The advent of electronic sensors and data loggers, with their ability to record and store information in database ready digital format, made the importing and compilation of recorded values much easier and faster. While there was the benefit of collecting data faster there were problems to overcome. The first devices were usually more expensive than their mechanical counterpart and were not easy to program.
Extreme environmental operating conditions, (i.e. temperature) revealed problems in getting electronics to operate reliability and accurately. However, as the use of these electronic automated systems increased during the 1980's, advancements in both sensor and data logger design overcame these problems.
Today there are a variety of sensors and data loggers on the market. In addition to recording water level, one can now include water quality and meteorological parameters as well. What used to require a separate recorder for each parameter now can be collected in one data-logging device. To determine what automated system best suits the need of the end user will depend upon their present and future monitoring plans.
There are a variety of different methods for measuring the change in water level. Some of the products currently used include:
Ultrasonic Sensors send out a series of sound waves, which travel through the air and strike a target (water surface). An echo is returned back to the sensor and the transit time taken to send the wave and return is related to the distance traveled. The advantage of this sensor is that it is non-invasive, having no physical contact with the media being measured. In areas where periodic flooding may carrying away a conventional gauge station, a ultrasonic sensor could be safely mounted above the high water. The main disadvantage of the ultrasonic sensor is that the sound wave can travel at different speeds depending upon the prevalent environmental conditions (temperature, pressure, and humidity), thus affecting accuracy.
Submersible Pressure Sensors represent the largest segment of the sensors market. As the name implies the system consists of a pressure transducer immersed in the water at a fixed depth. The sensor transmits an analogue or digital signal via underwater conductors back to the data logger. In addition to the signal wires there in normally a vent tube as well. The vent tube allows the sensor to equilibrate itself to changes in barometric pressure. If the sensor did not do this then an increase or decrease in barometric pressure would be reflected in the recorded readings, creating another source of error. Submersible analogue sensors can have an accuracy as good as 0.1% of Full Scale Output (FSO), while digital sensors are available with accuracy's of 0.02% FSO or better. The main advantage of the submersible sensor is that it is relatively inexpensive and easy to install. Some of the disadvantages include variances in accuracy (depending upon make and model). A pressure transducer inaccuracy is usually based on the following characteristics:
In addition there are errors caused by sensor drift (especially analogue sensors), and the realization that, should the sensor leak, in most cases the electronics are damaged beyond repair and the sensor must be replaced. Digital submersible sensors offer high accuracy and excellent long-term stability, but usually at a substantial cost over their analogue counterparts.
The Pressure Measurement (Bubbler) Sensor is an extremely accurate digital sensor which is used to measure the gas pressure required to generate a bubble at the end of a submerged orifice line. The pressure required to create the bubble is proportional to the water head above the orifice. These "bubbler" gauges are similar to the submersible digital sensors, with the exception that they are typically mounted in a walk in shelter along with the pressure source (nitrogen tank or battery compressor) and pressure regulator. The main advantages of this type of sensor is its lower cost compared to its submersible counterpart, and that the only component in the water itself is the low cost orifice line. The main disadvantage is the requirement of a bulky pressure tank or external pressure source.
The use of Shaft Encoders was a natural progression from the mechanical chart recorders. The existing float and pulley arrangement mounted could be removed from the mechanical recorder and mounted directly to the shaft of the encoder. The encoder would typically provide a pulse corresponding to the smallest measured increment (1 mm). As the float raised and lowered the encoder would provide a positive or negative count depending upon the direction of movement. The main advantages of the shaft encoder are the ease to which it can be retrofitted to stations already using a float and pulley set-up, and the excellent linearity and accuracy provided. The main disadvantage would arise from the installation costs as float sensors do require a stilling well to operate.
In addition to measuring and recording the physical aspects of the water, the simultaneous collection of the water's biochemical data is desired. Water quality sensors are available in single parameter and multi-parameter styles. Some of the more common parameters measured include Temperature, Dissolved Oxygen, Specific Conductance, pH, Turbidity, and Redox. Coming onto the market now are: ammonium, nitrates, chlorides, total dissolved gas and fluorescence, to name but a few, and the list keeps on growing. Not unlike the submersible pressure sensors the signal outputs from these sensors can either be analogue or digital. Most single parameter sensor utilize an analogue output, while the multi-parameter sensors use a digital format known as SDI -12 (Serial Digital Interface, 1200 Baud), which allows the data from the various sensors to be sent over the same set of signal wires.
Having decided on what parameters will be monitored, and subsequently selected the appropriate sensors, one needs to look at the various data loggers available.
