EQUIPMENT AND SUPPLIES: Field evaluation of New England Research Associates (NERA), Inc. Model 4 Water Quality Monitoring System March 14, 1975 QUALITY OF WATER BRANCH TECHNICAL MEMORANDUM NO. 75.16 Subject: EQUIPMENT AND SUPPLIES: Field evaluation of New England Research Associates (NERA), Inc. Model 4 Water Quality Monitoring System Short-term, intensive measurements of various water-quality characteristics are an integral part not only of special studies but also of some parts of the continuing basic data collection program of the Geological Survey. Instruments designed to facilitate such measurements have been developed by several companies. These units convert analog signals received from. a sensor unit to digitized signals which are subsequently recorded on magnetic tape. One such instrument is the Model 4 Water Quality Monitoring System manufactured by New England Research Associates (NERA), of Bedford, Mass. The Pennsylvania District recently had the opportunity to field test a NERA unit at three dissimmilar sites: 1. a stream which receives acid mine drainage, 2. two lakes that are characterized by temperature inversions and permanently anaerobic hypolimnions, and 3. the Delaware River estuary. The attached report, prepared by Robert Bubeck, Harrisburg, summarizes the field test of the NERA monitoring system. In his evaluation, R.C. Bubeck addresses many of the questions which will arise when such equipment is being considered for purchase. The evaluation is forwarded for your information. The Quality of Water Branch would appreciate additional comments on the performance and usefulness of this particular water quality monitoring system. R. J. Pickering Attachment WRD Distribution: A, B, FO-L, PO A FIELD EVALUATION OF THE NERA, INC. MODEL 4 WATER QUALITY MONITORING SYSTEM Robert C. Bubeck, Harrisburg, Pa. Introduction Northeastern Regional Headquarters requested the Pennsylvania District to evaluate a new portable water quality monitoring system for use in lakes and streams. The new system is actually a coupling of three subsystems: a portable measuring device, a portable recording device, and an office software package used to process and illustrate the field data. In December 1974, a Model 4 Water Quality Monitoring System was loaned to the Pennsylvania District office by New England Research Associates (NERA), Bedford, Mass., developer and distributor of the system. The purpose of the equipment loan was to allow District personnel to make an objective field evaluation of the system, in any field condition available, without the aid of a NERA representative. A NERA representative presented a two-day school on the calibration, use, and maintenance of the field and office equipment. The presentation included a field trip to a local lake where District personnel operated the field equipment, conducted measurements, recorded the data, and then returned to the office to practice the data processing and software manipulation with the NERA office equipment. Thereafter the equipment was left with the District for about three weeks and the evaluation was conducted without NERA supervision. Description of the System The NERA System consists of two classes of equipment: the field equipment, which is taken to the specific field site to conduct measurements and to record results; and the office equipment, which is used to process the recorded field data after the field activity is completed. A. The Field Equipment The field measurements are made by a Hydrolab Corporation in- situ water quality analyzer known as the Hydrolab Surveyor. The Surveyor consists of three major parts: 1. The Surface Unit: The control unit contains operating controls, surface electronics, and read-out meter. 2. The Sonde Assembly: The submersible unit houses seven sensors for the measured variables, electronic pre- amplifiers, and electrically driven water circulator. The sensors available are depth, temperature, electrical conductivity, DO, pH, oxidation-reduction potential, specific ion, and the required reference electrodes. 3. The Instrument Cable: The connecting cable transmits electrical signals between Surface Unit and Sonde Assembly and is used to control the depth of the Sonde. The remainder of the field equipment consists of the following: 4. The NERA Model 4 Controller/Digital Logger: This device controls the timing sequences and power application to the probes and associated equipment, converts analog signals from the Surveyor Surface Unit to digitized signals, displays the signals, and records the signals on magnetic tape. The unit includes a malfunction warning flasher. 5. Data Cassette: This cassette package contains computer- grade, environmentally protected magnetic tape on which the digitized signals are recorded in the field. 6. Portable Power Supply: The power supply provided was a 12 volt DC, heavy duty, 80 amp-hour auto battery. An optional 110 volt power source is available. 7. Weatherproof Carrying Case: This is a portable carrying case which contains all above equipment with the exception of the Sonde Assembly and Instrument Cable. 