Saltwater Intrusion Succumbs to the Pressure of Advanced Monitoring Techniques

As seen in Waterworld, June 2008

Freshwater has become an increasingly important resource throughout many areas of the world due to factors including drastic climate changes as well as population growth.  The largest available source of freshwater lies underground in aquifers and it is critical to sustain this supply by observing and addressing those forces acting negatively upon this resource with the use of advanced monitoring techniques.

Freshwater aquifers are susceptible to the impacts of saltwater intrusion along the coasts of large bodies of water.  The phenomenon of saltwater intrusion occurs when the saltwater, with a denser composition, intrudes into the freshwater portions of the aquifer.  This process occurs when the freshwater level or head declines from either climatic or man-made (i.e., pumpage) stresses, having a negative impact on the natural balance between fresh and saltwater.  To effectively manage coastal groundwater resources, it is critical to have a quantitative understanding of the occurrence of the freshwater/saltwater interface as well as those factors that influence the process of saltwater intrusion.

Specifically in Southwest Florida, saltwater intrusion has been an ongoing issue among the southern west-central Florida ground-water basin (SWCFGWB), which serves as a public water source for domestic, utilities, recreational, industrial and agricultural applications.

In the early 1900s, the Upper Floridian aquifer in the SWCFGWB discharged approximately 530,000 cubic meters per day of freshwater from the aquifer into the coastline. The decline of groundwater levels within the aquifer has resulted in saltwater intrusion.  With goals to minimize or even reverse the impact of saltwater intrusion, water managers must monitor the aquifers closely. 

Originating in 1974, the Southwest Florida Water Management District (SWFWMD) undertook a comprehensive assessment of the risk saltwater intrusion posed to the Upper Floridian aquifer water supply. The main goals of this extensive program were to explore and test existing conditions as well as to construct dedicated monitor wells for future monitoring of water levels and water quality. As analysis techniques become more sophisticated through computer modeling, there is an even greater demand for advanced monitoring.

Today, using some of the latest technologically advanced level and pressure measurement instrumentation, the SWFWMD is expanding and improving this hydrogeological exploration program with the use of dedicated monitoring wells,
aquifer performance tests and a water-quality sampling network.

The SWFWMD has constructed coastal monitor well transects consisting of two to three monitor well sites that serve as monitoring and testing sites.  This coastal monitoring program provides the opportunity to observe the movement of saltwater either landward or seaward.  The data collected from these sites is used in the various water resource studies conducted to better manage the sensitive coastal aquifers of the SWCFGWB.

The first phase of this investigation includes discrete zone testing within a borehole with the use of a packer assembly.  Water levels above the packer within the annulus are monitored to ensure the packer was properly sealed to the formation.  Any water level fluctuation within the annulus would be very small, requiring precise water level measurements. 

To achieve exact measurements, the SWFWMD called upon Pressure System’s KPSI™ Series 335 Submersible Level Transducer for its high level of accuracy in environmental test applications.  This small-bore transducer is ideal for monitoring this annulus and many other water level monitoring applications requiring high precision static accuracy.

The SWFWMD also chose the Series 335 because it offers a super high precision static accuracy of +0.05% FSO, is 100% computer tested and calibrated, and is fully temperature compensated and data logger compatible.  In addition, this transducer features high performance internal signal conditioning and is available with standard voltage and current loop outputs. 

During the packer test, the SWFWMD conducts a program of four or more slug tests consisting of a near-instantaneous stress or a slug being added (or removed) from the well.  With a very permeable formation, these low magnitude slug tests result in a rapid water level response. 

This program of slug tests is repeated 50 to 100 feet as the borehole is advanced to depths of up to 3,000 feet below the land’s surface. In order to monitor the rapidly changing water levels during these tests, the SWFWMD again turned to Pressure System’s transducer for its high-speed data acquisition. 

