Water Treatment System
One possible configuration for a water treatment system is shown in the diagram below. This sample system includes at least one of the options from Sections 1-4 below.
Tap water can often contain significant amounts of dissolved ionic material. The most obvious side effect of this is an increase in the water conductivity, which can result in a "short circuit" path between electrodes of some co-planar membrane hydrophones. The increased conductivity can provide a conduction path for Electro-Magnetic Interference, which will increase the noise received by the hydrophone. There is also a likelihood that these dissolved solids will crystallize out onto any objects left within the water. This is a particular problem in "hard-water" areas where chalky deposits can build up on anything left within the tank. If hydrophones are subject to such deposits then there will be a gradual reduction in the sensitivity of the hydrophone as the thickness of the contamination increases.
The forthcoming IEC62127-part 1, detailing the use of ultrasonic hydrophones, recommends a conductivity of no more than 5 mS for measurements involving electrically unshielded membrane hydrophones. The same figure is also recommended by IEC61102, and IEC61161 also recommends the use of distilled water for the same reason.
Ion Exchange Resins
These resins contain a mixture of anionic and cationic reagents that replace all dissolved ionic content with their respective hydrogen or hydroxide ions leaving pure water. The initial equipment costs for an Ion Exchange system are relatively low, but the exchange resin is a consumable item that must be replenished. This process is suitable for infrequent use as running costs (due to resin replacement) can be expensive if larger quantities of water are required.
By boiling, and then separately re-condensing water, any dissolved solids are left on the inner surfaces of the still and the emerging water has a lower ionic content. Whilst the capital cost of a still is small, distilled water is not as effective as de-ionisation and water must often be double or triple distilled before it approaches the purity of an ion exchange resin. This process is best suited to production of larger volumes of water when poorer levels of deâ€‘ionisation are acceptable.
Reverse osmosis uses a membrane that is semi-permeable, allowing the fluid that is being purified to pass through it, while rejecting other ions and contaminants from passing. Reverse osmosis systems are often quite expensive, but have relatively low running costs. This process is best suited for applications that demand high large volumes of very well de-ionised water and can tolerate the high initial equipment costs.
If calcium carbonate (chalk) deposits have built up on the surface of a hydrophone, then it may be possible to gently remove them by soaking the affected part of the hydrophone in lemon juice. This works because the citric acid in the lemon juice reacts with the alkaline calcium carbonate to remove it - but the solution is sufficiently weak acid that it will not damage other components of the hydrophone. Leave the hydrophone in lemon juice for no longer than 30 minutes and then rinse thoroughly with de-ionised water before allowing to air dry.
Under no circumstances should the hydrophone be rubbed with a cloth or paper towel, as this is likely to abrade the gold electrode and damage it.
Tap water is often super-saturated with dissolved gases (primarily oxygen). Bubbles can be a cause of major experimental problems since they act as near perfect reflectors of ultrasound. This can perturb the ultrasound field being measured. Worse still, if a bubble forms directly in front of the active element of the hydrophone it will prevent any propagating ultrasound from being measured by that hydrophone. Finally, in high-pressure fields bubbles are able to cavitate. This will result in the production of spurious acoustic signal at both lower frequencies (stable cavitation) and higher frequencies (inertial cavitation). Particular care should be taken to avoid inertial cavitation as the bubble collapse is a particularly destructive event. If such a collapse happens on the surface of a hydrophone, damage to the hydrophone may well occur. It is useful to note that macroscopic bubbles are visible to the naked eye. However, microscopic bubbles may be much harder to visually detect, but can be just as much of a problem.
Many international standards require the use of degassed water including IEC61161, IEC61689, IEC61102, IEC61220, IEC60854 and the forthcoming IEC62127-part 1. This topic is sufficiently important to warrant a dedicated annex in IEC61161 Ed 2.0 discussing methods of water degassing.
There are many possible methods of degassing water, but the reader is referred to IEC61161 Ed2.0 Annex D or Fowlkes and Carson (1991) for a more rigorous review of available methods. It is also important to note that re-absorption of gas occurs at all exposed water surfaces therefore water surfaces should be covered where possible to prevent reabsorption. A brief review of some degassing methods follows.
When a vacuum is applied to a standing body of water, the reduced pressure will prevent dissolved gases from remaining in solution. Under these conditions the water will appear to boil as the gas bubbles rapidly expand and then break at the water surface. When subject to hard vacuums, levels of dissolved oxygen can be reduced below the 1 mg/litre level. This method of degassing is suited to small volumes of water (such as would be needed for a power balance), but can be difficult to implement for larger volumes (such as would be needed in a scanned hydrophone measurement tank).
Reduced Pressure Re-circulation
This can be considered as being a derivative of vacuum degassing, but provides an inexpensive means of degassing on a much larger scale. The system comprises a high volume pump connected to a standing body of water by rigid walled tubing. A pressure restrictor is fitted to the inlet tube, such that the pump is attempting to draw water through the tube faster than water is allowed in. This creates a partial vacuum within the rigid walled tube and any dissolved gas bubbles increase in size, and eventually nucleate to form larger bubbles. These larger bubbles (due to their lower surface area to volume ratio) are less able to dissolve back into the water, and thus remain as bubbles as they pass through the pump into the outlet stream. The outlet stream will then contain larger (non-dissolved) bubbles which then float to the water's surface and escape to the atmosphere. It is important to note that both inlet and outlet points should be as low as possible in the water tank to prevent the turbulent flow nearby promoting gas re-absorption. The effectiveness of this method depends upon the pressure drop that can be achieved within the inlet hose, but with appropriate configuration oxygen levels of 2-3 mg/litre can be achieved.