8. Maintenance Kit: A tool kit is provided which contains the items necessary for routine field calibration and maintenance. B. Office Equipment This portion of the system processes the field data, converts it to engineering/scientific units, and displays the data to the user. 1. NERA DTR-5 Playback/Interface Unit: This unit provides for playback of the cassette tape and interfaces with a standard send-receive time share terminal and telephone modem. Time Share Terminal and Telephone Modem: Standard terminals and modems are used. Users rent the terminal of their choice and bear the cost of the rentals. Users may elect to use their own in-house equipment. NERA, Inc., will furnish technical advice and assistance to users who elect to use their own terminals or teletype printers. Telephone: A standard telephone is required. Costs are paid by the user. NERA Clear Waters 1 Software Package: The package includes computer programs stored in the GE Time Share which allow the user to process data for review and to illustrate the data through various packaged plot routines. Data from the Model 4 System is in a format which users may employ at their own computer facilities, if desired. NERA, Inc. charges the user on a monthly as-use basis for the Clear Waters 1 Software. Time Share Subsidiary Account: If requested, NERA Inc. will establish an account on G.E. Time Share for the user. This account permits access to the stored Clear Waters 1 Software. Cost is paid by the user. Technical Manual, Field Assistance and Service: A user's technical manual is provided which gives step-by- step procedures for operation, calibration, and servicing of the field equipment. Procedures are outlined for the use of the hardware and software to process and display the recorded data. NERA, Inc. also offers field assistance and consultant service for all sub-units of the system. III. Instrument Evaluation A. Laboratory Testing Prior to taking the field equipment to the field, the instrument was operated in the continuous record mode for 3 days. The Sonde was set in a laboratory sink filled with water and the instrument was operated in the Auto-Scan mode; that is, every 30 minutes, the internal controller turned on the system, allowed the probes to equilibrate, made measurements, recorded them on magnetic tape, and turned-off the system. Two or three times a day warm water (30!-35!C) was poured into the sink to change the temperature of water and probes. Cold water and ice were added late in the afternoon to vary the temperature in the early evening. Each morning and afternoon the system was calibrated according to the methods outlined in the instruction manual. Temperature and conductivity calibrations are actually internal resistance checks; therefore, T and EC were checked against laboratory Leeds and Northrup equipment. pH and DO calibrations were checked against a laboratory pH meter and Winkler titrations. The instrument's temperature and conductivity readings remained within the accuracy of the read-out meter. Its pH readings agreed within +0.2 pH unit of the readings of the laboratory pH meter. Occasional fluctuations of the Surveyor unit pH meter were noted which are typical of field instruments with excessively long lead cables, coiled in air, and not earth grounded. Comparison with laboratory Winkler DO titrations indicated that the instrument DO probe read slightly high by 0.1 or 0.2 mg/l during the initial testing. Comments: (l) The purpose of this testing procedure was to observe the internal consistency of the field equipment, to become familiar with the calibration procedures, to check the logic of the operating instructions and claims made in the manual, and to become familiar with the mechanics of the system. No attempt was made to conduct a rigorous evaluation of the electronic components or the circuitry of the system. (2) At this point in the evaluation, the field equipment operated within the specifications claimed by the vendor. (3) No attempt was made to calibrate the redox-probe during this evaluation. The redox probe was operated in the field, however, to serve as an indicator of change from aerobic to anaerobic environments. Also, no specific ion electrode was furnished by the vendor. Therefore, only six of the seven advertised sensors were used in the field. B. Field Testing The instrument was field-tested at three sites: (l) submerged in an acid mine drainage stream for 10 hours, operating in auto-scan mode; (2) used to profile two lakes which were characterized by high electrical conductivity, temperature inversions, and permanently anaerobic hypolimnia; (3) used to profile a grid of stations comprising cross sections of the Delaware River estuary. 1. Stream affected by acid mine drainage The Sonde (sensor package) was placed in the middle of the stream and the cable was stretched to maximum extent (20 m) up an embankment to the permanent carrying case housing all equipment. The case was set on the tail-gate of a station wagon, exposed to 6!C air temperature and constant rain and snow. The equipment was set in the Auto-Scan mode and measurements were made automatically every 30 minutes and recorded on tape. Measurements were made manually by the operator every 15 minutes and logged for later comparison to the taped data. The system was run for 10 hours, simulating part of a continuous 24-hour study of T, EC, DO, pH, and redox in an acid stream with a rising stage. Comments: In general the instrument performed well. The data recorded manually on site matched well with the data recorded on tape and later processed in the office. Some specific comments on the use of the instrument at this site are: (l) The equipment case is relatively large and heavy (80 to 90 lbs). The equipment would be easier to use in the field if it were packaged in a smaller container. The excessive weight of the case is due mainly to the mass of the heavy-duty battery. An alternate power supply, which is lighter and rechargeable, is recommended. (2) The Sonde assembly is well designed, built well, and able to withstand rough field treatment. It is relatively light and small. The probes are well protected, yet exposed to the in-situ sample. In the stream test, the Sonde was intentionally pushed over at an angle to the vertical (and then on its side) a