The SWFWMD uses a different Pressure System KPSI™ transducer, the Series 735 Submersible Pressure Transducer, along with a ¼-inch nominal pipe thread, to measure air pressure within the pneumatic slug testing head to record the slug magnitude of the test.  The submersible pressure transducer was used to accurately measure the water level displacements that occurred within the test interval.  Constructed in a small, rugged package, this pressure transducer is ideal for accurately recording pressure measurements within a close proximity, which is exactly what the SWFWMD needed.

The SWFWMD also relies on this transducer for its ability to read pressure ranges from 0-2 psig through 0-100 psig as well as its advanced on-board electronics that provide outputs of 4-20 mA.  Provided in a small rugged package constructed of corrosion-resistant 316 stainless steel, the Series 735 offers precise level measurement in hostile environments.

Approximately 30 years of exploration combined with highly developed monitoring technologies implemented by environmental professionals have helped the SWFWMD to collect accurate and reliable data for groundwater resource studies involving saltwater intrusion related issues. These advancements in monitoring techniques, in turn, have enabled the SWFWMD to be one step closer to achieving its mission of ensuring continued availability of this natural resource. 

REFERENCES:
Butler, J.J., Jr., E.J. Garnett, and J.M. Healey, Analysis of slug tests in formations of high hydraulic conductivity, Ground Water, v. 41,
                                                                                                                                           no. 5, pp. 620-630, 2003.

Enhanced Accuracy of Ocean Level Data Available with Hydrostatic Submersible Transducers

Ocean level monitoring has become increasingly important among scientific areas of study due to factors including global warming as well as seasonal changes that result in extensive flooding and damage. For physical oceanographers, ocean level data is imperative to their research and therefore, they are in search of the most accurate, quickest, cost-effective data acquisition equipment in the field.

Over the years, water level monitoring equipment has evolved from a mechanical means of measurement involving buoys and manual data collection, where human error was a significant factor, to electronic techniques involving radar and acoustic sensors, as well as level transducers that automatically capture data and transmit it to a host PC.

Finding The Best Solution

When AmberJackSolutions, a company specializing in custom data acquisition and telemetry solutions, wanted to expand the capabilities of its B600 versatile tide gauge, the company looked at different aspects of the system’s water, atmospheric, wind speed, wind direction and solar radiation monitoring functions. 

AmberJackSolutions focused specifically on the B600’s water level sensors and compared the radar, acoustic and pressure technologies typically used to capture water level data in oceanic applications. The company found that the pressure sensor technology used in hydrostatic transducers not only provided data in a shorter amount of time, but the information was also more accurate, as compared to the other alternatives.

Greater Accuracy with Pressure Systems

AmberJackSolutions invested in Pressure Systems’ KPSI™ 735T hydrostatic submersible level transducer for use in its tide gauge system because of its exceptional ability to deliver precise ocean level measurements.  The 735T transducer offers a static accuracy of 0.05% FS and analog outputs of 4-20 mA as well as 0-5 VDC or mV, ideal for rigorous settings encountered in liquid level measurement and control. 

“The 735T provides greater accuracy, including much smoother curves, as compared to the results we received from other data collection instrumentation, giving us a more realistic picture of the actual sea levels being measured, “ said David Mendes, hardware engineer with AmberJackSolutions.

The 735T features titanium construction, which enables the transducer to hold up exceptionally well under prolonged exposure to caustic environments, such as salt water. Each transducer also includes a SuperDry Vent Filter that utilizes a unique water block feature that self-seals in case the electrical cable is cut, as well as prevents moisture from entering the vent tube for at least one year. This double protection against moisture is crucial, considering the units are submerged in ocean waters.

“The 735T requires underwater work to install the transducer, but the extra work is well worth the effort when the results of the level transducer are compared to radar and acoustic sensors.  Our tide gauge systems collect data used to make informed decisions about sea levels and, in conjunction with seismic activity, to detect tsunamis.  The installation costs are offset by the validity and timeliness of the information generated by the transducers and our customers’ subsequent ability to act on their data,” mentioned Mendes.

