By: Max Hansen and Karsten Matthiesen Danfoss A/S.

The problem is to assess the feasibility of the below proposed method.

A tube is bend and secured to a common block in a manner shown in figure 1.

Fig 1

Beakers are filled with fluid from a supply tank.

The valve opens when an empty beaker is under the outlet. Since the fluid in the tubes were at rest before opening the valve, the fluid must gain momentum until it reaches a equilibrium between head in the tank and the drag in the tubes.

The change in momentum creates forces on the tube. The forces are measured with four stain gauges glued to the tubes. The voltage from the strain gauge bridge is amplified.

The amplified signal is supplied to an integrator and the result is assumed to be proportional to flow. The flow is integrated and the result is the amount of fluid delivered to the beaker.

When the amount is equal to wanted amount in the beakers the valve is closed. The integrators are reset while the valve is closed and the flow is known to be zero.

• The relation between the weights of fluid delivered to the beakers and the signal from the bridge.
• The relation between head in tank and signal from the strain-gauge bridge just after opening the valve.
• The error due to the drag in the tubes.
• The error due to changing temperature and density during filling.
• The error due to offset drift in amplifier and strain gauges.
• The error due to changing pressure while the valve opens.
• The error due to loop constantly rotating around vertical axis.
• Other errors not thought of yet.

Karsten Matthiesen

Max Hansen

6 July 2002.