number of times by the operator. The data remained consistent (as long as probes were in water) in spite of the change of orientation. (3) The Surface Unit and the Controller/Digital Logger were exposed to rain and snow continuously. Recommended precautions were taken to keep them as dry as possible. Frequent manual measurements and checks to see that the system was actually working precluded keeping everything dry; however, the equipment worked well in wet conditions. (4) The plexiglass cover and gasket material which covers the Surface Unit and the Controller/Digital Logger provide excellent protection against foul weather. All shafts of control switches which pass through the face plate have waterproof fittings. In addition, each "calibration pot" which needs to be set by the operator in the field has a small screwdriver mounted above it. The screwdriver fittings are waterproof. Calibration controls which may need adjustment only occasionally in the field have screw fittings which are also waterproof. During the last hour of the test, a bucket of water was intentionally poured over the top of the equipment. No leaks were noted in the face plates of the two components during the following 2 hours. (5) When the system is in Auto-Scan and making measurements under the control of its internal timer (ex., every 30 minutes), the operator can manually over-ride the timer and take more frequent measurements (ex., every 5 minutes). If the timer has been interrupted to make a manual measurement, it is automatically started again when the mode switch is re- set to Auto-Scan. Thus, the user can take more frequent samples during a particularly interesting event or he can check on the operation of the system anytime he wishes during a long period of study. (6) The operator must keep a log of start times and stop times of the Auto-Scanner. He should also keep a record of initial and final dial settings of the Surface Unit and any intermediate changes he makes to the controls. In addition, a record would have to be kept of the time of any intermediate manual readings, if the operator wished to interrupt the auto-timer and then re-start it. Although the instrument recording device and timer work well, they do not keep real- time records of when the instrument is on and off. As in any field or laboratory experiment, the more complete the records and notes which the operator keeps, the better use he can make of the recorded data. (7) Removing and inserting the tape cassette is cumbersome in the field. Remembering to insert the clutch mechanism can be a problem and doing so in the field is rather difficult. It is best to prepare the cassette and clutch mechanism beforehand and insert both in the office or laboratory before going to the field. (8) There are methods of "tagging" certain information on the tape cassette. For example, the user may set one or more parameters to zero by switching them off, and then manually pressing the profile switch to record a cycle. "Zeros" will be recorded on the tape and can be used to indicate a change of site position or station in a stream or its cross section. However, it is also most helpful to maintain a written log of any "tagging." The operator can record the time and position change which are represented by the zeros he places on the tape. This allows the user to label data properly when it is processed later in the office. (9) For field and laboratory calibrations of the DO probe, the manufacture's instruction book recommends an air calibration similar to that of other DO probes. In this evaluation, the standard Winkler DO determination was found to be easier and more convenient in the lab and field. In a field situation where a 24 or 48-hour study might be conducted, occasional independent Winkler and pH measurements could be made while the system is in continuous operation and undisturbed. Then the independent measurements could be compared to the instrument readings at the site by switching to the particular parameter on the Surveyor Control Unit and comparing immediately. The independent data could also be logged for further evaluation with the recorded data. (10) The pH callDration jig supplied by the manufacturer to hold the pH, reference, and temperature probes during field or lab calibration is awkward to use. It required some adjustment and balancing to use it properly and can probably be redesigned fairly easily for more convenient use in the field. The instrument was re-calibrated in the lab on the morning following this test. Calibration results remained within ranges described above. 2. Profiling of Lakes The field equipment was taken to two lake sites chosen because: (l) The electrical conductivity of these lakes often exceeds 2000 micromhos. (2) These lakes are meromictic (permanently anaerobic) and offer excellent natural conditions to observe unique changes in temperature, EC, pH, DO, and redox as a function of depth. (3) Early winter conditions provided air temperatures between 20! and 39!F (-6.5! and -1!C) snow and rain, thus providing a fairly severe environmental test for the instrument package. The instrument was calibrated in the lab the day before the trip to the field site. Prior to the trip to the first lake, the instrument package remained in the field vehicle all night at 25! to 30!F (-4!C to -1!C). Only the battery was removed to room temperature where it was charged. On the following day, the instrument case and Sonde were put aboard a Boston Whaler. Due to problems with the boat battery in cold weather, the battery for the NERA system was used to start and operate the boat. The boat was then used to gather limnological data for about 2 hours before the NERA