Success with Hydrostatic Transducers

With the use of Pressure System’s 735T level transducer, data can now be conveniently transmitted through GPRS protocol, saved on an SQL database in real time and downloaded for graphical viewing. Website:(http://oceano.horta.uac.pt/azodc/tidegauge.php)

The B600 tide gauge from AmberJackSolutions has been installed on several islands in the Portuguese Azores and has proven to be stable, self-sufficient and cost effective. The success of the B600 is due in part to the superior 735T transducer, which provides accurate ocean level measurements in a timely manner.

AmberJackSolutions Website: http://amberjacksolutions.com
AmberJackSolutions E-mail: info@amberjacksolutions.com

Transducer Cleaning Schedule

If no historic data exists on which to base a maintenance schedule then a baseline must be established. After an evaluation of the conditions at each site has occurred a preliminary schedule can be developed. This schedule should be re-evaluated periodically to ensure that it is still valid. The following steps can be used to begin the initial evaluation.

Visit each site and determine the physical and electrical condition of each transducer. Document that information for future analysis.
Among the conditions that should be noted are:

1. The appearance of:
   1. The cabling - damaged or fouled
   2. The transducer housing - fouled and extent and type of that fouling
2. The performance of the transducer
   
   1. Lift the unit from the liquid and note the output on its indicator or using a handheld DVM.
   2. Return the unit to its installed position and note the output and its accuracy (i.e. measure the fluid level and compare it to the output of the transducer note any deviation and its magnitude)
3. Any other parameter that may be appropriate such as the excitation voltage and an indication that the units performance is:
   1. acceptable
   2. marginal
   3. unacceptable
      * Be sure that the inspector notes why the device falls into the evaluation categor

Visiting each site on a regular basis for the purpose of this examination will provide trend information on which a more appropriate schedule can be based. The initial evaluation interval should be frequent until sufficient data has been accumulated. Although it may be onerous, an examination interval of 1 month or less for a period of 6 months would be a good starting point. After this initial evaluation has occurred the data should be analyzed to determine:

1. The rate of fouling
2. The rate of change in performance of the measurement parameters in the log sheets.
3. Any degradation in performance indicated by the technician.
   
   * Degraded performance can point to the cause of failure allowing the user to be proactive and avoid system downtime.

At this point a regular cleaning cycle can be scheduled based on the observed rate of fouling and problematic sites can be targeted for remedial action. The evaluation procedures should be maintained although their period can be extended or reduced to maintain an accurate picture of the system as a whole.

Sealed-Vented-Absolute: How To Know Which Transducer to Specify

Challenge: "A job to design a liquid level measurement system has just come in. The client requires a submersible hydrostatic pressure transducer in the system, but these transducers can be built in three different pressure formats-psig (vented gage); psis (sealed gage); or psia (absolute). Which format is best for this job? The most important factor in deciding which transducer to use is to make sure the difference between the three pressure formats is understood."

The Differences Between Pressure Transducers
By far, the most common format used in making a hydrostatic level measurement is vented gage. A vented gage pressure transducer is constructed so that the reference side of the actual pressure sensor in the transducer is open to the atmosphere. This is accomplished via a vent tube integral to the transducer cable. When the cable enters the body of the transducer, the tube is connected to a nipple that enters the reference side of the internal pressure sensor. In this configuration, the sensor reference and the atmospheric pressure acting on the surface of the liquid being measured is the exact same pressure. Unlike vented gage, sealed gage and absolute pressure transducers are identical in their physical construction. They do not have a vent into the reference cavity of the sensor. The sensor is sealed with a vacuum on the reference side. The absolute pressure transducer's zero output reading is relative to the vacuum. A sealed gage transducer has its zero output reading electronically elevated to simulate a reference to one nominal atmosphere. This is fine in many applications where the effects of daily fluctuations in the actual atmospheric pressure will not induce unacceptable errors in the level measurement being made.

