Flow measurement

Flow sensor ( also flowmeter ) is a collective term for all sensors that measure the flow of gas or liquid through a pipe.

Next flowmeter is called a device in medical technology, which measures blood flow, and so derive different values. ( Flowmetry ).

A flow measurement is made essentially for three reasons. Firstly, there are commercial reasons. The flow rate is then part of a contract or basis for taxation. Then, the flow rate is integrated over time to give the total amount. Second, there are recipes where multiple quantities of substances shall be mixed to an approach in batch processes. Third, material flows can be directly mixed in a predetermined ratio, even without batch process.

Indicator species

The basic distinction between two types of outputs:

Important characteristics

In the industry employed sensors usually have standardized output signals: either currents from 0 to 20 milliamps or 4 to 20 milliamps or voltages from 0 to 10 volts.

The measuring range can be specified by:

Lower measurement limit, for example, 1 l / min
Upper measurement limit eg 100 l / min

The conventional measuring principles described herein allow reliable measurements down to flow rates of a few ml / min. Below it takes the reliability and accuracy of this method sharply. For smaller flow rates, down to the range of nanoliters per minute, based sensors are used on microsystems technology. These usually work with thermal measurement methods.

The measurement accuracy is usually specified as a relative error, such as 1 % of the current measured value.

The pressure drop is an important parameter because it always means an energy loss in the pipe system. Various types differ in their characteristic pressure loss:

Ultrasonic flow sensor based on the transit time principle without flow straightener: almost no pressure loss
Differential pressure orifice measurement: high, strong with the flow rate increasing pressure loss
Thermal air mass sensor in automobiles: low pressure drop, because small measuring elements protrude into the flow channel

The non-contact ultrasonic flow meter by determining the transit time is a non- invasive flow meter, as no objects interfere with or change the flow in the pipe. There are two formats: Transit time and Doppler. The accuracy is about 0.1 to 2 % relative error. The span, which is the ratio of the smallest to the largest measurable speed is about 1:100.

A basic distinction between clamp-on (that is, the sensors are on the pipe from the outside strapped ) and in-line devices.

Differential pressure method

There are a variety of flow meters that operate on the differential pressure process, such as Pitot tube, Prandtlsonde or orifice plate. All work on the energy conservation law ( flow Bernoulli and Venturi) with the result that the volume or mass flow rate Q is proportional to the square root of the pressure difference between two measuring points. To make the final formula for the user to handle, one often combines all constant values of the measuring device ( pipe and throttle cross sections ) and the measured medium (density) as a constant c together:

The equation shown above is true when the density of the fluid density in the calibration procedure. However, since the density of gases is strongly dependent on the temperature and pressure compensation of this effect can be achieved within certain limits by the following equation.

Wherein air flow meters is usually used for the density calibration.

Besides the classical pitot tubes, there are other vendor-specific types, which can have various advantages under certain conditions, such as lower demands on the inflow conditions, lower pressure drop or simpler installation. However, these devices require, in contrast to the primary elements of a calibration for the particular use condition

The kinetic energy of a fluid (such as water flowing in a pipeline ) is converted to potential energy ( pressure). The best known of the primary element, the shutter, is nothing more than an orifice plate for narrowing the pipe to the fluid to increase velocity forces (increase of the kinetic energy). Thereby, the pressure downstream of the orifice low (reduction of the potential energy ). The span is based on the primary element and quality of sensors and analysis used is between 1:3 and 1:20.

Requirements

For normal measurement points for accurate flow values can be achieved even without calibration in place in operating condition if the following conditions are met:

Flow measurement

Indicator species

Important characteristics

Differential pressure method

Requirements

Primary elements