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Going With the Flow - Status Instruments DM4000 Instrument

The measurement of flow is one of the most fundamental process measurement parameters and is used for a diverse range of applications- from the sale of goods such as petrol through to the dispensing and monitoring of sewage sludge.

Flow can be measured to provide either a volumetric quantity or an instantaneous flow velocity which is then normally translated into flow rate.

Over the years, a wide variety of methods have been developed to produce a signal proportional to flow, each with its own merits, shortcomings, complexity and accuracy. The choice of flow measurement methods will depend on a number of criteria including;
Material
Flow rate
Accessibility
Accuracy required
Cost

The wide variety of flow metering methods has resulted in an almost equally wide range of accompanying instrumentation, which can present the user with two main problems.

1. Upgrading of current instrumentation can be difficult or expensive, especially if existing instrumentation has become obsolete within the useful life of the meter.
2. Spares holding on sites where a wide variety of flow measurement methods are used, can be a major headache.

To overcome these problems, Status Instruments have developed their universal DM4000 instrument which gives the flexibility to be able to be integrated with most types of flow meter. Both the DM4000A and DM4000C are smart indicators and can display either flow rate or total. One of these can be chosen by the operator to be the prime display while the other can be viewed by pressing a front panel key

The 4000A accepts analogue signals, typically 4-20mA or voltage. Built into the unit is the ability to enter non-linear functions; square root, power 3/2 and power 5/2 are standard while the user can also enter their own 13 point non-linear characteristic. The 4000C accepts virtually all common types of pulse signals within the frequency range 0.03 Hz to 20 KHz without loss of resolution or accuracy ( Relay, TTL, NPN, PNP, Low Level and NAMUR ).Option cards are available for relays and re-transmission of data which can be assigned to either the rate or the total.. The 4000C also has an innovative non-linear feature whereby a custom characteristic can be applied to the incoming frequency, greatly improving the accuracy of turbine flow meters over the measuring range.

A typical flow measuring system consists of an appropriate sensor, signal conditioning electronics and a display, control or alarm instrument.

Sensors usually measure flow by measuring flow velocity which is directly related to flow rate when the cross section of the pipe is constant.

Flow measurement can be split up into four main categories;

Differential Pressure
Open Channel Flow
Positive Displacement Meters
Velocity Meters

DIFFERENTIAL PRESSURE
This method measures the pressure drop across some form of restriction in the flow line. The relationship between differential pressure and flow rate depends on the type of restriction, the most common being square root, power 3/2 and power 5/2 (rectangular or V)

Because flow is often proportional to the square root of pressure, small zero offsets can cause significant flow errors. The design of the DM4000 compensates for this by changing from a square root function to a linear function below 10% flow rates. In addition, any input below a programmable level may be ‘clamped’ to zero to prevent small offsets at zero flow from adversely affecting the totalised value.

The DM4000A can be used with the most common form of restrictions including:
Orifice Plate
An orifice plate is simply a flat plate installed in a pipe between two flanges, with an accurately sized hole bored in it. The pressure drop across the plate is measured and is proportional to the square root of the flow.

Venturi Tube
These provide a gradual reduction in pipe diameter followed by a similar expansion. The flow rate increases through the neck causing a pressure difference between this point and the inlet pressure. As with the orifice plate, the pressure drop is proportional the square root of the flow.

Pitot Tubes
Here a tube is inserted into the flow at right angles. Holes facing the flow measure the impact pressure and this is compared with the static pressure. Again the difference is proportional the square root of the flow. Although they are low cost and easy to install, small holes make them susceptible to blockage.

OPEN CHANNEL FLOW
Open channel flow refers to flow which has one free surface. Several types of restriction are in common usage which cause a ‘head’ of fluid to back-up, the height of which is proportional the flow rate. The most common types of restriction are:

Weir
This is the open channel equivalent to the orifice plate and is normally either rectangular or 45 degree ’V’ notch. The DM4000 instrument has the required power laws as standard.

Flume
This is the equivalent of the venturi and a narrowing of the channel produces an increased flow rate which helps to keep the flume clean.
The flume produces a smaller loss of head than the weir and uses the 3/2 power conversion, again a standard 4000 feature.

POSITIVE DISPLACEMENT METERS
These split the flow in small discrete ‘packets’ and count each one as it passes through the flow meter. Each packet has an accurately defined volume and therefore these meters are usually very accurate making them ideal for automatic batching or dosing of clean liquids. The DM4000 can accept incoming pulses from this type of sensor to provide flow rate and totalising information. The common types of this type of meter are:

Oval Gear Meters
Two closely meshing oval gears allow a measured quantity of fluid to pass with each revolution. The DM4000 can accept a wide range of pulse input types to measure shaft rotation and hence compute flow rate.

Reciprocating Piston Meters
Here an osculating piston or pistons are moved as the fluid passes the meter and a magnetic pick up counts the number of collations. The DM4000 accepts low level signals from magnetic pick ups and converts the count to flow rate.

Rotary Vane Meters
The passage of fluid through this type of meter forces the vanes around, the number of rotations being directly proportional the volume passing through.
The DM4000 is compatible with most methods of measuring shaft rotation and coverts this to flow.

VELOCITY METERS
This type of meter uses a variety of techniques to measure the velocity of liquids in pipes and hence flow. These can vary from turbine meters, which have a turbine blade rotating in the fluid, to magnetic or acoustic flow meters, which are totally non invasive and are therefore ideal for retrofit and hygienic applications.

Turbine Meters
This type of meter performs well in clean fluids and can be very accurate (0.25%).
The speed of rotation of the turbine in measured via a magnetic pick up or optical means and the resulting pulses can be processed by the DM4000.

Mag Flowmeters
These are non invasive hence can be used for aggressive fluids or where hygienic installations are required. They are suitable for most electrically conductive fluids and are based on Faraday’s law, which states that of a conductor (the fluid) is moved through a magnetic field, the induced voltage is proportional to the magnetic field and speed (flow) at which it traverses the field. Most Mag flowmeters provide a 4-20mA output directly proportional to flow.

Vortex Shedding Flowmeters
This method involves the artificial creation of vortices in the fluid by inserting a small strut across the flow. The rate of shedding of these cortices is proportional to the flow and is measured by a sensor. Most types provide a pre-calibrated 4-20mA output proportional to flow.

Doppler Flowmeters
In Doppler Flowmeters, an ultrasonic transducer excites the fluid with waves of a known frequency and then measures the reflected waves. Because the fluid is moving, the returned frequency is altered and the degree of this frequency change is proportional to flow. Doppler Flowmeters have built-in electronic conditioning and most provide a 4-20mA output proportional to flow.

Time of flight
These use an ultrasonic generator and a down-stream receiver. The time taken for the pulse to reach the receiver is proportional to the speed of flow and most meters provide a 4-20mA output which can be processed by the DM4000. Distortion and scatter of the ultrasonic signal can be a problem.

Conclusion
The DM4000 offers a comprehensive solution for processing signals from flow measurement equipment and can readily be integrated into existing installations. The unit provides excellent user flexibility and is easy to install and set up. Key features ensure optimum performance and long term reliability. The DM4000 is part of Status Instruments’ wide range of instrumentation and data logging equipment, designed specifically for discerning engineers.
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For more information, please contact :-

Status Instruments Ltd
Green Lane, Tewkesbury, Glos GL20 8DE
Tel: +44(0) 1684 296818
Fax: +44(0) 1684 293746
Email:
sales@status.co.uk
Web:
www.status.co.uk

May 2007

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