Measuring and testing – Liquid analysis or analysis of the suspension of solids in a... – Viscosity
Patent
1996-06-27
1997-11-11
Brock, Michael
Measuring and testing
Liquid analysis or analysis of the suspension of solids in a...
Viscosity
73 5303, 73 5433, G01N 1116, G01N 1100
Patent
active
056866607
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to concentration measurement transmitters, more specifically to an arrangement for a static blade-type transmitter, which includes a blade active in the medium to be measured and suspended by a spindle, such that shear force moment at the blade is translated, via the spindle, to a measurement converter for determining fiber concentration in the fiber slurry medium, this converter preferably being situated in the transmitter housing, into which the spindle also extends, and against which it is sealed with the aid of a lead-through included in the arrangement.
2. Description of the Related Art
There are today several different main groups of devices for measuring concentration in fiber suspensions. One of these groups covers the so-called static blade-type concentration measurement transmitters, hereinafter denoted "blade transmitters". It is generally considered that the transmitters of this group usually have a performance inferior to that of transmitters in the other groups. In the cellulose industry there are applications with different requirements, however, with respective to such as measurement accuracy, repeatability, sensitivity to flow, temperature and pressure. Blade transmitters have consequently become established on the market as a relatively large group. Since they are given comparatively unqualified measurement tasks, these transmitters have a low price level in relation to that for the other groups. In turn, this means that manufacturers are not able to justify technical solutions notably increasing the cost of existing devices. The result has been that the drawbacks always associated with blade transmitters still remain.
There are about ten different blade transmitter manufacturers supplying the world market at present. As mentioned above, it is generally recognized that these transmitters have lower performance than is to be found in transmitters from the other groups. In most cases users accept this situation, since the transmitters are selected for less demanding applications, and have a comparatively low price. However, breakdowns occurring too frequently and costing expensive production interruptions cannot be tolerated. In this respect users demand useful life spans comparable with those for transmitters in the higher price ranges. Large sensitivity to pressure and temperature are also factors only very reluctantly accepted by users.
The majority of defects in blade transmitters, leading to breakdowns and possible production losses, are those that can be traced to the arrangement permitting the spindle to enter the transmitter housing while isolating the dry housing interior from the wet medium being measured. This arrangement is generally denoted "lead-through" hereinafter. Some form of seal must be provided between these two environments, while the shear force moment, inter alia varying as the concentration of the fiber suspension acting on the blade, must be translated to a measurement converter in such a way that measurement accuracy is not degraded. On the accompanying drawings FIGS. 1 and 2 illustrate conventional methods of arranging the lead-through. The latter figure depicts a relatively recent implementation, but if the lead-through itself is examined it will be understood that no technical advance has been made for the last twenty years.
The known implementation in FIG. 1 illustrates the most usual lead-through. Without exception it is also provided with a bearing means, inter alia for taking up axial forces. The sealing element consists of a ring made from an elastomer suitable for the purpose. Correctly implemented, this element is not sensitive to pressure. On the other hand, it is often sensitive to temperature, since the elasticity of the elastomer varies with temperature, and the transmitters are implemented using the weighing balance principle. As will be understood, this means that the measuring principle cannot compensate for variations of the kind mentioned, nor for ageing of the
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Brock Michael
BTG Kalle Inventing AB
Wiggins J. David
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