Determining Which Transducer to Use
Now that the differences in construction between the different pressure formats have been identified, it is time to consider other factors to help determine which will be right for the job. An absolute pressure transducer is best suited for jobs that involve a system or tank that is under pressure and not open to the atmosphere. If it were to be used to measure a liquid level where the liquid is exposed to atmospheric pressure, a secondary barometric pressure measurement would have to be made so that it could be subtracted from the absolute measurement, leaving only the pressure exerted on the transducer by the liquid being measured.

Vented gage is the most common format used for a hydrostatic level measurement because of the open reference side of the sensor. Since atmospheric pressure is also acting on the surface of the liquid being measured, the effects of changes in atmospheric pressure on the measurement are negated. The end result is that the measurement is the most accurate possible reflection of the level of liquid above the pressure transducer. The one drawback to vented gage, however, is that the vent tube provides a possible path for moisture from the atmosphere (i.e. humidity or rain) to enter the pressure transducer. The accurate performance of a vented gage transducer depends upon keeping moisture out of the vent tube since the weight of the accumulated liquid will cause level readings that are lower than the actual liquid level. It is therefore necessary to use a moisture protection device to prevent moisture-laden air from entering the transducer.

Extending the Life of the Transducer
The most common method for preventing moisture incursion is the attachment of a desiccant-filled cartridge to the vent tube at the cable's electrical termination end. This will allow air to pass through the desiccant, which absorbs the moisture in the air, as the barometric pressure changes. Some manufacturers use an indicating desiccant that changes colors as it becomes saturated so that cartridge replacement is apparent upon inspection.

Another method to prevent moisture from entering the transducer is to attach an aneroid bellows to the vent tube. The bellows, a closed system that prevents moist air from entering the vent tube, expands and contracts with changes in the barometric pressure, thereby equalizing the pressure in the vent tube with the current barometric pressure. Either the bellows or the desiccant cartridge can be mounted in a junction box or panel near the electrical termination of the transducer cable.

The longevity of the sensor also depends upon keeping moisture out of the vent tube. Gold bonding wires internal to the sensor are only a few mils in diameter; direct exposure to water will cause their rapid corrosion and result in failure of the pressure transducer.

A couple of in-the-field remedies can be attempted if moisture does get into the vent tube and pressure transducer. The transducer cable can be coiled and the cable and transducer placed in a pan. The pan should be placed in an oven at 50°C for two hours to dry the transducer and its components. It is important that the temperature does not exceed 50°C or damage to the transducer and cable may occur. Alternatively, the cable and transducer can be suspended in a vertical position (transducer end up) overnight to allow the water to drain out. There may be cases where it is impractical to use either the desiccant cartridge or the aneroid bellows. The job may be in a remote location where it is not possible to periodically check the desiccant and see if it needs replacement or there may not be a suitable location to mount the aneroid bellows. Also, the level to be measured may be so deep that the effects of daily atmospheric pressure changes will have a negligible effect on the measurement. In instances such as these, a sealed gage pressure transducer is the best option.

Additional Considerations
Since a sealed gage transducer is electronically set to simulate the effects of one nominal atmosphere, it is important to take into account the elevation where the transducer will ultimately be installed. Normally, sealed gage transducers are calibrated with the zero output compensated for one nominal atmosphere at sea level.However, if the installation is to be made in a remote mountainous area or in a town that is well above sea level, it is imperative that the manufacturer of the transducer is aware of the elevation at the installation site. If the transducer were constructed without taking these factors into consideration, there would be considerable error induced in the level measurement. Using the elevation information supplied by the customer, the manufacturer calculates the nominal atmospheric pressure at the installation location and sets the zero output for that pressure.

