Liquid Level Measurement Made EasyFollow article
"The application conditions pre-determine the technology that can be used."
This is a phrase one of my colleagues frequently uses during the training sessions we run for solving level, flow, pressure and temperature applications. It is a statement I agree with 100%. It is also the reason why there are so many different Instrumentation Technologies available on the market.
IMPORTANT PROCESS VARIABLES FOR LEVEL APPLICATIONS
If you are looking to solve a level application there are a number of key parameters which will become apparent when looking at the application. Some of these parameters will discount certain measurement techniques, others, for example, process temperature, will define whether a particular sensor model will be suited to the application. Here are some examples of the important questions to ask when looking at a level application:
Point Level or Continuous Level? -
What is it you actually need to detect? Do you want to know once the level in a vessel has reached a certain point? i.e. do you need to detect when the level gets too high for overfill protection / too low to indicate that you are about to run out of liquid or cause a pump to run dry ? Alternatively, you may wish to continuously monitor the level or volume of liquid in the vessel.
How tall is the vessel and where can a sensor be mounted? -
Where can a sensor technology be mounted is key. Can sensors be mounted in the side of the vessel? This is very common for point level tasks. Or is it only possible to mount a sensor from the top of the vessel? How tall is the vessel and is it necessary to measure over the full height of the vessel? This is very important when considering continuous level measurement tasks.
What is the liquid and what are its characteristics ?-
As well as considering if the liquid is corrosive or inert, which of course will impact what materials can be used in any sensor solution, some technologies are sensitive to the conductivity, density or foaming properties of the liquid. Certain sensing technologies will be sensitive to different levels of these properties resulting in erratic readings or even an inability to measure at all.
SIMPLIFYING THE CHALLENGE
It used to be the case that Guided Wave Radar was the preserve of high end process applications with a cost to match. When SICK launched its first LFP Guided Radar probe in 2011 all that changed. For the first time, a guided radar sensor was available for Factory Automation applications at a price point and performance which matched. But how does this technology and the LFP, in particular, solve the challenge of finding a suitable level sensor for your application?
Guided Radar Sensors and the LFP, in particular, are typically mounted into the top of a vessel. The guided radar principle is based on a low power pulse of electromagnetic waves which is guided down a metal rod or rope to the liquid. The liquid reflects the pulse back towards the sensor head. Once received the sensor head turns the pulses time of flight into a level value.
The level value detected by the LFP can then be transmitted to a control system as a continuous analogue or IO-Link level value. In addition via the keypad, the user can configure either 2 or 4 switch outputs (depending on the version) to generate point level information for demand or overfill indication.
But how does the LFP cope with the wide variety of application criteria?
The principle of guided radar is not sensitive to liquid conductivity or density and in the case of the SICK LFP a patented foam algorithm also copes with foaming liquids.
What about chemical resistance?
The wetted parts of the probe have been designed to be very resistant to chemical attacks. The main material in contact with the liquid is 316-grade stainless steel. The chances are, if the tank and pipework are made from stainless steel, the probe will probably not be corroded by the liquid.
Is there anything about the liquid we need to consider?
LFP will work on most liquid types, but the dielectric properties of the liquid are something to be mindful of. The di-electric of the liquid affects the strength of the radar pulse reflected back to the sensor. High di-electric liquids are typically water-based and these provide a strong reflection. Low dielectric liquids are typically oil-based and these provide weaker reflections. For this reason, when using LFP on oil-based liquids the SICK Coax Tube accessory should be used.
Are there any considerations in terms of the vessel?
Many of the vessels used in industry are closed metallic vessels. These are perfect at ensuring a good strong signal returns to the LFP sensor head. In the case of plastic or concrete vessels or vessels with an open-top, the Coax Tube accessory ensures that strong signals return to the sensor.
But what about different vessel heights?
The rod or rope fitted to the guided radar sensor determines the measurement range of the sensor. But this rod or rope is only a guide, it is a piece of metal to guide the radar pulse. In the case of the LFP, this can be cut to length by the installer using a hacksaw. Once complete the installer enters the new length into the sensor menu so that the sensor knows its new length. If a Coax tube is used, this can also be cut to length.
What about other process conditions?
There are 2 LFP product families. The LFP Cubic is suitable for standard Factory Automation processes requiring process temperatures up to 100ºC. The LFP Inox is a full stainless steel model for higher process temperatures up to 150ºC and it is also approved for use in hygienic applications due to the smooth design of its wetted parts and washdown capabilities of the housing.
|LFP Cubic||LFP Inox||Coax Tube accessories|
"The application conditions pre-determine the technology that can be used." Although true, the SICK LFP solves almost every liquid level sensing application. Point level, continuous level, conductive or non-conductive liquid, liquids with or without foam in a wide range of vessel types. On water-based liquids in closed metal tanks, the sensor can be used without a Coax tube accessory and for Oil-based liquids or applications in open or non-metallic vessels the Coax tube ensures reliable measurement results.