- 1 percent
- 2 percent
- 5 percent
- 10 percent
- None of the above

The 10 inch error at full scale
flow is 10 percent of the full scale differential pressure. This error represents an approximate 5
percent flow error at the full scale flow rate.
However, the process pressure will compress the bubble that is initially
present at startup so Answer C and Answer D are not correct.

At operating pressure, the
non-condensable gas will be compressed by a factor of approximately [(4 bar +1
bar)/1 bar] or 5. Therefore, the height
of the bubble will be approximately 10/5 inches, or 2 inches. A 2 inch error is 2 percent of the full scale
differential pressure and represents approximately 1 percent flow error at the
full scale flow rate. Therefore, Answer
B is not correct.

The approximate 2 inch water column error associated
with the bubble is independent of the operating flow rate. By way of example, the error associated with
the bubble is 2 inches of water column when the flow is the 10 percent or 100
percent of full scale flow. At 100
percent of full scale flow, the flow error is approximately 1 percent. However, at 10 percent of flow, the flow error
attributable to the bubble (2 inches of water column) is larger than the
differential pressure measurement (1 inch of water column). In other words, the flow measurement error
varies with the operating flow rate so Answer A is not correct. Answer E is correct (except at 100 percent of
full scale flow).

Additional Complicating Factors

The above analysis presumes that the bubble
height is a function of starting up the impulse tubing from an empty
condition. In operation, non-condensable
gas can be bled from the system and additional non-condensable gas can
accumulate in the bubble. Both of these
mechanisms can present additional measurement error.

This article originally appeared in Flow Control magazine.