Conclusion
In summary, the most accurate pressure format to specify for any open-atmosphere system is vented gage. Whether used in a lake, river, reservoir, well, or sewage lift station, a vented gage transducer will negate the fluctuations in atmospheric pressure and ensure the most precise indication possible of the actual level of the liquid being measured. If it is not practical from a maintenance point of view to use a vented gage, then sealed gage is the next best option. Absolute transducers are best suited for closed systems that are under pressure or vacuum and not subject to the effects of atmospheric pressure.

Now that the three pressure formats have been defined, it should be clear which is appropriate for the liquid level measurement system that is being designed. Selecting the proper format for the application will ensure that the client receives a transducer from the manufacturer that will provide years of uninterrupted, stable operation and the accurate level measurement required.

Ship Draft

Challenge: Seawater-compatible level transmitters for thru-hull mounting in ocean-going vessels.

Detail: Transmitters must have materials that will withstand long-term exposure to seawater combined with high accuracy and ability to withstand severe pressure spikes as a result of the pitching of the vessel in rough seas. Units must retrofit into existing receptacles to minimize downtime.

Solution: A special version of the Series 730 provides better than 0.1% static accuracy with titanium outer housing and diaphragm. A high range sensor is deranged to provide the overpressure capability. Field trials prove the design to be successful.

Roadside Drainage Ditches

Challenge: Monitor water depth in roadside drainage ditches to warn of imminent "wash-overs".

Details: A municipality that experiences frequent roadway "wash-overs" during periods of heavy rain requires a submersible transmitter to monitor depth of water in roadside drainage ditches. The transmitters must interface with telemetry that relays data to a monitoring station.

Solution: KPSI's Series 730 with optional piezometer nose cap allows for burial in the roadside ditch (with sand and gravel fill above transmitter). Reliable signal conditioning circuitry provides a 4-20 mA signal proportional to the water level above the transmitter. The piezometer nose cap prevents solids from clogging the pressure port of the transducer. The all-welded 316 SS housing completes the package.

Sealed-Gage Transducer Configured for Altitude Above Sea Level

Challenge: Monitor water depth in a remote location in the Rocky Mountains where periodic vent filter maintenance is not practical.

Details: Transducer will be used to monitor the depth of a lake high in the Rocky Mountains. Due to the remote location, the customer desires to use a sealed-gage transducer to alleviate the need for a vent filter and the periodic maintenance it requires.

Solution: A Series 700 constructed in a sealed-gage configuration with the zero output compensated for local barometric pressure. Since sealed-gage transducers are normally calibrated with the zero output compensated for 1 nominal atmosphere at sea level, there would be considerable error induced when used at the higher elevation if the transducer was constructed without taking into consideration the difference in atmospheric pressure at sea level and the atmospheric pressure at the higher elevation. In order to eliminate error due to the difference between atmospheric pressure at sea level and atmospheric pressure at the altitude where the transducer will be installed, the customer is asked to identify the elevation of the lake where the transducer will be used. Using the supplied elevation information, the nominal atmospheric pressure at the installation location is calculated and the zero output set for that pressure.

Zero Check for Sealed Gage Transducers

A good way to verify the stability of a pressure transducer is to periodically check the output at zero pressure input. With a gage-type transducer, this is easily accomplished by applying the same pressure to both the pressure input and the reference sides of the transducer, simply by laying the transducer on a table with nothing attached to the pressure input! However, checking the zero on a sealed gage transducer is a bit more difficult, since the reference side of the sensing diaphragm is sealed under hard vacuum. When a sealed gage unit is calibrated at the factory, the zero pressure output is set to correspond to one standard atmosphere, or 14.696 PSIA (a.k.a. 1013 millibars, or 29.92 inches of mercury.) So to check the zero on a unit of this type, one must control the input pressure to exactly 14.696 PSIA, or, one can call the local weather service and get the current barometric reading (in units of millibars), then calculate what the transducer zero output should be by the following formula:

Wet Well Level Transmitter

The liquid level or pressure shall be sensed by a pressure transducer certified by FM, UL, and CSA for installation in a Class I, Division 1, Groups A, B, C, and D, Class II, Division 1, Groups E, F, and G, Class III, Division 1 hazardous location when connected to associated apparatus manufactured by R.G. Stahl and others. The transducer shall be installed in accordance with manufacturer's instructions.