instrument was used in a small anaerobic cove, adjacent to the larger lake. During this time period, the NERA instrument was exposed to below freezing temperatures, snow, and rain. The instrument was then used for about 1 hour to make profiles in the small anaerobic cove. The following observations were made on the performance of the equipment: (l) The DO probe took a relatively long time to stabilize (4 to 5 minutes) as DO concentrations decreased with depth. While this observation may be a common characteristic of all DO probes, it was especially noticeable in this situation. Within the chemocline and at the known depth of oxygen depletion, the DO probe took about 6 minutes to stabilize and always indicated some apparent "residual oxygen" in the anaerobic zone. The anaerobic zone in this lake is known to contain high concentrations of CH4, C02, and H2S gases which may have affected the equilibrium rate of the DO probe. (2) pH decreased, as expected, but the pH meter's needle movement was relatively unstable in the chemocline and at anaerobic depths; however, the readings measured by the pH unit were realistic. (3) The redox probe (not calibrated) indicated some changes as it passed through the chemocline into the anaerobic zone. However, past experience with studies on this lake suggests that the probe was not performing well in this chemical environment. The behavior of the redox probe cannot be commented on with much confidence, however, because no attention was given to its initial calibration. (4) The temperature and conductivity probes appeared to operate well, but occasional erratic needle movement occurred on the meter during conductivity readings. Following the trip to the first lake, the battery was charged overnight and the equipment was kept at room temperature. The next day the equipment was taken to a second lake site where limnological data were being collected at four stations. The NERA equipment was put aboard a small boat and rowed to the position of a station (18 m deep) in a meromictic lake with a known extensive anaerobic zone. Profiles were made in a manner similar to those of the day before. The equipment was exposed to temperatures of 20 to 25!F (-6.5! to -4.0!C) during the day. The following observations were noted: (l) Again the DO probe took a relatively long time to stabilize. In the anaerobic zone, the probe continued to indicate a few tenths of a ppm of oxygen. The other probes reacted as expected but with more stability than the day before. (2) After returning to the office and processing the recorded data from these two lakes, it appeared that spurious signals were superimposed on the data of the first lake. This seemed to agree with the instability noticed on the meter readings at the first lake. After consultation with NERA representatives, it was determined that the use of the instrument battery as a power supply for the boat (in the first lake) caused spurious signals to be introduced in the boat and instrument circuitry. These signals were picked up by one or both of the surface units and recorded as noise on most sensor signals. Therefore, a user should check the operation of the instrument carefully if he considers using a boat battery directly or a boat's secondary power supply with the instrument. An independent power supply is probably best for the instrument. Some boat power supplies can probably be used if proper filtering is available and if some experimenting is carried out with the instrument beforehand. Other comments applicable to the use of the field equipment in boats are the following: (l) The present size and weight of the instrument case are a definite disadvantage in a field situation where small boats are the only boats available. A method of decreasing the size and weight of the case through the utilization of a smaller, lighter battery was discussed with representatives of NERA, Inc. They were amenable to the suggestion and have recommended a specific battery which could be substituted. (2) It would be advantageous to have the connector of the main cable linking the Sonde to the Surface unit fitted into the outside of the carrying case, rather than into the side of one of the components which is placed within the case. The present hook-up method is awkward in many field situations. (3) If it is necessary to use a power boat's battery as the instrument power supply, disconnect both positive and negative leads of the boat engine from the battery before energizing the NERA instrument. The manufacturer, however, recommends a separate battery for the monitoring system. (4) As in the previous field test, the use of a log to record time, place, equipment dial settings, etc., at all profile stations is an essential part of the data collection procedure. The use of an automatic recording instrument should not eliminate constant checks on the operation of the system and ample field notes on its behavior. Upon completion of this field trip, lab calibrations were repeated and found to remain within the ranges described above. 