliver.
How PV and YP are measured
PV (cP) = dial600 − dial300
YP (lb/100 ft²) = dial300 − PV
What each tells you about solids
| Parameter | Driven by | Rising trend means | SC response |
|---|---|---|---|
| PV | Total solids volume | Solids building | More centrifuge hours, better screens |
| YP | Colloidal/reactive clay (MBT) | Clay dispersing into mud | Centrifuge fine cut, chemistry review |
The key split: PV climbs when you have too many particles — a volume problem, fixed by removal. YP climbs when particles are reactive — a chemistry problem, fixed by centrifuge plus mud chemistry. The Marsh funnel sees neither distinctly.
PV rising — what to look at first
- Check LGS from the retort split — if LGS is climbing, the removal train is behind.
- Walk shakers: screens, beach, bypass gates.
- Check cone feed head and centrifuge hours vs fines generation.
YP rising — what to look at first
Rising YP is the signature of reactive clay. MBT confirms it. The centrifuge is your mechanical tool; inhibitor chemistry (PHPA, KCl, glycol) stops the clay dispersing in the first place. Check pH — out-of-range pH drives both dispersion and flocculation.
Key takeaways
A rising PV sends you to shakers, cones and centrifuge hours. A rising YP sends you to MBT, centrifuge fine cut and the mud chemist. Track both every tour and act on each independently.