The transducer shall sense the liquid level or pressure variations and convert these variations into a linear 0-5 VDC or 4-20 mA signal. The transducer shall be solid state with no mechanical linkages or moving parts. Static accuracy of ±1% includes the combined errors due to nonlinearity , hysteresis and nonrepeatability on a Best Fit Straight Line basis at 25ºC, per ISA S51.1.

The pressure sensing element shall incorporate a four active arm Wheatstone Bridge strain gage diffused directly into a silicon diaphragm. The sensed media shall exert its pressure against a 316 SS barrier diaphragm. The sensing element shall exhibit non measurable hysteresis, withstand overpressures to 200% of rated range without damage and have a life expectancy in the tens of millions of cycles.

Transducer shall operate from a 9-30 VDC regulated or unregulated power source. On-board signal conditioning shall also include overvoltage and reverse polarity protection.

The transducer housing shall be assembled with 360º circumferential welds. Wetted materials may include 316 SS and Viton only. Transducer housing diameter may not exceed 1.0 inch.

Factory-attached 0.30" OD polyurethane or Tefzel jacketed cable shall have pull strength of 200 pounds. Cable construction to include non-stretch Kevlar stiffeners, cable shield and vent tube for atmospheric reference with moisture barrier. Water tight cable seal shall be via compression type fitting.

Cleaning Series 700 Submersible Pressure Transducers

Materials Required

• plastic bowls 8-12 inches in diameter and 4-6 inches deep
• supply of clean, lint-free cleaning rags
• 1 pair of internal retaining ring pliers with 90 degree tip angle, ring range 1/4" - ".1"
• 1 32 oz. Bottle of "The Works-Tub and Shower Cleaner" manufactured by Lime-O-Sol Company in Ashley, IN 46705 and locally available through WalMart, Kmart, Target and ACE Hardware stores at $2 to $4 per bottle

Preparation

Prior to the cleaning of the pressure transducer, ensure that all procedures have been followed in the proper cleaning of the cable and transducer to remove any hazardous materials. The Series 810 vent filter must be properly attached to the vent tube exiting the end of the cable. The cable should be coiled to ensure ease of handling and it must be protected against the possibility of accidental abrasion and/or penetration of the cable jacket by sharp objects. A lead length of 1 to 1 ½ feet of cable from the transducer should be allowed to facilitate handling during cleaning. The grey protective covering that is shipped with each transducer should be attached to the transducer at all times. It should only be removed prior to installation or cleaning.

Your work surface needs to be clean and free of clutter and large enough to accommodate all materials required in addition to the transducer and cable. Fill one of the bowls with fresh water, one with a mild detergent mixed with water and the last with 16 oz. of "The Works".

Cleaning

Step 1: Holding the cable 6 inches from the transducer, immerse the unit in the bowl containing the mild detergent and stir for 20-30 seconds. Remove and rinse in the bowl containing the fresh water using the same stirring motion used in the mild detergent. Rinse and wipe dry.

Step 2: Holding the body of the transducer with one hand so that you are looking at the retaining screen protecting the sensor, carefully remove the retaining spring using the retaining ring pliers. Keep the tips of pliers as close to the inside edge of the transducer as possible so that the tips will not penetrate the protective screen and damage the sensor diaphragm. Also, use minimal pressure needed to remove the ring. Do not remove the protective screen. Let the protective screen fall into the plastic bowl containing a mild detergent as you invert the transducer and repeat the instructions in Step 1.

Step 3: Place the transducer in a vertical position with the pressure sensing end facing downwards, in the bowl containing "The Works" solution for approximately 15-20 seconds. Rinse in the bowl containing clean water and wipe dry the external casing only. Place the protective screen and retaining ring in the same solution for 15-20 seconds, rinse and wipe dry.

