Level measurement can be achieved using many different forms of electrical and mechanical devices such as ultrasonic, radar, guided wave, capacitance, micro-wave, hydrostatic, float, vibrating probe and many more.  The first decision is to establish if you want single point level measurement to just trip and alarm response when a level hits a certain point or a continuous output of actual level for calculative, predictive and preventative measures to be made.

One common technique is to use pressure sensors which will determine the level by measuring the head of pressure present above the device, this can be useful in applications such as open tank level measurement and borehole submersible applications where the device is submersed to the bottom of the tank or borehole and measurements are returned based on the hydrostatic head above the sensor.  In applications where you may have a closed tank or pipe then a differential pressure transmitter can be used.  the following describes the typical installations for such an application:

Closed pressurised vessel with ‘non-volatile fluid’

In closed vessels any pressure at the top of the vessel will affect the pressure at the bottom. To measure the true level, the vessel top pressure must be subtracted from the pressure reading taken at the bottom of the vessel.

The level may be measured by taking a pressure tap from the top of the vessel to a differential pressure transducer which is also connected to a pressure tap at the low point of the vessel. The vessel pressure is now applied to both the measurement and reference sides of the transducer. The resulting differential pressure is proportional to the liquid height multiplied by the specific gravity of the liquid.

A typical installation arrangement for low viscosity liquids using a standard DP transducer is shown in the diagram.  If a high viscosity or liquid with solids in suspension is to be measured, a standard DP transmitter cannot be used as the process port connected to the bottom of the vessel could become blocked. A flanged DP or a transmitter with a remote seal should be used in these circumstances.

In closed vessels any pressure at the top of the vessel will affect the pressure at the bottom. To measure the true level, the vessel top pressure must be subtracted from the pressure reading taken at the bottom of the vessel.

The level may be measured by taking a pressure tap from the top of the vessel to a differential pressure transducer which is also connected to a pressure tap at the low point of the vessel. The vessel pressure is now applied to both the measurement and reference sides of the transducer. The resulting differential pressure is proportional to the liquid height

Differential pressure transmitter installation

multiplied by the specific gravity of the liquid.

A typical installation arrangement for low viscosity liquids using a standard DP transducer is shown in the diagram.

If a high viscosity or liquid with solids in suspension is to be measured, a standard DP transmitter cannot be used as the process port connected to the bottom of the vessel could become blocked. A flanged DP or a transmitter with a remote seal should be used in these circumstances.

Closed pressurised vessel with ‘volatile fluid’

The basic measurement configuration is similar to the non-volatile fluid arrangement described above, however if the gas above the liquid condenses at the system operating temperature, the piping to the reference side of the transducer will slowly fill up with liquid creating a measurement error. To eliminate this potential error, the impulse pipe to the transducer reference port should be filled with condensate or a compatible fluid which has a specific gravity at least as high as the process fluid. This provides a reference port pressure that varies only with the top pressure of the vessel.

In this configuration the fluid-fill in the reference leg will exert a head pressure on the reference side of the transducer causing the output to be negative when the vessel is empty. This should be considered when selecting the FSD range of the DP transducer as DP transducers by design will only respond to a maximum negative pressure input of 50% FSD. It should also be remembered that the transmitter electronics will need to be have a zero offset applied to provide zero output at minimum vessel level.

Differential pressure transmitter in volatile media

If a high viscosity or liquid with solids in suspension is to be measured, a standard DP transmitter cannot be used as the process port connected to the bottom of the vessel could become blocked. A flanged DP or a transmitter with a remote seal should be used.

Alternatively a DP transmitter with two remote seals can be used. Whilst the initial transmitter cost is greater, the installation costs and long term costs of ownership are lower as the installation and maintenance of the fluid filled reference leg is no longer required.

Closed Pipe Flow Measurement

Principle of measurement

All pressure based closed pipe flow measurement systems use either an Orifice Plate, Pitot Tube, Venturi or Nozzle, to produce a differential pressure which is linearly proportional to the square of the flow. In order to obtain a linear to flow signal, a square root function must be applied either in the DP transmitter itself or to the output signal from the transmitter.

To meet basic accuracy requirements British and International standards specify a minimum straight pipe length both before and after the flow meter. Most general industrial applications this can be approximated to a minimum of 10 times pipe diameter upstream and 6 times pipe diameter downstream.

In order to specify a complete flow measurement system the following information needs to be established.

1. Internal diameter of system pipe work at the point of measurement.
2. Flowing media.
3. Maximum and minimum flow rates.
4. Media temperature and pressure.
5. Flange details for mounting of Orifice plate, Nozzle or Venturi or any special boss details for a Pitot tube assembly.

Open Channel Flow Measurement

Principle of measurement

Open channel flow is undertaken by measuring the height of liquid flowing over a weir or through a flume.  The level can be measured by a low range submersible transducer immersed either directly up stream off the flow centre line or in a stilling well formed to the side of the channel. Alternatively a bubbler system can be used.

Weirs are most often employed for clean water applications or on large flows such as rivers of culverts. For a weir two types are normally employed; Rectangular (including full channel width) in which the flow is proportional to head generated raised to the power of 3/2. 90°V-notch in which the flow is proportional to head generated raised to the power of 5/2.

Flumes are most often used for dirty liquids with solids in suspension as their smooth shape minimisessolid build up. The detail design of flumes can vary but generally the flow is proportional to the head generated raised to the power 3/2 approximately. The selection chart identifies suitable transducer types and relevant transmitter series to allow selection of an appropriate measurement solution.

The Impress Sensors range of indicators can be used with remote transducers & transmitters or any of the loop powered transmitters to form a complete cost effective measurement and control package where a system solution is required to give an output linear to flow.

If you have any applications you wish to discuss or any questions questions regarding this product then please contact sales here sales@impress-sensors.co.uk