This is a simple but effective way of degassing water. Boiling periods as short as 5 minutes are sufficient to yield acceptable levels of degassing (oxygen levels below 2 mg/litre).The water should NOT be stirred whilst cooling as this will promote re-absorption of gases.
Addition of Sodium Sulphite
Sodium sulphite works by reacting with the dissolved oxygen in the water. Concentrations as low as 4 g/litre will keep oxygen levels below 2 mg/litre for at least 40 hours. However this technique introduces significant ionic content and the conductivity will increase to about 5 mS/cm - which is in contradiction to the efforts made in section 1. Furthermore sodium sulphite solution is alkaline and will result in corrosion on many metals including Aluminium and Nickel.
Prevention of Biological Activity/Growth
Whilst not explicitly mentioned in ultrasonic measurement standards, prevention of biological growth is an important issue. The most obvious side effect of biological activity within a water tank is that the water tends to develop a cloudy yellow/green appearance, and can start to smell. A slimy film tends to accumulate on the surfaces of any items that are exposed for long periods. These deposits will serve to compromise the performance of a hydrophone in the same manner as calcification (see Section 1). Most importantly however, is that bacterial growth within the water can be a health risk to users of the measurement tank.
Removal of dissolved solutes and gases (see Sections 1 and 2) will serve to slow down biological growth by reducing the available "food sources". However, these methods will not kill off any bacteria already present and a selection of methods is listed below. Please also note that either of the chemical methods will re-introduce ionic content into the water and will thereby compromise the deionisation discussed in Section 1.
Exposure to high intensity ultraviolet is a very effective means of destroying water-borne bacteria. There are numerous commercially available UV filters that can be fitted in-line with a pre-existing re-circulatory water conditioning system.
Chlorine Based Chemicals
Chlorine based chemicals are commonly used to control biological activity in Swimming Pools. These chemicals are sometimes used as antibacterial agents in measurement tanks, although elevated concentrations of chlorine solutions can cause bleaching of fabrics. Furthermore prolonged skin exposure may lead to skin sensitisation and irritation for users of the tank.
Copper Based Chemicals
Copper based compounds have been used extensively in the marine industry as anti-fouling agents and there are some copper based solutions that are designed for addition to Swimming Pools. Care should be exercised though since copper can react with other metals (particularly Aluminium). It has also been know to react with the thin gold electrodes that are commonly found on hydrophones and transducers, thereby reducing the sensitivity of the devices.
Removal of suspended particulates
The presence of suspended particulate is also not mentioned explicitly in ultrasonic measurement standards. However, increased particulate load in the water can result in an increase of scattering at higher frequencies and thereby an increased attenuation. The commonest source of suspended matter is airborne particulates (dust) that fall into the tank. Suspended particles are also often introduced as transducers and hydrophones are moved into and out of the measurement tank. Since many of these particulates are organic, they are a further "food source" for biological activity. Consequently, removal of suspended particulate matter inhibits biological growth, as well as improving the optical clarity of the water.
A re-circulation system that incorporates particulate filters will significantly reduce the suspended load. A common approach is to employ a two-stage filtration system: a coarse (e.g. 5 mm) filter will remove the majority of particulates, whilst a subsequent fine (e.g. less than 1 mm) filter will remove any residual suspension.
Covering the surface of the water.
Thin plastic film, or hollow plastic balls (e.g. polypropylene) provide an effective means of stopping airborne debris falling into the tank. These methods also serve to reduce the surface area exposed to the air and thus reduce gas re-absorption.
Fowlkes J.B. and Carson P.L. (1991), Systems for degassing water used in ultrasonic measurements, J. Acoust. Soc. Am, 90(2) pp. 1197-1200
IEC60854 (1986), International Report- Methods of measuring the performance of ultrasonic pulseâ€‘echo diagnostic equipment, IEC, Geneva
IEC61102 (1991), Measurement and characterization of ultrasonic fields using hydrophones in the frequency range 0,5 MHz to 15 MHz, IEC, Geneva
IEC61161 Ed 2.0 (2007) - Ultrasonics - Power measurement - Radiation force balances and performance requirements up to 1 W in the frequency range 0,5 MHz to 25 MHz and up to 20 W in the frequency range 0,75 MHz to 5 MHz, IEC, Geneva
IEC61220 (1995) - Ultrasonics - Fields - Guidance for the measurement and characterisation of ultrasonic fields generated be medical ultrasonic equipment using hydrophones in the frequency range 0,5 MHz to 15 MHz, IEC, Geneva
IEC61689 (1996) - Ultrasonics - Physiotherapy Systems - Performance requirements and method of measurement in the frequency range 0,5 MHz to 5 MHz, IEC, Geneva
IEC62127 (forthcoming) - Ultrasonics - Hydrophones - Part 1: Measurement and characterization of medical ultrasonic fields up to 40MHz using hydrophone, IEC, Geneva