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

Level measurement using pressure transmitters! is a post from: Impress Sensors

XMD Smart differential pressure transmitter

The SMART differential pressure transmitter, XMD has been designed and developed for applications in the process industry, it has various possible applications such as level measurement of closed, pressurized tanks, pump and filter control, closed pipe flow measurement in conjunction with orifice plate, venturi or a nozzle.  It’s especially suited to the flow application due to the ability to switch the output from a standard linear output to a square root extraction where flow rate can be output.

The XMD has a differential pressure range from 75mbar through to 2 bar and with a static over pressure capable of 130 bar line pressures.  The calibrated pressure range can also be turned down to 10:1 times so for example a 75mbar range can give a fullscale output at 7.5mbar and still maintain the 130 bar line pressure!

The outputs available are 4-20mA as standard with the option of having HART® communication.  If this option is selected then ATEX approval comes as standard.  There are two possibilities for ATEX approval:

The construction is rugged and designed for harsh environments, the electrical enclosure is aluminium die cast, powder coated and has teh option of integrating a smart display which makes programming of the following parameters easy: electronic damping of signal, offset adjustment up to 90% of FS and turndown of span 10:1 times.  The housing is a two chamber aluminium die cast case and is prepared for fitting of chemical seals to extend the application possibilities.  Applications for the XMD Smart differential pressure transmitter include:

The optional display of the XMD is LCD with a visible range of 32.5 x 22.5 mm, the main display is 5-digit 7 segment with a height of 8mm whilst the secondary line is 8-digit 14 segment with a high of 5mm used for textual indication.  There is also a 1mm high 52 segment bar graph which runs along the bottom for clear and easy interpretation of the output from the device.  The whole device is rated to IP67 even with the display option!

Product Datasheet: XMD Smart differential Pressure Transmitter

XMD Smart differential pressure transmitter with display! is a post from: Impress Sensors

IMD Differential Pressure Transmitter

The IMD Differential pressure transmitter has been developed to cater for applications that may only be looking at a low level of differential pressure (from 350mbar) but with a HIGH static line pressure up to 200 bar!

The IMD has a differential pressure range from 350mbar through to 35 bar and can have a nominal static line pressure of up to 200 Bar which makes this product ideal for low differential pressure measurement across a filter on a pipe or manifold that may have excessively high line pressures.  This has always been an issue for applications and usually results in customers compromising on the resolution and accuracy by using a higher pressure range than what they actually need to achieve the high line pressure requirements!  Outputs include 2 and 3 wire 0/4-20mA and voltage outputs which are ideal for most conventional HMi’s, PLC’s, signal conditioners and data loggers.

The sensing technology used within the IMD is a strain gauge fabricated as part of the silicon chip.  This is then isolated from the media by means of an oil filled cavity and a stainless steel diaphragm which enables the product to be used with liquids or gases.  The housing and sensor is made from stainless steel and incorporates an internal Viton O ring seal making this product suitable for a wide range of applications.  Every device is temperature compensated and calibrated and supplied with a traceable serial number and calibration certificate. The integrated electronics can be configured to have either voltage and current outputs with zero and span adjustability via the internal Pots.

With optional ATEX approval to EEx ia IIC T6 extends the possible applications to Petrochem & Process Industry, other applications include the following:

• Hydraulics
• Pneumatics
• Filter condition monitoring
• Agricultural machinery
• Laboratory testing
• Mechanical engineering
• Environmental engineering
• Automotive testing
• Pumps & compressors
• Medical testing
• Waste water & sewage

Other applications for differential pressure measurement include tank level measurement in a sealed vessel where the air pressure in the top of the tank is measured relative to the hydrostatic pressure of the liquid in the vessel.  Flow measurement in a pipe by means of an orifice plate which creates a differential pressure dependant on the size of the orifice plate.  More can be seen about applications and typical installations following this link.

Product Datasheet: IMD Differential pressure transmitter

Product Page:  Differential pressure sensors

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

IMD Low Differential pressure transmitter with 200 Bar line pressure! is a post from: Impress Sensors