Step 4: Holding the transducer in a vertical position so that you can see the face of the pressure sensor, slide the protective screen across the edge of the transducer and let it drop into place. Replace retaining ring using the retaining ring pliers. Make sure the tips of pliers do not touch the sensor diaphragm.

Submersible Polyurethane-Jacketed, 4 Wire Cable

Specifications

Cable is constructed of 4 conductor, 22 AWG 19/.34 tinned copper with .009" polyethylene insulation. The conductors are cabled with fillers and a polypropylene vent tube (.100" x .060"). A clear mylar is added followed by a 36 AWG tinned copper wrap that provides minimum coverage of 95%. An additional clear mylar is added followed by a Kevlar member (200 lb. pull strength), a water block layer and a 0.30" black polyurethane outer jacket.

Chemical Resistance

Polyurethane is chemically resistant to potable water (less than 2 ppm chlorine content), waste water, borax, butane, animal fat, carbonic acid, citric acid, cod liver oil, corn oil, glycerin, glycol, mineral oils, potassium nitrate, potassium sulfate, silicone oils, Stoddard solvent, tannic acid (10), tartaric acid and turbine oil.

Installation

Most installations either suspend our submersible transducer in a perforated 1 ½" or 2" PVC instrumentation still well or attach the transducer (using our optional ½" MNPT fitting) to a rigid conduit. Other applications use our optional mounting bracket to clamp the transducer to a fixed object (i.e., wall, ladder, step) or require the unit to be suspended without any protective still well or attachment device.

Bending of Cable

Maximum bending radius is 1 inch.

Cable Compression

Cable can be used in conjunction with compression-type fittings; e.g., ingress to a junction box. We recommend only those fittings that utilize a plastic or rubber female, no metal. Also, the installer should avoid over-tightening, which can result in a pinched vent tube.

Cable Lengths

For a 0-5 VDC output, a maximum cable length of 100 feet is recommended. A 4-20 mA signal can be sent thousands of feet, depending upon the power supply available. Longest continuous submersible cable available is 2500 feet.

Cable Splicing Kit

A field installable cable splice allows splicing to our polyurethane cable. It is commonly used for well applications where expensive tefzel cable is required for suspension in corrosive media and the liquid level is shallow but the well is hundreds of feet deep. It also is used in those emergency situations where cable must be spliced together to get an application up and running.

Tank Level Measurement

Submersible or above ground pressure transducers can be used for tank level measurements. Application specific circumstances dictate which approach is most feasible. For example, buried tanks usually require submersible pressure transducers while high water towers are best served by above ground transducers attached to the wall or a pipe coming from the bottom of the tower.

Occasionally, one finds a high pressure line running through the bottom of a tank filled with liquid. In such instances, we recommend a submersible pressure transducer with a threaded pressure port (e.g., Series 720 with a 1/4" male NPT pressure cap).

Submersible installations require suspension of the cable via one of 2 cable anchoring schemes. The first is via a wire cable hanger in an open top configuration. The cable hanger slides onto the cable from the bare-wire end. It can be positioned anywhere on the cable by pushing the ends of the hanger together. Second, a compression fitting is used in a closed top configuration. We recommend only those fittings that utilize a plastic or rubber female, no metal. Also the installer should avoid over-tightening, which can result in a pinched vent tube.

We supply, free of charge, a moisture and vapor trap with each submersible pressure transducer to ensure reliable operation and long life. This device protects sensitive electronic components from mildew, corrosion, rust and prevents the formation of a liquid column in the vent tube. Any such column directly affects calibration. This trap connects to the existing cable vent tube via a user-installable pin connection. Two common reference connections schemes are presented below.

