Ultrasonic, Transit-time Flowmeters

Ultrasonic flowmeters measure fluid velocity by passing high-frequency sound waves along the fluid flow path. Fluid motion influences the propagation of these sound waves, which may then be measured to infer fluid velocity.

Transit-time flowmeters, sometimes called counter propagation flowmeters, are an alternative to Doppler ultrasonic flowmeters. A transit-time ultrasonic flowmeter uses a pair of opposed sensors to measure the time difference between a sound pulse traveling with the fluid flow versus a sound pulse traveling against the fluid flow. Since the motion of fluid tends to carry a sound wave along, the sound pulse transmitted downstream will make the journey faster than a sound pulse transmitted upstream:

The rate of volumetric flow through a transit-time flowmeter is a simple function of the upstream and downstream propagation times:

Q = Calculated volumetric flow rate
k = Constant of proportionality
tup = Time for sound pulse to travel from downstream location to upstream location (upstream, against the flow)
tdown = Time for sound pulse to travel from upstream location to downstream location (downstream, with the flow)

An interesting characteristic of transit-time velocity measurement is that the ratio of transit time difference over transit time product remains constant with changes in the speed of sound through the fluid. When this equation is cast into terms of path length (L), fluid velocity (v), and sound velocity (c), the equation simplifies to Q=2kv/L, proving that the transit-time flowmeter is linear just like the Doppler flowmeter, with the advantage of being immune to changes in the fluid’s speed of sound. Changes in bulk modulus resulting from changes in fluid composition, or changes in density resulting from compositional, temperature, or pressure variations therefore have little effect on a transit-time flowmeter’s accuracy.

Reprinted from "Lessons In Industrial Instrumentation" by Tony R. Kuphaldt – under the terms and conditions of the Creative Commons Attribution 4.0 International Public License.

Industrial Point Level Measurement

As opposed to continuous level measurement, point level measurement is used to measure liquid or bulk solids in a tank or vessel to a single, precise point. Most often used as a notification for high or low level, they are placed at the top or bottom of the vessel. When activated, the point level switch can activate an audible or visual alarm, start or stop a process, and/or notify a supervisory control system. There are several technologies that provide point level control, each with its own advantages and disadvantages. This post focuses the more advanced technology used for point level control, namely capacitance, vibration, microwave, and radiometric.

Capacitance Switch
Capacitance Switch

Capacitance Switches

  • Measure the capacitance of the process medium’s ability to hold an electrical charge.
  • Can be used with both conductive or non-conductive liquids and solids
  • Looks for step changes in standing capacitance.
  • When media reaches the area between the two plates, the capacitance changes, and the switch output changes state.
  • Must be installed and set up in process to know the different capacitance values for a vessel that’s full and a vessel that’s empty.

Vibration Switches

Vibration Switch
Vibration Switch
  • Constantly vibrate at a specific frequency.
  • The frequency or amplitude is monitored to determine when a liquid or a bulk solid has reached the switch point. 
  • Once the medium contacts the sensing probe, the frequency or the amplitude changes and this change activates the output.
  • Easy installation.
  • Are "plug and play".
  • No media calibration requirement.
  • Little or no maintenance costs.

Microwave Switches

Microwave Switch
Microwave Switch Emitter
  • Non-contact.
  • Utilizes an emitter and receiver.
  • Use a microwave beam to determine point level. Once the bean is broken, a switch changes status.
  • Not only used for high or low level, but also for determining id material is filling a vessel or falling to a conveyor.

Radiometric Switches

  • Non-contact
  • Utilizes an emitter and receiver.
  • A detector measures the amount of gamma radiation from an emitter and determines point level when a threshold is met.
  • Can be installed externally, with no vessel penetration.
  • Good for volatile and hostile processes.
  • Radiometric Switch
    Radiometric Switch
  • Can measure changes in gamma radiation and be used to detect building vapors.

For more information or any process level requirement, contact Classic Controls by visiting https://classiccontrol.com or by calling 863-644-3642.

Industrial Control Valve Body and Actuator Operation

Control valve
Control valve (Masoneilan)
The design and operation of industrial control valves  is very important to understand if you work as a process engineer, a plant maintenance person, or if you design process control loops.

Control valves are used extensively in power plants, pulp and paper mills, chemical manufacturing, petro-chemical processing, HVAC and steam distribution systems.

There are many types, manufacturers, body styles, and specialized features, but the they all share some basics operating principles. The video below explains components, operation, and fundamentals.

Classic Controls provides control valve systems for all major industries including chemical, pulp and paper, petro-chemical, power generation, and water treatment.  Classic Control’s experience and engineering background make them a uniquely qualified partner for your next automated valve requirement.