Data Loggers, also known as Data Collection Platforms or Data Acquisition Systems, store readings electronically taken from the sensors mentioned previously. Most loggers are programmed through a computer or keyboard. The signal can be from a variety of sensors whose output corresponds to a specific engineering value, whether it be water temperature or turbidity. The recorded information is then stored in the data logger's memory, where it remains until its downloaded by the end user into his/her computer.
Programming the Data Logger can often be a trying experience, as with any new product, which uses software. It is good practice to first review the manuals which come with the data logger and become familiar with the various menus and commands. A lot of valuable time will be saved should you have to explain your problems to the equipment supplier if can navigate your way through the software. Most data logger manufacturers utilize a menu driven format with "pull down" sub-menus to access specific functions (Figure C-1).
Software is installed by placing the manufacturer's disk into the appropriate disk drive of the computer (i.e. Drive A). If you are using a windows based operating system use the "Run" command to access that drive (Drive A) then using the "Browse" command select the software installation program and press the Enter [( ] key to begin the process. If you are using a DOS based operating system, change your drive directory to that containing the software (i.e. cd\A Drive):, then select the installation program and press Enter [( ].
The transfer of information between the data logger and your computer is accomplished using the computers RS-232 serial port. The port can be either a 25 pin, or more commonly, a 9-pin connection. Always ensure that you have selected the correct communications port under the setting sections of the logger manufacturer's software and at the same time check the baud rate (speed at which data is exchanged between the data logger and the computer), (Figure C-2).
Having set up the computer to enable it to communicate with the data logger, the next step is to create your sensor's characteristics. While each logger has its own menu structure, the steps required to create a logging program tend to follow the same basic steps. Typically this will included putting information on Sensor Type (i.e. Temperature, Level...), Signal output, Excitation, Warm-up time, Conversion into Engineering Units etc.
Once the sensors types have been established, the next step is to specify the sampling and reporting intervals for the parameters being recorded. Simple data loggers may only allow a single sampling and recording rate, which would apply to all sensors. More sophisticated data loggers can be programmed to sample individual sensors at various intervals and then calculate and record averages, and maximum and minimum values. Once the program is created, it is saved for uploading into the data logger.
Remember, if you require technical assistance, do not hesitate to contact the data logger supplier. Nothing is worse then getting frustrated over a problem which can be solved relatively quickly using the data logger supplier's expertise. Inquire about training courses as many suppliers can provide training suited to your specific monitoring program.
Prior to connecting the data logger to the computer, make sure that all the connections to the power source and sensors are secure. The data logger can then be connected to the computer, and communications established (Figure C-3). The data logger's time clock should be synchronized with that of the computer or a suitable standard (i.e. Greenwich Mean Time) and then the program can be uploaded into the data logger. Before disconnecting the data logger from the computer, make one last check to ensure that the program is operating properly. Use the software's real-time display capabilities to ensure that the sensors and the data logger are functioning properly. Make sure that your battery is fully charged and able to provide power until you make your next service trip.
Stored data can be retrieved by a number of means. The most common method is by directly connecting the data logger to the computer. However, there are applications where the site is so remote that access is limited. In these cases the data can be sent from the remote location by phone modem, radio transmission, or satellite telemetry. If you plan to use the direct hook-up protocol ALWAYS ensure that you download any recorded values BEFORE proceeding to make any physical adjustments to the logger or the sensors.
Figure C-1. Typical menu-driven software package.
Figure C-2. Check that the correct communication port is selected.
Figure C-3. Programming screen showing types and allocation of sensors being used.
As mentioned previously, sensors will either provide an Analogue or Digital output signal. The data logger stores all the readings in a digital format. Therefore, an analogue signal must first be converted before it can be stored. The data logger uses an Analogue to Digital Converter to perform this process.
The important aspect when choosing the appropriate data logger is to look at the number of "bits" used convert the analogue signal to a digital. Suffice to say the larger the "bit" number (i.e. 14 bits vs. 8 bits) the higher the accuracy. A detailed explanation on Analogue to Digital conversion can be found in the Ministry of Environment, Land and Parks "Interim Data Logging Standards for Water Quality Monitoring" manual.
Recorded data is typically stored in RAM (Random Access Memory). The size of available RAM will dictate how long your data logger can remain in the field before your memory is filled and you have to download the collected readings. Check with the data logger supplier on how their machines records and stores readings and confirm whether the logger will have sufficient capacity for the application it was intended. As you usually only get one try at collecting your data, the safe storage of that data is very important. Most logger manufacturers use a lithium battery to maintain the logger's internal time clock and ensure the integrity of the data. Recently, data logger manufacturers have been using the computer industry PCMCIA type RAM cards as a way of providing additional storage space for readings. In most cases, the card with the stored readings can be replaced in the field with a new one, thus avoiding the problems of taking a computer into a potentially hostile environment.
