Water Level Sensors Overview

Stevens is the original water level measurement instrumentation company with the introduction of the widely known chart recorders introduced in 1911. Today, Stevens offers a wide selection of water level measurement sensors including robust ceramic pressure sensors, shaft encoders, acoustical sensors, and visual reference staff gages. Stevens still offers the low-powered, mechanical chart recorders for long-term uninterrupted, real-time chart of water level.

Here is an overview of the different types of instrumentation for monitoring water level.

Pressure Transducers

Pressure sensors like Smart PT perform liquid level measurement by having the sensor submerged at a fixed level under the water surface. The pressure sensor measures the equivalent hydrostatic pressure of the water above the sensor diaphragm, using this to calculate the total liquid depth. This function of a pressure sensor can be compared to “weighing the water”. Pressure sensors are ideal for ground and surface water level applications.

Variances in accuracy of measurement depend on the model of pressure sensor used. The following parameters will influence the accuracy of the pressure sensor:

  • Non-linearity: the deviation of the sensor’s signal curve from that of a straight line.
  • Repeatability: the ability of the sensor to reproduce an output reading when subjected to identical pressures.
  • Hysteresis: the difference in value for the same measured point when pressure is rst increased, then decreased past the point.

Applications

  • Ground water level monitoring
  • Ground water slug testing

Advantages

  • Output can be analog or digital depending on model
  • Smaller diameter stilling well or pipe can be used for installation.
  • A low profile installation site can be achieved using pressure sensors with internal data logging.
  • Easy to install, maintain and calibrate.

Limitations

  • Typically subject to long-term drift and variations with temperatures. However, they are in the water where the temperature is usually fairly stable. It’s a good idea to check calibration every 6 months.
  • Fouling or corrosion with direct exposure to the water can affect the readings.
  • Models are available in a broad pressure range that needs to be known at time of purchases.
  • Some models require breather tube in the cable to reference to atmospheric pressure for best accuracy.
  • Some models have a sensor head that can be easily damaged by human touch or other objects.

Absolute (non-vented) and vented (gauged) pressure sensors

Absolute pressure sensors respond to both atmospheric (barometric) pressure as well as the pressure head of water above the sensor. Therefore, an absolute sensors installation will work provided that a barometric measurement is made at the time of the pressure sensor measurement to compensate for the barometric fluxes. This typically requires a separate barometric measurement instrument and post-processing of the data to compensate for the barometric pressure.

Vented pressure sensors utilize a vented cable—a very small vent tube that run down the length of cable from the surface and terminates behind the pressure transducer. This vent tube acts as a conduit to compensate for barometric pressure changes at the surface, thereby, allowing the barometric pressure on the water column to be cancelled out by the pressure transmitted in the tube.

Comparison of absolute and vented pressure sensors

  • The suspension cable or direct read cable deployment for absolute sensors is less costly than that required for vented communication cable.
  • An absolute sensor typically requires less maintenance, whereas the use of vented cable requires at the very least annual vent tube maintenance and assessment, the purchase of hydrophobic membranes or desiccants to keep moisture out of the tube. However, the absolute method requires a separate barometric measurement that may require additional calibration and maintenance.
  • Vent tubing can crimp, become flooded or blocked by moisture, debris or provide a conduit for water to enter the instrument.
  • Vented sensors require no post processing of the data to compensate for the effects of barometric pressure.
  • Vented sensor measures true water level elevations in that no separate sensor to measure and report on barometric pressure is required. Also, there are no maintenance or calibration requirements of a separate sensor.
  • With absolute sensors there is a higher risk of less accurate results when correcting absolute data with barometric data.

Logging Level Sensors

Cost-effective and reliable, compact groundwater logging sensors like the Van Essen TD-Diver provide accurate and reliable long-term measurements of water levels, temperature, and conductivity—essential data needed for effective water resource management, environmental remediation, mine dewatering, and slope stability.

The Diver family of pressure sensors from Van Essen provide robust level measurement for environmental professionals.

  • Hermetically sealed to external influences—electrical and/or environmental effects cannot affect the measurement results.
  • Include an internal data logger that measures up to 48,000 measurements per parameter.
  • Measure temperature and provide temperature compensated level measurement.
  • Come with internal battery with an extended life up to 10 years.
  • Absolute pressure sensors with ceramic pressure sensor head.

