| Particle Number Density Meter: This meter measures the number density of gas particles. You must attach the number density meter to an upper orange hotspot on the Ultrascope (see Ultrascope View (Evaporation)). The number the meter displays roughly equals the number of gas particles of the ty pe being measured that are visible in the Ultrascope. In the Properties Box of the number density meter, you can choose to measure the numbers of air or liquid vapor particles. You must place an number density meter to plot number density using the graph tool.