Reliable Level Control requires reliable level measurement. There are many ways to measure level. If you ask someone what you should do, they’ll recommend what they’ve had good experience with, or what they’re selling. Simple float switches will work in most applications, but sometimes applications require a detailed analysis to determine the best way to measure liquid level.
Level Control • Liquid Level Measurement Application Details
- What are the properties of the liquid you want to monitor ?
- Is the liquid water, oil, a corrosive chemical, etc. ?
- Is the liquid under pressure, or exposed to atmospheric pressure ?
- Is the liquid hot, or cold, or exposed to ambient temperature ?
- What are the properties of the container ?
- Is the container a vertical cylinder, a horizontal cylinder, or box shaped ?
- What is the container depth, and depth range to be monitored, in inches or feet ?
- Is there a source of turbulence (pump, mixer, etc.) in the container ?
- Where do you want to locate the measurement device within the container ?
- Through Top (Liquid Contact) – Float Switch, Conductance Probe, Submersible Transducer, Bubbler
- Through Side Wall (Liquid Contact) – Float Switch, Capacitance Probe, Differential Pressure
- Through Top (No Liquid Contact) – Ultrasonic, Radar
- How do you want to use the liquid level information ?
- Point Level – Control devices (pumps, valves, alarms, etc.) at pre-determined levels
- Continuous Level – Monitor level, calculate volume and/or flow rate, control devices.
Each level measurement technique and device has advantages and disadvantages relating to cost, ease of installation, ease of troubleshooting, and maintenance. Below are some of them.
Point Level – Float Switches
Float Switches are simple and will work in most point level control applications. They are inexpensive and available in many mechanical and electrical configurations.
- Construction – Float Switches typically consist of an electrical switch inside an air filled bulb at the end of a long electrical cord with a weight located near the end of the cord. The electrical switch changes state (opens or closes) based on the orientation of the air filled bulb.
- Installation – Float Switches are typically installed by using its cord to lower it into the liquid container until the float switch is hanging at a pre-determined level.
- Operation – When the liquid level is below the hanging float switch (air filled bulb), its internal electrical switch is open or closed, and the cord mounted weight is above it. When the level rises above the hanging float switch, the air filled bulb rises above the weight. This changes the orientation of the air filled bulb and the internal electrical switch changes states (opens or closes).
- Advantages – Inexpensive and readily available from many manufacturers and vendors. Simple to install and troubleshoot. Low Maintenance.
- Disadvantages – Makes contact with the liquid so it can be negatively affected by the liquid or any suspended impurities. Must be mounted within the container at pre-determined levels.
Point Level – Conductance Probes
Conductance Probe are relatively simple and will work in many point level control applications. They are more expensive and more complicated than float switches, but they provide more precise level detection.
- Construction – Conductance Probes typically consist of a conductance probe (electrical rod) holder designed to accommodate two or more conductance probes (electrical rods) cut to specific lengths, and one or more conductance sensing relays.
- Installation – Conductance probe holders are typically flange mounted through the top of the liquid container. Conductance probes (electrical rods) are cut to length and installed so they hang from the holder to each desired level detection point, with an extra (longest probe) cut to length and installed so it always hangs in the liquid.
- Operation – The longest probe is always in the liquid acting as an electrical common. As the liquid level rises above each shorter probe, the liquid acts as an electrical conductor connecting it to the longest probe. A conductance sensing relay is used to detect the change in electrical resistance between each shorter probe and the longest probe where low / high resistance means the liquid level is below / above the shorter probe.
- Advantages – Relatively inexpensive and available from a few manufacturers and vendors. Relatively simple to install and troubleshoot. Low Maintenance.
- Disadvantages – Makes contact with the liquid so it can be negatively affected by the liquid or any suspended impurities. Must be mounted within the container with fixed probe lengths.
Continuous Level – Submersible Transducers
Submersible Transducers are relatively simple and will work in many continuous level control applications. They are relatively inexpensive and available in many mechanical and electrical configurations.
- Construction – Submersible Transducers typically consist of a pre-calibrated pressure sensor inside a stainless steel housing at the end of a long vented signal cable. A port (hole) on the stainless steel housing exposes one side of the pressure sensor to the liquid and the vent (hole) in the vented signal cable exposes the other side of the pressure sensor to atmosphere. Many options are available.
- Installation – Submersible Transducers are typically installed by dropping it into the container. It is typically secured to the container side wall to fix its location below the lowest level to be monitored.
- Operation – A Submersible Transducer measures liquid level by measuring the weight of the liquid above it. As the liquid level rises and falls, the weight of the liquid above it rises and falls. A DC Power Supply, Signal Conditioner and Controller are required to power it, interpret it’s signal and control devices.
- Advantages – Relatively inexpensive and readily available from many manufacturers and vendors. Relatively simple to install and troubleshoot. Low Maintenance.
- Disadvantages – Makes contact with the liquid so it can be negatively affected by the liquid or any suspended impurities. Must be mounted within the container at a pre-determined level.
Continuous Level – Ultrasonic / Radar
Ultrasonic and Radar are relatively complicated, but will work in many continuous level control applications. Both are relatively expensive and use reflected waves to measure distance. Ultrasonic is typically less expensive and simpler to install than Radar, but Radar can handle more difficult applications.
- Construction – Ultrasonic and Radar level measurements systems typically consist wave generator, a wave receiver, a wave guide, and a means of determining the time between when a wave was generated and when it receives the generated wave’s reflection.
- Installation – Ultrasonic and Radar level measurements systems are typically flange mounted through the top of the liquid container at a location where they have a straight down clear line of light to the bottom of the container. A wave guide is typically provided to keep stray reflections from creating interference. In Ultrasonic applications the wave guide is also called a stilling well because it also keeps the liquid surface still (flat) so the wave reflects back to the unit.
- Operation – Ultrasonic and Radar level measurements systems operate by emitting a wave and timing how long it takes to receive its reflection.
- Advantages – Does not make contact with the liquid so it isn’t negatively affected by the liquid or any suspended impurities. Low Maintenance.
- Disadvantages – Both are relatively expensive. Both are relatively difficult to install and troubleshoot. The presence of fog or foam above the liquid creates a problem for Ultrasonic. Both have a minimum sensing distance and Radar’s minimum sensing distance is longer than Ultrasonic’s.