What to consider when choosing a Water Level Sensor for your Project.
When looking for a submersible water level sensor for your application, there are several factors to consider. Due to the variety of choices on the market today, it is best to sit down and asses your needs vs what is available today.
Understanding Submersible Water Level Sensors and Pressure Transducers
Submersible Water Level Sensors measure hydrostatic pressure (also known as head pressure) exerted by the fluid above the sensing element. The level of hydrostatic pressure is measured by the sensor is affected by two main factors; The density of the liquid being measured and its height above the sensing element. Provided the liquid density remains at a constant, the changes in height can be accurately measured.
Temperature Compensation
There are of course external factors that affect the measurement of liquids. As with many sensors, pressure sensors are sensitive to temperature changes. Often, this sensitivity is significant to the accuracy of the sensor and therefore must be compensated across the expected temperature range of the installed condition. Some sensors have a fixed temperature compensation such as being compensated to 20 degrees Celsius. Others use internal temperatures sensors and logic to measure the temperature of the liquid directly and actively compensate based on temperature fluctuations. This is most important in applications where temperature can vary over time and seasonality.
Barometric Pressure
Another influence is barometric pressure. Water is essentially incompressible under normal conditions. The lack of compressibility of water is useful. It is why we can move water so easily moved through water delivery systems under pressure. Under extreme pressure water can become more dense, but not by much. A square foot of water at the oceans surface is nearly the same size if sent to bottom of the ocean. This means that when barometric pressure changes, and pushes on water a sensor will feel the direct effect of this change in pressure on the sensing element. This is why compensation for barometric pressure is important on water level sensors. Especially those in shallow water applications that require a high degree of accuracy.
In our article on “To Vent and Not to Vent” we previously discussed two main ways of compensating for Barometric Pressure. The traditional method uses a vent line. This allows barometric pressure from above the waters surface to push through the vent line and equalize the pressure behind the sensing element. However, vent lines require desiccants to dry out air in the line or bellows to keep water from condensing in the vent line which can create blockages or worse, introducing water into the electronics. Barometric sensors can be used as well to compensate for the readings on non-vented sensors but require software and logic on the sensors to be able to take this reading and compensate for barometric changes.
Sensor Construction
Often, people forget to consider the construction of materials and make decisions solely based on the cost of the sensor. However, the construction materials are important to the long-term ability of the sensor to perform on site.
Fresh water is relatively gentle on most materials of construction. A sensor made of 316 Stainless Steel for example will resist corrosion in this environment handedly, and the need for a coated sensor or a sensor made of Titanium is not needed. In rivers and lakes where biofouling could be a concern. The build up of biological materials on the sensor may coat the sensor and could grow into the sensing chamber or block the ports to the sensor. In this situation, an antifouling screen, typically made of copper or an alloy of copper will reduce the fouling.
Saltwater and brackish environments, however, will heavily corrode sensors made of 316 Stainless Steel. Add in higher temperatures and the corrosive effect is strengthened even further. Biofilms, Chemicals, Galvanic Corrosion, are all potential as well. In situations such as these it would be best to find another material of construction.
Coated stainless teel, ceramic, and titanium sensors are often used in situations where corrosion would do damage to Stainless Steel. However, each has its advantages and drawbacks. Coating a stainless steel sensor is one way of dealing with the corrosion. The coating process if done correctly seals the steel off from the liquid its emerged in. Coatings, however, are often just as expensive as ceramic and titanium for sensors can be damaged if used in situations where abrasion on the body of the sensor can damage the coating. Ceramic sensors offer high corrosion resistance for a sensor body, however, by their very nature, ceramics can be brittle and should be handled carefully. They can shatter when dropped from low heights. Titanium, known for its strength and light weight possesses the ability to resist the corrosive environment of aquatic environments without being fragile or rubbing off due to abrasion.
