Coriolis balance
A Coriolis balance , Coriolis flow balance or rotor balance is a balance that measures the weight of a mass flow using the Coriolis force . The mass flows measured by means of the Coriolis balance are mainly bulk goods , in particular dusts, powders and flours, but also granulates and grain. The Coriolis balance should not be confused with the Coriolis mass flow meter , which is used to measure liquids and gases.
construction
The Coriolis balance essentially consists of an inlet pipe, a rotor plate driven by a motor via a shaft and an outlet funnel. There are different designs, especially with regard to the feed and the rotor plate. However, the basic structure is comparable to that shown here.
function
The bulk material to be weighed is fed through the inlet pipe into the center of the rotor plate. The rotor plate rotates at a fixed speed and accelerates the mass flow flowing through it through radially arranged blades. The mass flow then leaves the rotor plate in a tangential direction and slides out of the balance in spirals through the discharge funnel .
The torque required to accelerate the mass flow is measured and evaluated. It is proportional to the amount of mass flow conveyed.
There are various methods for measuring this torque.
One possibility is to measure the current drawn by the motor. Due to various loss and interference influences, this method is relatively imprecise.
Another possibility is to mount the motor in a pendulum fashion and to measure the force acting on it with a strain gauge . This enables a direct force measurement to be achieved, but has to accept the effects of friction caused by the bearing . This method is therefore only suitable for mass flows greater than approx. 1 t / h.
Other methods can eliminate these influences through a precise suspension and bypassing the engine mounting and enable the weighing of mass flows down to the gram range.
advantages
The measurement of mass flows with the Coriolis balance is almost unlimited. Scales for mass flows of up to 150 t / h have already been implemented.
One advantage of the Coriolis measurement is the elimination of frictional forces. Since frictional forces act in the radial direction along the blades, they are not included in the tangentially directed Coriolis acceleration. Thus, bulk goods can with varying product characteristics (for example density , viscosity ) predominate, without the need for renewed calibration is necessary.
Another advantage of the Coriolis balance is its wide range of uses. It is suitable for determining the mass of both free-flowing ( e.g. granulates , grain ) and cohesive , poorly flowing bulk materials ( e.g. coal dust , flour , bran ).
Compared to impact plate scales , smaller measurement errors are achieved (in the range of approx. 0.5%).
Further developments have made the Coriolis balance into a robust measuring device that delivers the same accuracy even under the influence of vibrations and shocks .
disadvantage
The disadvantage of Coriolis balances is their high price compared to other non-verifiable balances.
In addition, the measurement of the mass flow is negatively influenced in the event of a poor feed. Therefore, an optimal product feed should always be ensured.
The verification of Coriolis balances is difficult because the accuracies required in the corresponding verification classes are not always achieved. Nevertheless, this has already been achieved for certain calibration classes.
The fact that the Coriolis balance weighs the entire mass conveyed by it shows that high accuracy can have negative effects . As a result, the air flowing through it is also weighed. If the air flow is constant, it can be eliminated by taring . A fluctuating air flow, however, leads to measurement errors.