MEASUREMENT OF VELOCITY

Generally it is the volume flow rate which is the most important

quantity to be measured and from this it is possible to calculate a mean

flow velocity across the full flow area, but in some cases it is also

important to know the velocity at a point. A good example of this is in

a river where it is essential for the captain of a boat to know whatstrength of 

current to expect at any given distance from the bank;

calculating a mean velocity from the volume flow rate would not be

much help even if it were possible to measure the exact flow area over

an uneven river bed.

It was exactly this problem which led to one of the most common

velocity measurement devices. A French engineer called Pitot was given

the task of measuring the flow of the River Seine around Paris and

found that a quick and reliable method could be developed from some

of the principles we have already met in the treatment of Bernoulli’s

equation. Figure 3.2.17 shows the early form of Pitot’s device.

The horizontal part of the glass tube is pointed upstream to face the

oncoming liquid. The liquid is therefore forced into the tube by the

current so that the level rises above the river level (if the glass tube was

simply a straight, vertical tube then the water would enter and rise until

it reached the same level as the surrounding river). Once the water has

reached this higher level it comes to rest.

What is happening here is that the velocity head (kinetic energy) of

the flowing water is being converted to height (potential energy) inside

the tube as the water comes to rest. The excess height of the column of

water above the river level is therefore equal to the velocity head of the

flowing water.

LIQUID DENSITY MEASUREMENT

During gas density measurements, when variations in pressure and temperature are small, the temperature and pressure act Difficulties in THE measurement of densities of fluids are due to complexities in processes, venation of fluid dens[i.e!; witting the process, and the diverse characteristics of the process lndfluids governesses. Some of these methods arc: custom designed and applicable to special cases only. Others are very similar in principles and technology, and can be used for many different types of fluids. Presently, apart from conventional methods, many advanced techniques have been developed, for example, density meters based on electromagnetic principles, which are intelligent instrumentation systems. 

Depending on •the applications, fluid densities can be measured both .in static or dynamic forms. In general, static density measurements of fluids are well-developed, precise, and have greater resolution than most dynamic techniques. Chronometer and buoyancy are examples of static techniques that can be adapted to cover small density ranges with a resolution and precision, 

'Today, many static density measurement devices are computerized, coming with appropriate supporting hardware and software. In general, static type measurements. are employed in laboratory conditions, and dynamic methods are employed for real-time measurements where properties of fluids vary from time to time, . 

Density can also he detected indirectly through the measurement of some other process property: Measurement of boiling point elevation is one of the common methods of indirect density detection. Here resistance elements com¬pare the temperature of the boiling process sample with that of boiling water at the same pressure. The differential temperature scale for a particular 'solution can be calibrated in terms of density. This method is also used for end-point determination in evaporators.