3. The Delaware Estuary The field equipment was taken on a routine sampling trip conducted weekly on the Delaware River estuary by the USGS and Philadelphia Water Department. Weekly water samples are taken at surface and bottom in a line of stations along an east-west cross-section of the river. Individual cross sections run the length of a particular north-south reach of the estuary. In a study such as this, the NERA instrument package could be used to gather a vertical profile at each station of a given cross section. This would provide time- frequent and depth-frequent data for many points in a three dimensional grid of the estuary. In this case, the instrument was carried aboard a 30-foot vessel especially designed for water-quality research. Size and weight of the field instrument were not limiting factors in this case. More important considerations in this type of a field test were: (a) The speed with which data could be taken at each desired depth in a profile of a particular station; (b) The availability of a quick method of identifying, on the magnetic recording tape, the position in the grid of the station being measured. The instrument was calibrated in the laboratory the day before the field trip in the manner described above. The following observations were made of the instrument's performance: (l) In a study in which data is to be gathered quickly in a three dimensional grid, the time on station is the sum of the time required to measure data at each depth of interest. At a given depth, the seven parameters can be measured and recorded in about 3 seconds after the "step switch" is pushed. However, the operator must hold the Sonde at each depth for the length of time necessary for all sensors to approach equilibrium with the water they are measuring. In this field situation, as the others before it, DO was the slowest sensor to respond. Experiment showed that about 3 minutes was necessary to insure that the DO probe reading was at or near its equilibrium value at any given depth. What this means is that at the various levels within a vertical profile, readings should be taken only after the time period required to equilibrate the least responsive sensor has elapsed. In this case, it would take about 3 to 4 minutes to characterize each level within the profile. Profiling in this manner requires that the boat be held in a fixed position by anchors and/or engines, that the operator pay close attention to the instrument, and that a longer time on station be acceptable. (2) Many spurious electrical signals probably exist in the type of vessel used in the estuarine study. No adverse effects were noted on the instrument or in the data recorded on the cassette. However, the instrument was operated from its own power supply. No attempt was made to use the boat's power supply. (3) In this field case, as others, the use of a field book is necessary to record the time, place, cross section station number, and the tape identifier for each group of data placed on the tape. The user can assign a number to each cross section and a number to each station within the cross section. The user can then key-in zeroes on the magnetic tape immediately before each set of profile data. The position of the zeroes or the numbers of lines of zeroes would correspond to a particular station number. Later, with the office equipment, the data can be edited and actual identification assigned to each group of data. The instrument was calibrated in the laboratory the day after the field trip and was found to conform to the limits described above. IV. Office Equipment The office equipment consists of the NERA DTR-5 Playback/Interface Unit, a printer and telephone modem. The latter are used to access the G.E. Time Share system which holds the software package. Directions for use of the playback equipment and printer are fairly straightforward and advice is available from NERA software consultants. The versatility of the system becomes more evident as one uses the programs and practices handling the data. Programs available in the NERA software package allow the (l) display raw data from the tape, store it, edit it, and process it to usable data with proper units assigned; (2) display, store, and edit processed data; (3) plot processed data--variables vs time, or variable vs variable. The NERA software package also allows the user to add additional data which has not been produced by one of the sensors to the data fiie. For example: if a profile were made in a lake 10 metres deep, and if in-situ data were taken every one metre in depth with the instrument, and if water samples were taken for chemical analyses every 2 metres, the results of the chemical analyses could be entered in the data file at the 2 metre intervals at a later time. The amount of additional data that can be entered appears to be limited by page width. When using the printer and software to illustrate data or to view various relations between variables, much header information is required to be entered via the keyboard printer in order to make the plot routines run. Users' time, computer time and paper are wasted to make these entries and to have them printed out again, when all the user really needs to see is the plot of "A vs B". Short cuts to more efficient use of the plot routines are desirable. The printed format of the plotter output seems somewhat awkward to understand at first glance. In some of the instruction manual directions for use of plot routines, the labeling of the X and Y axes seems to be confused, either in the directions given to the user or in the manner in which the computer program is written. The capability to list, store, and add to the data which were recorded in the field by the NERA system is extremely versatile. If this capability could be coupled to statistical packages and plot routines available in the WRD computer library, the value of this system could be enhanced greatly. While it is necessary for the user to purchase the DTR-5 Playback/ Interface Unit, the WRD user may utilize his own printer or teletype device which is often already available in the District Office. The use of in-house computer facilities or terminals already installed at a District Office may open up many avenues for more efficient, adaptable, ar_d less costly use of the system. The printer sold by NERA, however, is a relatively