flow and pressure measurement

Question 1

how does a rotameter work?

Answer 1

variable orifice, fixed pressure device

has a needle valve controlling flow of gas through a taped transparent tube with a bobbin inside and a calibrated scale.

As the needle valve is opened, the flow will push the bobbin up the tube until the forces pushing upwards and down equilbriate at that particular flow. the user can then read off the calibrated scale the flow rate that corresponds to this position.

if the needle valve is opened further the increase in flow has more force to push the bobbin higher, until a new equilbrium is met.

Question 2

how accurate are rotameters?

Answer 2

+/- 2.5%

Question 3

How are rotameters calibrated?

Answer 3

calibration is specific to each gas this is because the visocity and density of each gas will vary.

At the bottom of the tube, the gap between bobbin and the tube is narrow - this promotes laminar flow. therefore flow is inversely proportional to viscosity and the tube will be calibrated in this way.

At the top of the tube, the gap gets wider and turbulent flow exists whereby flow is inversley propotional to root density so it is calibrated in this way.

the scale on the tube is therefore non-linear and gas specific