HyQuest Solutions HS40C, Руководство пользователя, Страница: 28 / 37

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Appendices
It is the “rising” or “increasing head” situation that is critical for the correct operation system. Consider a situation
where the gas supply is cut off and the level rises- refer
Fig. With gas supply cut off. Head increased from H1 to H2, gas
is forced down the tube by an amount X .
Figure 5 – With gas supply cut off. Head increased from H1 to H2, gas is forced down the tube by an amount X.
In this situation, it can be seen that whilst the system pressure has increased, it is not equal to the new head H2, but
rather H2-x. This condition will remain as long as the gas is shut off. In other words, we now have a “closed” system,
which respond to changes in accordance with universal gas law.
To revert back to the open system, we must restore gas supply, which will eventually build up pressure in the system,
until the gas escape from the orifice and the system pressure again becomes the precise head measurement.
The time taken for the pressure system to respond to a given head pressure is commonly referred to as “follow rate”.
The normal procedure is to establish the maximum rate of rise to be measured and adjust the gas flow so that there is
no significant lag in the pressure system.
However, in order to conserve the gas supply, the flow rate (bubble rate) is set to the minimum that will meet these
conditions.
“Falling“ Situation
The “falling” situation is of course the reverse of the rising situation except that there is now excess pressure in the
system. If we were to suddenly lift up the orifice in Fig. 4, the pressure would drop to the new head, seen by the
sudden rush of bubbles.
Accordingly, even the lowest bubble rate will suffice for falling conditions.
In practice, it may take some time to the pressure to “bleed away”, but this is usually less than the response time of the
sensing unit.
Field Requirement of Gas Purge Systems
The principle requirements of gas purge are :
I. A fine metering valve and sight feed for accurately setting the bubble rate.
II. Constant regulator to maintain a constant bubble rate with changes of head.
III. Pressure connections and valves for connecting the tubing to the river and the instrument.
With the overall provision that the unit must be “leak proof”, otherwise the true head will not be recorded.
The configuration most widely used is shown in Fig 6.
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