At high altitudes, what is the behavior of a rotameter flow for gas through the device?

At high altitudes, what is the behavior of a rotameter flow for gas through the device?

• A. Greater than expected
• B. Less than expected
• C. Greater than expected at high flows but less at low flows
• D. Greater than expected at high flows but accurate at low flows

Answer: (D)

Rotameters measure flow by balancing upward drag against the downward effective weight of a float. The device is calibrated for standard density conditions, so the relationship between float height and actual flow assumes those ambient conditions. At high altitude the ambient gas is less dense. That reduces buoyancy on the float (since buoyancy depends on the density of the surrounding gas) and also reduces the drag produced by a given flow. To raise the float to a particular height (the reading on the scale) under these lower-density conditions, you must push a higher actual flow than at sea level. Because the scale is calibrated for denser gas, that higher real flow shows up as more flow than the reading would suggest, and this discrepancy grows as you go toward higher flow settings. Near the low end of the scale, the difference is small, so readings are fairly accurate. So, at high altitude, the rotameter tends to deliver more gas than the dial indicates at high flow settings, while remaining fairly accurate at low flow settings.

Rotameters measure flow by balancing upward drag against the downward effective weight of a float. The device is calibrated for standard density conditions, so the relationship between float height and actual flow assumes those ambient conditions.

At high altitude the ambient gas is less dense. That reduces buoyancy on the float (since buoyancy depends on the density of the surrounding gas) and also reduces the drag produced by a given flow. To raise the float to a particular height (the reading on the scale) under these lower-density conditions, you must push a higher actual flow than at sea level. Because the scale is calibrated for denser gas, that higher real flow shows up as more flow than the reading would suggest, and this discrepancy grows as you go toward higher flow settings. Near the low end of the scale, the difference is small, so readings are fairly accurate.

So, at high altitude, the rotameter tends to deliver more gas than the dial indicates at high flow settings, while remaining fairly accurate at low flow settings.