RADIOMETRIC MEASUREMENT OF DENSITY
WHY SHOULD DENSITY BE MEASURED ?
In many industrial processes knowledge about the density is required - be it specific gravity or bulk density - to draw a conclusion on process parameters.
In many instances density is only a measure for other properties: By using density it is possible to determine the concentration of a mix consisting of two materials or the content of solid matter when transported through pipes.
DENSITY CAN BE MEASURED CONTINUOUSLY IN RUNNING PROCESSES.
WHAT ARE THE CAPABILITIES OF RADIOMETRIC DENSITY MEASUREMENT?
The result of the absorption measurement is influenced by the average density between the detector and the source of radiaton. All those parts which are crossed by the measuring beam and do not change
during operation are included in the calibratetion. Since only the meduim to be measured is subject to change, it is possible to determine exactly the density.
It is important that the average density is used. Thus one can conclude to other properites from this measurement: specific density, concentration of solid matter (i.e. coal, ore, salt) and concentration of mixes (acids, lye, content of sugar).
HOW DOES IT WORK ?
An electromagnetic wave called Gamma radiation with the same properties of X-rays is weakened according to its density by passing through matter . When a fixed intensity is know relating to its density, the ratio to every other density can be calculated according to the intensity it lets pass through. Like an x-ray photograph has its dark parts where the density is higher also the intensity of the Gamma-rays becomes lower with a higher density. A high accurat determination of the intensity therfore is able to determine the density to a point where laboratories sometimes have difficulties to undercut the error. To achieve this only a few components are necessary: The source, installed in a secure lead shielding, further a high sophisticated detector and an evaluation unit.
WHERE CAN MEASUREMENTS BE PERFORMED ?
A constant measuring distance is necessary to determine the density exactly. The simplest way to achieve this is measuring through a pipe. But it is also achievable in a tank or a bunker by a special construction. Contactless measurement is also possible in a container: Neither the source of radiation nor the detector touch the medium.
THE SOURCE OF RADIATION:
To obtain a simple source of radiation an artificial radioactive metal or metal oxide is used. It is welded into a high security capsule made from stainless steel. The capsule is thoroughly checked
according to international law - for example it was heated up to a very high temperature and then put into ice water, a shock test was performed etc.
The radiation can emit through the wall of the capsule, so the radioactive material can remain in it forever. Since the source emits radiation in all directions, it is built into a container made of lead. This container has a small channel facing the detector. The channel can be closed for transport or during work near to the measuring beam.
When radiation is to be measured it must be absorbed. It passes through a gas detector nearly aneffected so to measure a certain amount it needs a bigger source. A high dens material can absorb all the radiation comming in. This type is called scintillation detector with crystal which converts radiation into flashes of light. The light is amplified by a photomultiplier and is then registered electronically. A special stabilizing circuitry which analyzes the spectrum of the radiation, and keeps constant the characteristics, provides for exact measurement in spite of altering temperature and aging.
Since the detector already delivers a digital signal, the successing circuitry only has to perform the mathematical conversion and the calibration. A processor card built with SMDs allows for all functions to reside on a single card. This device takes into account all possible applications. Included are the power supplies with 230, 115, and 24 V AC or DC.
A backlit alpha numerical LC display allows an effective dialogue and a very simple way of operation in spite of the complex task. After installation the level of density is displayed, which only has to be corrected in teamwork with the laboratory - through automatic calculation of the required parameters.
The registered value is put out digitally, by a bar graph and with a potential free 0/4-20 mA output . Of course there are also freely adjustable relais switches.
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