levw iet hlm e CLN | ISSUE 03 | OCT-JAN ‘16
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ou or possibly your kids may have heard the urban slang ‘on the level’. It’s used to be completely honest or above-board... givin’ it to you straight with no exceptions, you know what I’m sayin’? Ahem… Basically they are asking if the information is accurate. Engineers with a large tank can relate because they need to know if their media is ‘on the level’. What is hydrostatic measurement? Hydrostatic level measurement is a really simple and reliable method of measuring level. A submersible pressure transmitter is lowered into the media to a specific depth typically known as the zero level.
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The first step to accurately measuring ‘level’ is to understand it.
The transmitter then measures the pressure caused by the weight of the liquid directly on top of it. Due to the hydrostatic paradox, the pressure sensor does not measure the complete volume in the tank, but only the liquid column vertically above it therefore it’s completely unaffected by the shape of reservoir or vessel.
either reach the bottom of your tank or to your desired zero level. Run the cable to your control system to connect the level probe. Use an additional sink weight for a turbulent medium to stabilise the level probe in order to get a stable pressure reading.
“Hydrostatic level measurement is extremely tolerant to disturbances unlike many non-contact technologies...” 2
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How to select the right product for your hydrostatic level measurement
How to compensate for temperature effects
There are 4 main areas to look at which include the measuring range, type of media, accuracy and the electrical connection.
The medium temperature changes the specific gravity of the media. A simple solution would be to add a Pt100 temperature probe or even an integrated temperature sensor within the submersible pressure transmitter. By measuring the temperature of the medium you can easily compensate for the changes to the specific gravity in your control system and correct the level calculation accordingly.
The measuring range should be on the level you want to measure. Stand-alone tanks would be based on the height while underground basins or reservoirs are on how deep you want to submerse the level probe. Consider the media that will be measured and the compatibility of the sensor material and cable type. If the wetted material is not compatible, your sensor will degrade over time and could contaminate the media. Industry standard accuracy of <0.5% will give you a possible error of max. 5mm per metre of level, while highest accuracy variants with <0.1% will be as accurate as max. 1mm per metre of level.
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This leaves the electrical cable, all you need to consider is the appropriate length to allow wiring from the transmitter to the control system and to allow for the immersion length as well. How to install and set up a hydrostatic level transmitter Lower the unit via the cable until you
How to cope with steam, foam, dust, build-up and other disturbances Hydrostatic level measurement is extremely tolerant to disturbances unlike many non-contact level measurement technologies. Submersible pressure transmitters positioned in direct contact at a measuring point below the surface level are not affected by any disturbance above the surface level. Even with direct contact, any buildup or contamination does not disturb the sensor as long as they remain wet with the medium. If the tank or basin is emptied and build-up is allowed to dry, simply clean using a cloth or rinse it with water. For these reasons, hydrostatic level measurement is one of the most reliable technologies you can find on the market.
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