fast machine for listing and plotting operations. The vendor emphasizes the use of the playback unit and printer in the field to examine the field data immediately after it is collected and before the user returns to the office. While this is advantageous in theory, there may be some practical limitations: (l) A portable printer would be required in addition to any printer already available in the District Office. This would add to costs. (2) Many motel room telephone lines and switchboards are poor quality systems for data processing (as are some FTS systems), thus interruptions and noise can occur which eventually adds to costs. (3) Data processing, via telephone, from a small town's motel requires long distance calls to the nearest city in which a G.E. Time Share system is located. This adds to costs in the field. While no one of these three items may be cost-prohibitive in itself, each one adds to the cost of using the system, even after the equipment has been purchased. Other costs such as cassette tapes, time share costs, costs to use the NERA software package, printer paper, etc. must be considered (in addition to the initial purchase of the system) when a potential user considers the costs vs benefits of this system. Summary 1. Hydrolab Surveyor Package In general, the Hydrolab Corporation in-situ field package is well constructed, light, and easy to handle in the field. The fact that the electrical conductivity, dissolved oxygen, and pH probes of the unit are internally temperature compensated is an important and desirable characteristic of the field package. All probes which were calibrated in the laboratory and field maintained their calibration within the specifications listed by the manufacturer. The relatively slow response of the DO probe in anaerobic hypolimnia was not considered unusual; however, the DO probe tended to read slightly high by 0.1 to 0.2 mg/l during intermittent laboratory calibrations. a. Surface Unit The controls are easy to operate, accessible to the user, and offer versatility in operation and calibration procedures. The unit is waterproof and appeared to work well at temperatures to -6!C. An excellent feature is the use of small screwdrivers permanently mounted in the plexiglass faceplate for aid in calibration and adjustment. b. Sonde Assembly The submersible unit is relatively light, rugged, and easy to handle in the field. All sensors are well protected, yet well exposed to the in-situ water sample. The probes are accessible to the user for inspection and for servicing in the field. c. Instrument Cable The cable is relatively light and easy to handle compared to the connecting cables of some other systems on the market. Although its rated strength was not tested, the cable and supporting mechanism for the submersible unit appear adequate. 2. The NERA Model 4 Controller/Digital Logger (C/DL) The comments given above on the Surface Unit apply to this unit as well. Considering the rather complex nature of the electronics contained in this unit and the variety of functions it performs, the unit is well designed and fairly easy to use. The only awkward feature of the unit from the viewpoint of a field operator is the slight difficulty in moving the tape cassette in and out of the small recorder unit which is housed in the C/DL. With careful attention, this problem can be overcome. It is important for the field operator to know when the recorder is actually recording data from the sensors. At present the user must note the turning of the cassette sprockets and the fluctuation of the signal gain meter to ensure that the recorder is activated. It would be very helpful to have an indicator light in series with the recorder so that the user can tell at a glance when the recorder is operating. An indicator light is presently used by the manufacturer to indicate when the electric stirrer motor in the submersible unit is operating. This is a small but very desirable feature of the control unit. 3. Data Cassette The use of a tape cassette is convenient and efficient. The only problem (mentioned above) is the insertion and removal of the cassette from the C/DL recorder during unfavorable weather conditions. Also it is necessary to store and insert a small clutch mechanism in the cassette prior to placing the cassette in the C/DL recorder. To minimize these problems in difficult field situations, it is advisable to load the cassette in the C/DL while in the office or vehicle, rather than in a small boat in bad weather. 4. Portable Power Source The power source provided with the system worked well; however, it is too heavy and adds excessive size as well as weight to the field carrying case. This problem has been discussed with NERA representatives and they have recommended a lighter and smaller battery which is capable of providing the required power. It would be a further advantage to have a light weight battery with a built-in battery charger. This would eliminate the necessity to carry an external battery charger to the field. 5. Weatherproof carrying case The carrying case is durable and able to withstand rough field treatment. However, its present size and weight are somewhat of a disadvantage in a field situation where only small boats are available or long portage is required. Its weight can be decreased by substitution of the present battery, as discussed above. Its size may be decreased by repackaging the components it contains and rearranging inter- connecting cables. It would also aid the field user to have the connector from the main cable fitted into the outside of the carrying case, rather than into the side of one of the components which is placed within the field case. In general the design and construction of the carrying case is an important positive feature of the field equipment. 