- 0.60 percent
- 1.25 percent
- 2.50 percent
- 12.5 percent

For the purposes of this problem, let's assume that bar is a gauge pressure unit. In addition, one atmosphere is 1.01325 bar absolute. However, 1 bar absolute will be used in this problem to simplify the calculations. In addition, one atmosphere of pressure corresponds to approximately 10 meters of water column.

The pressure associated with the seal is approximately 1 bar. Therefore, the effect on the pressure measurement is approximately 1.25 percent (1 / 80). However, the effect on absolute pressure is important for pressure compensation. The effect on the absolute pressure measurement is approximately 1.23 percent (1 / 81). It should be evident that the steam pressure is sufficiently high such that the effect of the errors on the gauge and absolute pressures are similar. Answer B is correct.

Of particular interest is the magnitude of the error. Not accounting for the pressure head of a 10 meter high seal does not seem like a big problem when the pressure is 80 bar. However, its affect on the pressure measurement is over 1 percent of the measured pressure. Further, its affect on a differential pressure flowmeter will be approximately 0.6 percent. This error can cause the annual cost of the steam to be biased by over $100,000 in large steam headers.

Additional Complicating Factors

It is common for steam pressure to be measured in headers that are (say) 40 meters above grade. Many steam plants are designed such that the transmitter is located near the control room (say) 10 meters above grade. In such installations, the pressure measurement error will be approximately three times larger than calculated above.

This article originally appeared in P. I. Process Instrumentation magazine.