Why is backpressure needed when measuring dissolved gases with Orbisphere sensor?

Document ID

Document ID TE2317

Published Date

Published Date 12/03/2017
Question
Why is backpressure needed when measuring dissolved gases with Orbisphere sensor?
Summary
Orbisphere and back pressure
Answer
This information relates to recommended sample condition to measure dissolved gases. To measure a dissolved gas accurately with an Orbisphere analyzer, all gases in the liquid must be dissolved in solution. Any degassing causes error in the measurement to increase. Therefore a minimum backpressure, sometimes referred to as hydrostatic pressure, on a liquid sample is required. The minimum sample backpressure must be higher than the sum total of the partial pressure of each gas dissolved. For this reason the amount of sample backpressure required can vary from application to application.
The principle of the Orbisphere gas sensor is the measure of gas partial pressure. The hydrostatic pressure on the liquid does not influence gases dissolved in the liquid and therefore does not influence the partial pressure of the gases dissolved. When measuring dissolved gases in samples with dissolved gas pressure lower than atmospheric pressure, it is recommended to always have slight backpressure on liquid samples. The slight sample backpressure insures a consistent sample with no liquid flow problems. This slight amount of sample backpressure can be as little as 1 psig.
It is important to note that the sample backpressure should be relatively constant as rapid changes in hydrostatic pressure can cause noise in the dissolved gas measurement signal.