24 WATER / WASTEWATER


Tech Talk – What are the key differences between a Gauge (Vented) and an Absolute (Non-Vented) pressure water level sensors?


Firstly, how does a pressure water level sensor work?


A pressure water level sensor operates by measuring the force exerted onto the units’ diaphragm and converts this to a reading for water level. The further under water the units are the deployed the higher


the amount of pressure is exerted onto the units’ diaphragm. There are commonly two types of pressure sensors for the measurement of water level, these are Gauge, also called vented and absolute, which is also referred to as non-vented.


What is the difference between Gauge and Absolute Water level loggers?


A gauge water level sensor will not only measure the pressure exerted onto the sensor diaphragm, but it will also correct for changes in air pressure. This is important as it means that the unit can directly compensate for these air pressure changes, which directly have an affect on the water level if it is an open water body. To achieve this the unit has a cable which connects to the back of the sensor. The cable features a hollow tube inside that is vented to the atmosphere allowing for automatic compensation for changes in air pressure.


In comparison an absolute water level sensor will measure the pressure exerted onto the diaphragm but will not measure changes in barometric pressure as the unit is completely sealed. Because of this, users will also need to use a separate device to measure changes in barometric pressure; a LeveLine-BARO. Once water level data and barometric data is collected users can compensate the water level for the changes in barometric pressure within the LeveLink software.


Which one should I choose?


Your selection or water level sensor or water level data logger will really depend on your application and your requirements. If your deployment is going to utilise telemetry, then our advice would be to select a gauge or vented sensor. A telemetry system requires the use of a data cable to connect to the Leveline, a vented sensor requires the use of a vented cable. In addition, using a vented sensor means the data does not need to be compensated post collection. This makes data handling simpler.


However, gauge is not always best. As the units features a vented cable, the opposite end of the cable to the sensor will need to be kept dry, this is typically done using a desiccant cartridge, which stops water vapour entering the water level sensor trough the vent tube in the cable. These also need to be maintained, otherwise they can become saturated and allow moisture to enter the vent tube potentially blocking the vent. This would also need to be consider if the location is prone to flooding. In this instance the cable end with the desiccant should be installed at a height where water submergence can’t occur.


So, you may be thinking why would I use an absolute sensor? Well there are many advantages of an absolute / non vented solution. The first is that these water level sensors are completely sealed.


The advantage of this is they require less maintenance and can also be installed without needing a data cable. They can simply be hung on a steel wire or Kevlar chord. This can reduce cost, as the cables used to connect to the level sensors can be expensive. Another advantage is that you can use one barometric device to compensate more than one absolute sensor. This means you could have 10 or 20 devices located within a 10km radius all compensated by a single barometric device.


Absolute systems are also good for people who will be using the device for several applications, rather than having them permanently installed. In this instance a gauge type level sensor may become impractical as you may require varying cables lengths for different applications.


I have chosen between Gauge and Absolute, are there any other considerations?


There are a couple of further considerations to make. Firstly, how will I collect the data? Options include a water level sensor with an internal battery and datalogger, such as our Leveline, or a device which will connect to a telemetry system such as our Point Green, which can be combined with our Leveline Mini sensor.


The final consideration relates to the application itself. Will the conductivity / salinity vary at the location where I am monitoring? If you are measuring a marine or estuarine based application the answer will probably be yes, therefore our advice would be to have a device which can provide correction for density changes by measuring the conductivity and salinity. For this we would recommend a device such as our Leveline CTD, if a local datalogging model is required, or our Leveline Mini CTD with our Point Green telemetry system if you require remote access to the data from the unit. Both add a conductivity sensor to their measurement parameters to provide density correction.


If you require any further guidance or information please feel free to contact the Aquaread team – info@aquaread.com / 01843 600030


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World class multiparameter water quality testing probes now include built in data


logging capabilities The new AquaSonde range from Aquaread brings built in data logging capabilities to our range of reliable multiparameter water quality testing probes. The AquaSondes are powered by internal lithium batteries to extend the duration of your deployments for as long as 180 days, model and logging rate dependent. All AquaSondes feature an internal memory that is capable of storing up to 150,000 full data sets, that equates to over 3 years continuous data logging.


These logging devices can be deployed on their


own for a discrete deployment or they can be deployed with a vented cable allowing for barometric compensation of measurements, specifically depth and % saturation of dissolved oxygen. Each AquaSonde is supplied with a QuickDeploy Key, used to initiate the probe’s logging regime and SondeLink PC software for complete logger set up, sensor calibration and data collection.


The Aquasonds feature Programmable logging, event and cleaning rates. The Fastest logging rate is 0.5Hz and slowest logging rate is 120 hours. Event testing and logging is possible on any single parameter programmable between 1 minute and 99 hours. Programmable cleaning rate is available when using the AP-7000 with its built in self cleaning system.


SondeLink is the free PC application used with all Aquasonds to provide the following features via an integrated USB interface:


• Live data viewing • Live data logging directly to PC


• Full calibration with calibration report generation • Retrieval of logged data


• Logged data output to spreadsheet and text files • Full setup utility


• Site name and GPS geotagging


Each AquaSonde is supplied with a Quick Deploy Key. When fitted to the AquaSonde, this unique device performs the following tasks:


• Seals the connector • Automatically starts the pre-programmed logging regime • Provides instant visual indication of AquaSonde health, battery and memory condition.


This allows all programming of the AquaSonde to be performed at your office using the PC application and the logging regime to be started at the precise time of deployment. It also provides peace of mind that the AquaSonde is operating correctly at the time of deployment.


All models feature an internal barometric pressure sensor that is used when calculating Depth and percentage saturation of Dissolved Oxygen. If the AquaSonde is to be deployed for more than a day at a time and accurate Depth and %DO values are required, a vented cable is recommended. For profiling, dip testing or short-term deployment during which time the change in barometric pressure will be negligible, a vented cable is not necessary.


Vent / Data Hub


If a vented cable is to be used with the Aquasond then a special Data Hub is required. This option is a termination device for the vented cable that allows a desiccant bottle to be attached and provides a USB port for data retrieval and a visual indication of AquaSonde health, battery and memory condition.


By attaching a PC running SondeLink to the USB port, direct access can be gained to the AquaSonde allowing live data viewing, live data logging directly to PC, retrieval of logged data and full setup, all whilst the AquaSonde is submerged.


Finally there will soon be options for Bluetooth connectivity for use with mobile apps. Embedded site data and GPS geotagging via the app.


More information online: ilmt.co/PL/ZJjl For More Info, email:


distribution agreement Aquaread have recently signed an agreement with PID Technologia to represent them in Peru. This has strengthened their South American presence and PDI


Technologia join other distributers in Argentina, Brazil, Chile, Colombia and Uruguay representing Aquaread.


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PID Technologia has been supplying the Peruvian market with instrumentation, services, and bespoke monitoring systems for over a decade.


55881pr@reply-direct.com


Edward Lang, Aquaread’s South America Sales Manager stated, “Aquaread are going from strength to strength in South America, where we are establishing a good network of dealers, who have high levels of experience and enthusiasm’.


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