6. Maintenance Kit A tool kit is provided by the manufacturer. It contains all necessary equipment for servicing and maintaining the field equipment. A suggested addition to the kit would be a toothbrush. A small brush is a handy tool to remove sediment from around probe assemblies and to clear reference electrodes. 7. The Manufacturer's Instruction Manual The Manufacturer's Instruction Manual is designed to lead the user through most operation and calibration procedures in a stepby-step manner. Sections of the manual which explain the office equipment (hardware and software) include good examples as well as explanations for accessing the computer and running processing and plot programs. In some sections of the manual there are a few vague and confusing steps, but generally the manual is easy to follow and self-explanatory. 8. NERA-DTR Playback/Interface Unit This unit is a compact playback device for reading the tape cassettes and interfacing with the computer terminal and telephone. It is easy to use, occupies a minimum of space, and requires no maintenance by the user. 9. Time Share Terminal and Telephone Modem The terminal and telephone modem may be purchased through NERA, Inc., or the user may use his own terminal or teletype, if available. The equipment is relatively simple to operate and requires no maintenance by the user. If the user wishes to utilize his own in-house equipment, NERA, Inc., will interface the DTR-5 with the user's equipment. 10. NERA Clear Waters 1 Software Package Computer programs for processing field data are accessed through the G.E. Time Share System. Directions for use of the DTR-5, time share terminal, and telephone to access these programs are given in the Manufacturer's Instruction Manual. The directions are easy to follow and NERA, Inc., provides ample consulting services to the user. The programs provided for processing, editing, storing, and illustrating data are a primary service of NERA, Inc. The vendor is responsive to suggestions for updating, improving, or expanding software services. In conclusion, the NERA Model 4 Water Quality Monitoring System is an effective combination of a portable measuring device, a portable recording device, and an office data reduction device. The system would be useful for lake, reservoir, estuary, and short-term stream and river studies. VI. Some Comments Concerning the Use of NERA or NERA-type Systems Although this unit can be used effectively to make vertical profiles in lakes, rivers, and estuaries and to conduct short-term (24, 48 hr, etc) stream surveys, the instrument is not an end in itself. In any investigation in which it is used, additional chemical and biological data should be taken to supplement the automatic measurements which the monitor takes. In addition, the user must become thoroughly familiar with the equipment to be satisfied that the instrument is measuring real data. Only by taking occasional supplemental data, keeping a field record of changes made in instrument settings or probe alterations, and by knowing something about the characteristics of the water course being studied can the user take full advantage of this equipment. Consider the case of profile runs of lakes, estuaries, or deep rivers. Because there is a minimum amount of time that the user must wait at each depth of a profile for the sensors to equilibrate, there is merit to the suggestion that the user can just as well manually record his data as automatically record it on tape. A special advantage for manually recording is that the user will know immediately the quality of data he receives and whether it makes any sense. However, for a project in which many profiles would be highly desirable, a combination of occasional manual checks and recordings with frequent automatic tape recordings would be of great benefit. In the case of 24 hour (and longer) stream studies, the advantages of the automatic recording device and occasional manual checks are many. The versatility and utility of the interfacing of recorded field data to a computer facility seems obvious, yet it needs to be considered more fully. The user should try to make best use of computer facilities or terminals already existing within WRD offices. Processing of raw data with the NERA software package stored in the G.E. Time Share System is an integral part of the Model 4 System. The WRD user, however, should look to available, in-house WRD hardware (i.e. terminals, teletypes) for additional manipulation of the processed data. For instance, expenditures may be allotted for purchase of the field equipment and DTR-5 playback unit, but the possible use of in-house terminals or teletypes to access the G.E. Time Share System should be considered. Also, the use of statistical packages, plot routines, and modeling programs may best be handled within WRD operated facilities. The NERA, Inc. Model 4 Water-Quality Monitoring System is a useful tool for a variety of water-quality studies. However, any potential user should examine the system closely to determine whether it meets the specific needs of the investigator and the general needs of a District QW program. Interested Districts should also be informed that there are other manufacturers in the market place who offer in-situ measuring devices and digital recording devices. However, NERA, Inc. appears to be the only manufacturer to date (February, 1975) which offers these two devices plus a full hydrographic software package for processing the field data. Alternative systems should be evaluated and compared by potential users before purchasing decisions are made.