A test for oil-base and synthetic-base muds that indicates the emulsion and oil-wetting qualities of the sample. The test is performed by inserting the ES probe into a cup of 120掳F (48. 9掳C) mud and pushing a test button. The ES meter automatically applies an increasing voltage (from 0 to 2000 volts) across an electrode gap in the probe. Maximum voltage that the mud will sustain across the gap before conducting current is displayed as the ES voltage. The modern ES meter has sine-wave circuitry, whereas older meters used square-wave circuits. (The older units should not be used because they do not correctly address the theory described in the reference below. ) The ES sine-wave design and meaning of ES readings have been studied and were found to relate to an oil mud's oil-wetting of solids and to stability of the emulsion droplets in a complex fashion not yet understood.

Reference:
Growcock FB, Ellis CF and Schmidt DD: "Electrical Stability, Emulsion Stability, and Wettability of Invert Oil-Based Muds," SPE Drilling & Completion 9, no. 1 (March 1994): 39-46.