Sensors put out analogue and digital signals, which are in turn recorded by the data logger. However, not every data logger can accept the variety of output signals provided. Here are some of the most common types.
Two of most commonly used Analogue signals employed by sensor manufacturers are voltages (0-1 V, 0-2.5 V, 0-5 V) and current (0-20 mA, 4-20 mA). In each case the measurement range of the sensor is equal to its full-scale output. For example, a sensor with a 15 psi pressure range, and a 4-20 mA signal will output 4 mA at zero pressure and 20 mA at 15 psi. In most cases, adjustments to correct for minor changes in zero and slope values have can be done via the data loggers' software, but most sensors require shipment back to the manufacturer for proper calibration.
In addition to the two different analogue signals, there are three main types of wiring connections used when connecting a sensor to a data logger. Table C-1 illustrates the various signal types and their respective wiring conventions.
Digital signals are most commonly seen in a format called SDI-12 (Serial Digital Interface, 1200 Baud). SDI-12 sensors have their own A/D converter, allowing them to make corrections for changes in the ambient environment before sending the reading to the data logger. In addition, conversions to the engineering units and adjustments to zero and slope and can be directly inputted into the sensor. The result, is a "smart" sensor, which has a greater accuracy then those of an analogue nature. SDI sensors have an address number, which differentiates one sensor from another and each SDI sensor can have multiple fields (i.e. water level & temperature). SDI-12 sensors use 3 connections when connecting to a data logger.
Table C-1. Signal types and wiring conventions.
Sensor wiring |
Data logger connection | |
Single-ended voltage 1 |
Excitation voltage (+) |
Positive (+) leg from the power source |
Excitation ground (-) |
Negative or Return (-) leg to the power source or data logger ground | |
Analogue signal (+) |
Signal (+) transmitted by the sensor to the analogue input of the data logger | |
Analogue ground (-) |
Negative or Return (-) from the sensor to the ground terminal of the data logger | |
Single-ended current 2 |
Excitation voltage (+) |
Positive (+) leg from the power source |
Excitation ground (-) |
Negative or Return (-) leg to the power source or data logger ground | |
Analogue signal (+) |
Signal (+) transmitted by the sensor to the analogue input of the data logger | |
Differential voltage 3 |
Excitation voltage (+) |
Positive (+) leg from the power source |
Excitation ground (-) |
Negative or Return (-) leg to the power source or data logger ground | |
Analogue signal (+) |
Signal (+) transmitted by the sensor to the analogue input of the data logger | |
Analogue signal (-) |
Negative or Return (-) leg from the sensor to the differential analogue input of the data logger | |
SDI-12 sensor connections |
Excitation voltage (+) |
Positive (+) leg from the power source |
Excitation ground (-) |
Negative or Return (-) leg to the power source or data logger ground | |
Digital signal (+) |
Signal (+) transmitted by the sensor to the SDI data input of the data logger | |
1 The voltage signal is measured with respect to the analogue ground signal. In most cases, the excitation and analogue grounds are commonly connected on the data logger.
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Warm-up Time - Each sensor requires a period of time prior to the reading being recorded in which power is applied allowing the sensor to "wake-up" and the readings to stabilize. For most analogue sensors a warm-up time of one second is sufficient, while SDI-12 sensors may vary from 10 to 30 s. Check the sensor manufacturer's specifications for the correct time period.
Power Source - Most sensors and data loggers are designed to operate off a nominal 12 volt DC power source. In most cases the power supply will be a external battery. It is very important that one uses a deep discharge battery and not a shallow-cycle battery, such as those used in automobiles. The automotive batteries are not designed to withstand repeated heavy discharge/recharge cycles without damage. When hooking up your external battery, it is recommended that at least one lead, usually the positive (+) is fused to prevent shock damage to the data logger should a short circuit occur.
Wiring - When deploying sensors with long signal lines ensure that the wire gauge is large enough to minimize any voltage loss. Using too small a wire size can result in voltage drops, which may in turn manifest itself in inaccurate readings or damage to the sensor.
Operating Temperature - Most data logger and sensor specifications list an operating and storage temperature specification. As electronics are influenced by extremes in temperature, make sure that your selected equipment will operate reliably under the field conditions expected. Most data loggers used in Canada need to be able to operate down to -40°C.
Not unlike the first desktop computers, the first field data loggers were large, cumbersome, limited in capabilities and not very user friendly. Today's data loggers are smaller, faster, and easier to program. With the introduction of Windows 95, many data logger manufacturers are developing compatible software thus making programming much more intuitive.
