It consists of a graduated beam with a bubble level and a weight slider along its length and a cup with a lid on one end.
The cup is used to hold a fixed amount of fluid so it can be weighed.
Density is read at the point where the slider-weight sits on the beam at level.
Calibration is done using a liquid of known density (often fresh water) by adjusting the counter weight.
[3] There is no reliable visual method of determining the density of drilling mud; the mud balance is the most reliable and simple way of making the determination.