COMMON TECHNIQUES FOR MEASURING DEPTH/LEVEL
The following methods of measuring liquid level are some of the most commonly used in the industry today. We will discuss some of the advantages and disadvantages of each method. We start out with level measurement using pressure transducers then get into the other methods afterwards.
There are several ways to use pressure sensing to measure liquid level; measurement of hydrostatic head using bubble-tube (bubbler) systems, a pressure transmitter mounted outside typically at the bottom of a vessal (usually a tank), or submersible pressure transducers and transmitters.
THE PHYSICS OF DEPTH/LEVEL MEASUREMENT USING PRESSURE
In order to use pressure as a means of determining the level in a vented tank or any column of liquid vented to atmosphere, the following formula must be applied:-
H = P * C / Sg
where H = Height (in meters); P = Pressure in bar; Cf = Conversion Factor of Pressure in bar to meters; Sg = Specific Gravity of the Fluid Being Measured
Level may be calculated in inches, feet, meters, or any other units of linear measurement as long as the proper conversion factor is applied. Appendix A contains many of the most common conversion factors. In the case of water, using Appendix B takes into account the specific gravity changes due to temperature of pure water. If dissolved minerals are present, it is probable that the Specific Gravity is increased and this should be accounted for in the calculations.
Most manufacturers will calibrate their pressure sensors in whatever units the customer wishes. You must give them the proper conversion factors for whatever the units. If none are given, normally mH2O is used.
Specify the level pressure sensor ranged as close to the requirement as is possible in order to achieve the best accuracy possible.
In many applications, absolute accuracy is not critical. In those instances, it is an acceptable procedure to ignore variations in Specific Gravity.
TYPES OF DEPTH/LEVEL MEASUREMENT METHOSDS USING PRESSURE SENSORS:
By using a hollow tube and blowing dry nitrogen into it, measuring the back pressure generated by the depth within the liquid, a reasonable measurement of liquid level may be made. The major advantage of this method is that in extremely harsh environments, the tube, which is less expensive than the pressure sensor, is sacrificed to corrosion. The tube may also be immersed in high temperature liquids with application of the right materials. The acquisition cost is higher than a submersible sensor due to the addition of the Nitrogen or Dry Air source, plus installation and maintenance. Routine maintenance is required in order to maintain reliability. The accuracy is not quite as good as the directly submerged sensor.
This method is also used in open channel flow measurements where hydrostatic head is measured in flumes and weirs.
Tank Level using Pressure Transmitters
Typcially a flange mounted smart pressure transmitter with a self contained diaphragm seal attached to a flange on the side of a tank.
Submersible Pressure Sensors
This method is an accurate, cost-effective method of liquid-level measurement in most applications. A sensor with or without a gauge referenced breather vent in the cable is submerged in the liquid. Given the density of the liquid, the output is directly proportional to hydrostatic head.
Cable Venting Considerations: Shallow Wells
In shallow level (below 600 mH2O [1,969 feet H2O]) applications such as open tanks, open channels, rivers, lakes, canals, unpressurised wells, etc., a vented sensor or non-vented sensor may be used. To avoid the maintenance cost of using a Sensor Termination Enclosure (STE), one could use an absolute pressure sensor along with a barometer, where the barometric pressure is subtracted from the absolute pressure level reading to give gauge pressure. Otherwise if a vented sensor is used (gauge pressure) than an STE box is recommended to keep the vent tube dry.
Cable Venting Considerations: Deep Wells
In deep level (above 600 mH2O [1,969 feet H2O]) applications such as oceanographic surveying a non-vented (absolute) sensor may be used. In closed, pressurized applications, a differential pressure measurement must be made to allow for the pressure on top of the liquid.
This method allows accurate measurement in foaming liquids, in freezing conditions, in harsh environments, and is cost-competitive with most other methods. Major disadvantages are that many suppliers offer materials and designs unsuitable for the application. Corrosion is one of the major causes of premature failure. Leaking O-rings also are a major problem. However, GE Druck's submersible pressure sensors solve these problems by using all-welded titanium or 316 stainless steel metal parts.
GE Druck's small diameter and in some cases, short length, allows installation in the typical well without pulling the existing pump.
For more information, please refer to the "Pressure Level Application Guide" an extensive document (100+ pages) which goes into much more detail about level sensors.
OTHER COMMON TECHNIQUES FOR MEASURING DEPTH/LEVEL
Manual Logging: Graduated Stick
Liquid level can be determined by using a graduated line or pole, weighted at the end, which is dropped into the well. The line/pole is then retrieved and the level is noted where the wet/dry interface occurs. Its main advantage is its low cost. It also can be improvised from common materials. The disadvantages are that it is a manual system, with local indication, relying entirely upon the user for accuracy and measurement recording.
Manual Logging: Dip Meter
A variation is the use of a dip meter that uses a graduated cable and is normally wound on a spool. When the end of the cable (equipped with electrodes) touches the water, the circuit is completed and a galvanometer registers continuity. Some diameters have an audio tone that beeps when the circuit is completed at the surface of the water.
Visual Sight Gauges
A graduated column of liquid may be used to determine liquid level and are typically located outside of the tank. The level is visually checked and recorded. Some of these devices are quite inexpensive and no more reliable than the person taking the reading. In some circumstances, the marking becomes faded or the sight glass becomes clouded, making it difficult to read the level. It is also a local indication. The use of fiber optics allows remote readings to be made, but increases the cost dramatically.
Floats and Switches
A major step forward from the manual and visual systems. It is possible to set up alarms actuated by switches. This enables the system to turn pumps on or off or actuate alarm annunciators. Normally they are used as backup “last ditch” methods if all else fails. The major advantage is low cost. Problems: Sometimes the floats hang-up or freeze-up and will not actuate. It is impossible to test it without elevating the level, thus making it difficult to determine if it is working properly. It also does not provide suitable resolution to determine rate of change in level.
This method is quite reliable and is effective in tank level applications. The float contains a magnet, which slides along the outside of a transducer containing a resonated wire. The magnet’s location along the probe provides a reflection point that can be measured and related to displacement of the float. The major advantage is reliability. Disadvantages include cost, limited displacement, and difficulty of installation. Also in freezing or dirty environments, the float can become stuck.
A high frequency RF signal is generated and directed at the surface of the liquid. The time for the reflected signal to return is measured and related to the distance. Major advantages are accuracy and the fact that it does not contact the liquid, thus reducing corrosion effects. It also can be used on highly viscous slurries, where conventional methods of level measurement are not practical. Disadvantages include high cost, possible errors due to foaming liquids, and inability to penetrate ice layers.
Tank Weighing Systems
These are only effective where load cells can be placed underneath the tank. The tank plus its contents are weighed, subtracting the weight of the tank, leaving the weight of the liquid. By knowing the density of the tank plus the shape/volume, liquid level may be determined. These systems are relatively expensive, hard to install, and not suitable for applications other than tanks.