Power Sources
All submersible sensors require a power source in order to take a measurement. What power sources are employed is dependent on the sensor and the application, determining if the sensor if sensor is taking line/auxiliary power, running on batteries, or in some cases, both.
Often, “line power”, “auxiliary power” and “external power” are used interchangeably. They all refer to power coming in from outside the sensor itself. On 4-20mA or 0-5vDC sensors, the power supply is the communication protocol as well. The current or voltage loop will vary based upon the reading from the sensor. These sensors are most ideal for applications such as pump control, VFD valve and gate control systems.
Battery Power is commonly used for logging sensors. These are sensors that use internal or in some cases, externally batteries. The self-supplied power allows the sensor to be calibrated, programmed for the proper sampling interval, log the readings, and report them later when polled for the data.
Many battery-powered sensors are sealed units. This means that once the battery dies, the sensor must be replaced or hooked to an external power source to continue operating. Others, like Seametrics smart sensors, have user replaceable batteries. Replaceable batteries extend the useful life of the sensor and lower the long-term cost of owner ship for the user. Replacing a battery is often times much less expensive than buying a whole new sensor. An added environmental benefit is recycling the batteries.. Throwing away sensors with batteries could also be problematic and restricted by local regulations. Again, the user must decide which is best for them.
Analog VS Digital Sensors
Typically, a 4-20mA or 0-5VDC sensor, as described above, are considered analog sensors. These sensors are not designed to log their own data, be field calibrated, or programmed differently than the factory defaults. Any compensation needed on site is usually done on the SCADA, RTU, or VFD being used with the sensor. This includes scaling. For example, a 0-30 PSIG sensor on a 4-20mA system would be 0 PSI = 4mA and 30PSI =20mA. From there the system scaled correctly.
Digital Sensors, (which we refer to at Seametrics as Smart Sensors) have a lot more going on with them. In addition to the factory calibration and programming the user can make changes to these sensors as needed. A field zeroing of the sensor can be done to make it more accurate for that location. It can be programmed for taking readings and user selected intervals or based upon events the sensor detects. Data collected by these sensors can be downloaded for processing later. Many of these sensors are designed to be hooked up to external loggers and telemetry systems for remote or restricted access sites. There are some differences between the different digital smart sensors, so read their specifications to ensure that the sensor will perform on site as required.
Cable Type
Lastly, what type of cable will the sensor be deployed with? Vented (PSIG) sensors require a vented cable and will require a desiccant or bellows system to keep the vent line dry. Non-Vented (PSIA) cables are used with on-vented sensors. If the PSIA sensor is a self-powered smart sensor, then no cable is needed. Just a sealed cap, reliable wire or thread, and hanger system will be needed to deploy the sensor and retrieve it.
Cables can be made differently. The outer shell (Jacket) of the cable can be made of Polyurethane, Polyethylene, or ETFE. Polyurethane PU is flexible, easy to work with and is used most commonly in non-hydrocarbon or solvent contaminated waters. Polyethylene cable is more rigid, a little harder to work with and again not to be used on contaminated sites. ETFE is more rigid than PU but offers a high chemical resistance to contaminated sites. However, with PFAS and PFOS site requirements on the rise, ETFE is discouraged for use on sites where this is a concern or banned.
Reinforcing a cable is important as well. The longer the cable, the more it can stretch. This is due to the increased weight of the cable itself along with the weight of the sensor. Cable stretch varies, and this can be problematic. First, the placement of the sensor may not be accurate based upon the assumed length of the cable provided. Stretch can also damage the conductors inside the cable due to the tensile load. Proper cables should have an internal reinforcing layer, such as Kevlar or steel cables, to help prevent stretch and reduce possible damage to the conductors from cuts to the cable.
Summary
We have covered the most common considerations in purchasing a water level sensor. Other considerations may come into play, but these are the largest considerations we see while working with our clients. If you need more information or would like to discuss what sensor would work with you job requirements the best, please contact Seametrics or one of our trusted distributors.