Troubleshooting Submersible Pressure Transducers

Electrical Output Exceeds Range

Readings of 30 mA on a 4-20 mA output device indicates that the unit is no longer in calibration and needs to be returned to the factory. This condition is often caused by lightning strikes or other high-voltage transients that damage the instrumentation amplifier.

Actual Level Exceeds Indicated Level

Consistently lower readings can indicate a build-up of a liquid column in the cable vent tube. If you happen to get water in our vent tube and in the submersible pressure transducer, coil the cable and place the cable and transducer in a pan and place the pan in an oven at 50ºC for 2 hours. This on-site remedy may do the trick. Be careful that the oven temperature does not exceed 50ºC. Otherwise, you may damage the transducer and cable. Alternatively, you may suspend the cable and unit in a vertical position overnight to allow water to drain from the transducer and the cable vent tube.

Reading is Within Range But "Freezes" at One Point

Even though the input end of the level sensor is relatively self-cleaning, certain environments may induce the formation of "crust" over the sensor diaphragm preventing the sensor from identifying change in level. Removing the sensor from the media and cleaning it with a mild detergent will solve the problem. To combat marine growth, you might try wrapping our transducers with copper wire similar to that found in wire scouring pads for doing dishes. Marine growth occurs on the copper and eventually erodes the copper and drops off or the copper is manually removed during routine maintenance. Alternatively, there are various companies that will impregnate/coat the 316 stainless steel with antifouling chemicals or coatings. Similarly, level sensors temporarily removed from the well or sump should not be stored dry, but should be stored in a bucket of fresh water in order to prevent "crust" formation.

Readings Increasing Very Slowly Over Time

Our cable is shipped coiled and consequently takes time to straighten when installed. Attaching a weight to the transducer (i.e., one of our sacrificial anodes) will help. In order to prevent cable stretch with lengths greater than 200 feet, secure the Kevlar fibers (just under the black cable jacket) to the junction box or other immovable object.

No Electrical Output From the Transducer

Check all connections to ensure they are correct and secure. Double check your power supply or power the transducer from a battery (i.s., 9V lantern or 12V car). If everything checks out ok, the problem may be with our circuit board or sensor. The unit will need to be returned to the factory for evaluation. Most probable cause of this type of failure is damage to the submersible cable jacket allowing water to leak down the cable and into the transducer housing.

Lift Stations

The Series 700 submersible pressure transducer is suspended approximately 10 inches above the bottom of the wet well via a self-supporting cable. It can hang freely or be suspended in a PVC instrumentation wet well. Alternately, the transducer may be fitted with a ½" Male NPT conduit fitting and attached to rigid conduit.

The sensing element features a four active arm Wheatstone Bridge strain gage diffused directly onto a silicon diaphragm. Protected by a porous screen, this sensor assembly is housed in a rugged 316 stainless steel case with the barrier diaphragm in direct contact with the liquid media. Ported holes in the side of the case nearest the pressure sensor ensure clog-free reliable operation.

Optional pressure inputs and electrical output connections are available as are a variety of analog signals with current loop (4-20 mA) and voltage (0-5 VDC) output being the most common. Excitation voltage may range from 9 to 30 VDC. As the signal is proportional to the liquid column above it, lift station operators are provided with a continuous output that monitors wastewater levels. This facilitates the implementation of anticipatory control processes that can be related to alarm and other instrumentation as well as pump control mechanisms.

The factory attached self-supporting polyurethane jacketed cable has a pull strength of 200 pounds with integral Kevlar tension members to prevent cable stretching. The cable also contains an integral vent tube referenced to local barometric pressure to eliminate inaccuracies due to varying atmospheric conditions.

At the point of cable termination in the controller panel, a field replaceable Series 810 vent filter is connected to the vent tube. Designed specifically for those high humidity environments where water vapor may condense in the vent tube, this unit protects sensitive electronic components from mildew, corrosion, rust and other forms of deterioration while at the same time preventing the formation of a liquid column on the back of the sensor that could cause erroneous readings.