All Diver sensors have a 3 year warranty, up to 10 years battery life, and can be used from 300 m below to 5000 m above sea level.

We offer the entire family of Diver sensors and other logging sensors.

Non-Contact Sensors

Both ultrasonic and sonic level instruments like the APG IRU operate on the basic principle of using sound waves to determine fluid level. The frequency range for ultrasonic methods is ~20-200 kHz, and sonic types use a frequency of 10 kHz. A transducer directs sound waves downward in bursts onto the surface of the water. Echoes of these waves return to the transducer, which performs calculations to convert the distance of wave travel into a measure of height, and therefore water level.

Proper mounting is important to ensure that sound waves are reflected perpendicularly back to the sensor. Otherwise, even slight misalignment of the sensor in relation to the process material reduces the amount of sound wave detected by the transducer. In addition, the installation site should be relatively free of obstacles such as brackets or ladders to minimize false returns and the resulting erroneous response, although most modern systems have sufficiently “intelligent” echo processing to make engineering changes largely unnecessary except where an intrusion blocks the line of sight of the transducer to the target. Since the ultrasonic transducer is used both for transmitting and receiving the acoustic energy, it is subject to a period of mechanical vibration known as “ringing”. This vibration must attenuate (stop) before the echoed signal can be processed. The net result is a distance from the face of the transducer that is blind and cannot detect an object. It is known as the “blanking zone”, typically 150mm – 1m, depending on the range of the transducer.

The requirement for electronic signal processing circuitry can be used to make the ultrasonic sensor an intelligent device. Ultrasonic sensors can be designed to provide point level control, continuous monitoring or both. Due to the presence of a microprocessor and relatively low power consumption, there is also capability for serial communication from/to other computing devices making this a good technique for adjusting calibration and filtering of the sensor signal, remote wireless monitoring or plant network communications. The ultrasonic sensor enjoys wide popularity due to the powerful mix of low price and high functionality.

Applications

  • Water level measurement with the sensor attached to a bridge or structure directly over the water.
  • Flood applications to avoid damage from debris flow

Advantages

  • Easy installation on a bridge or structure over the water.
  • Reduces the problem of sensor fouling or corrosion. Also potential damage from debris is reduced.

Limitations

  • The speed of sound through air varies with the air’s temperature. The transducer may contain a temperature sensor to compensate for changes in operating temperature. However, this only takes into account the temperature at the sensor, which may be different as the sound wave approaches the water.
  • Debris, extreme turbulence or wave action of the water can cause fluctuating readings. Use of a damping adjustment in the instrument or a response delay may help overcome this problem.
  • Maximum distance to the water level surface is typically 30 feet or less.
  • Limited usage in shallow streams or in streams with very high velocities with minimum depth requirements.
  • Very high concentrations of fine sediment in suspension can scatter and absorb the sonic pulse, preventing reflection of a detectable echo.
  • Ultrasonics typically require more power than other water level sensors.
  • Build-up on the sensor head, even simple condensation, can cause problems with the sensor’s operation.

Bubblers

Bubbler systems measure hydrostatic back pressure in order to determine liquid level. A low cost tube (orifice line) is inserted into the liquid to be measured, and air or nitrogen is passed through the tube. The resulting back pressure against the air being pushed out the end of the tube is converted into a liquid level measurement by the device. As the pressure changes, the change in liquid level can be tracked. Typical systems include a pressure source (nitrogen tank or compressor) and a pressure regulator.

A bubbler is similar to a submersible pressure sensor, with the exception that they are typically mounted in a shelter with only the orifice line in contact with the liquid.

Bubbler systems like the OTT CBS are hydrostatic pressure sensors that are ideally suited for accurate liquid and water level measurement, especially in industrial process systems.

Advantages

  • Since measurement is performed by the low-cost tube, the bubbler unit itself can be stored safely away from the liquid.
  • Easy to install.

Limitations

  • Device must be cleaned periodically.
  • If there are changes in density of the liquid being measured the device must be re-calibrated.
  • Requires pressure tank or other external pressure source.

