An interesting variation on this theme of direct hydrostatic pressure measurement is the use of a purge gas to measure hydrostatic pressure in a liquid-containing vessel. This eliminates the need for direct contact of the process liquid against the pressure-sensing element, which can be advantageous if the process liquid is corrosive.
Such systems are often called bubble tube or dip tube systems, the former name being appropriately descriptive for the way purge gas bubbles out the end of the tube as it is submerged in process liquid. A very simple bubbler system may be simulated by gently blowing air through a straw into a glass of water, maintaining a steady rate of bubbles exiting the straw while changing the depth of the straw’s end in the water:
The deeper you submerge the straw, the harder it becomes to blow bubbles out the end with your breath. The hydrostatic pressure of the water at the straw’s tip becomes translated into air pressure in your mouth as you blow, since the air pressure must just exceed the water’s pressure in order to escape out the end of the straw. So long as the flow rate of air is modest (no more than a few bubbles per second), the air pressure will be very nearly equal to the water pressure, allowing measurement of water pressure (and therefore water depth) at any point along the length of the air tube.
If we lengthen the straw and measure pressure at all points throughout its length, it will be the same as the pressure at the submerged tip of the straw (assuming negligible friction between the moving air molecules and the straw’s interior walls):
This is how industrial “bubbler” level measurement systems work: a purge gas is slowly introduced into a “dip tube” submerged in the process liquid, so that no more than a few bubbles per second of gas emerge from the tube’s end. Gas pressure inside all points of the tubing system will (very nearly) equal the hydrostatic pressure of the liquid at the tube’s submerged end. Any pressure-measuring device tapped anywhere along the length of this tubing system will sense this pressure and be able to infer the depth of the liquid in the process vessel without having to directly contact the process liquid.
A key detail of a bubble tube system is to provide a means of limiting gas flow through the tube, so the purge gas backpressure properly reflects hydrostatic pressure at the end of the tube with no additional pressure due to frictional losses of purge flow through the length of the tube. Most bubble tube systems, therefore, are provided with some means of monitoring purge gas flow, typically with a rotameter or with a sightfeed bubbler :
If the purge gas flow is not too great, gas pressure measured anywhere in the tube system downstream of the needle valve will be equal to the hydrostatic pressure of the process liquid at the bottom of the tube where the gas escapes. In other words, the purge gas acts to transmit the liquid’s hydrostatic pressure to some remote point where a pressure-sensing instrument is located.
As with all purged systems, certain criteria must be met for successful operation. Listed here are some of them:
• The purge gas supply must be reliable: if the flow stops for any reason, the level measurement will cease to be accurate, and the dip tube may even plug with debris!
• The purge gas supply pressure must exceed the hydrostatic pressure at all times, or else the level measurement range will fall below the actual liquid level.
• The purge gas flow must be maintained at a low rate, to avoid pressure drop errors (i.e. excess pressure measured due to friction of the purge gas through the tube).
• The purge gas must not adversely react with the process.
• The purge gas must not contaminate the process.
• The purge gas must be reasonable in cost, since it will be continuously consumed over time.
One measurement artifact of a bubble tube system is a slight variation in pressure each time a new bubble breaks away from the end of the tube. The amount of pressure variation is approximately equal to the hydrostatic pressure of process fluid at a height equal to the diameter of the bubble, which in turn will be approximately equal to the diameter of the bubble tube. For example, a 1/4 inch diameter dip tube will experience pressure oscillations with a peak-to-peak amplitude of approximately 1/4 inch elevation of process liquid. The frequency of this pressure oscillation, of course, will be equal to the rate at which individual bubbles escape out the end of the dip tube. Usually, this is a small variation when considered in the context of the measured liquid height in the vessel. A pressure oscillation of approximately 1/4 inch compared to a measurement range of 0 to 10 feet, for example, is only about 0.2% of span. Modern pressure transmitters have the ability to “filter” or “damp” pressure variations over time, which is a useful feature for minimizing the effect such a pressure variation will have on system performance.