Encoders / Floats

A shaft encoder like the Stevens PAT is an electro-mechanical device used to convert the angular position of a shaft or axle to an analog or digital electrical signal. Part of the mechanical aspect of this device for level measurement utilizes a float and counter-weight attached to a line or tape placed around a pulley attached to the encoder’s shaft.

As the level changes, the float moves up and down and, thereby, rotating the pulley and the attached shaft—generating an electronic wave form for both rotating direction and amount. By converting shaft rotation into electronic signals, encoders are used to electronically monitor the position of a rotating shaft. There are two main types of encoders for liquid level measurements: absolute and incremental.

Absolute encoders provide a binary “word” for each position. Each bit requires a separate optical channel. The resolution is equal to the number of output bits. Absolute encoders constantly retain the correct position for one revolution. Therefore, the main advantage is that the output signal is not affected by a power shut-off. When power returns the encoder recognizes what position it is in based on the voltage measurement reference. Whereas incremental measurements rely on a referenced position pointer. Therefore, if power is shut off to an incremental encoder, the reference is lost and incremental pointer resets to zero.

Incremental (relative) encoders provide a contact or pulse for each increment of shaft movement. Usually this consists of two optical quadrature channels to enable the determination of the direction of rotation. The incremental encoder has a lower cost than the absolute encoder due to the limited number of channels, and the encoded position is not limited in revolutions.

Applications

  • Water level measurement with the sensor attached to a bridge or structure directly over the water.
  • Flood applications to avoid damage from debris flow

Advantages

  • Easy installation on a bridge or structure over the water.
  • Reduces the problem of sensor fouling or corrosion. Also potential damage from debris is reduced.

Limitations

  • The speed of sound through air varies with the air’s temperature. The transducer may contain a temperature sensor to compensate for changes in operating temperature. However, this only takes into account the temperature at the sensor, which may be different as the sound wave approaches the water.
  • Debris, extreme turbulence or wave action of the water can cause fluctuating readings. Use of a damping adjustment in the instrument or a response delay may help overcome this problem.
  • Maximum distance to the water level surface is typically 30 feet or less.
  • Limited usage in shallow streams or in streams with very high velocities with minimum depth requirements.
  • Very high concentrations of fine sediment in suspension can scatter and absorb the sonic pulse, preventing reflection of a detectable echo.
  • Ultrasonics typically require more power than other water level sensors.
  • Build-up on the sensor head, even simple condensation, can cause problems with the sensor’s operation.

Staff gages

Stevens’ environmentally rugged staff gages provide a quick and easy visual indicator of water level. Every water level monitoring station should include a staff gage from which the height of the water may be visually identified and easily compared to any data logger’s reported measurement.

Staff gages come in several standard styles and sizes:

Enameled iron gages are preferred over other type gages (such as painted gages) since they resist rust, corrosion or discoloration and will last almost indefinitely with proper installation and maintenance. Any algae, organic/marine growth or other dirt build-up on the gage is easily washed off.

Stevens gages are typically placed on a redwood, cypress, cedar or synthetic board of suitable width and the board itself it then attached or embedded to the wall. Mounting a staff gage directly to concrete or metal structures is also done, but care should be taken so that the mounting screws are not excessively tightened since this could chip or fracture the porcelain. In order to prevent this, rubber grommets should be placed immediately under the screw head before installing the gage. Stevens staff gages are designed to accept a #8 3/4” round head brass wood screw. Also, each Stevens staff gage includes pre-drilled mounting holes with a brass grommet ring to help avoid any porcelain chipping or fracturing from overtightening.

Contact Meter

Our contact meter is an ideal device for quick, portable measurement of bore holes, wells, reservoirs, and other applications where liquid level depth is needed. The probe is simply fed down and when it reaches liquid the light on the front of the unit illuminates. Depth can then be read from the easy-to-read tape.

The unit comes in several measurement tape lengths, including 15, 30, 50, 100, 200, 300 and 500 meters. The hand drum is attached to a sturdy coated aluminum frame with built-in carry handle. The probe and light are powered by 4 user-replaceable C-cell batteries.

Stevens also carries two optional accessories for the contact meter. One is an acoustic buzzer that emits an audible sound when liquid contact is made. Also available is a ground sensor, which will turn on the light or buzzer once the bottom of the well is reached.