The effects of wind are very important in selecting the right load cell capacity and determining the correct amount to use in outdoor applications. In analysis, it must be assumed that the wind can (and does) blow from any horizontal direction.

The figure shows the effects of wind force on a vertical tank. Note that not only is there force distribution against the tank on the windward side, but also a “suction” distribution on the leeward side. These forces tend to be addictive and can tip the vessel over in the direction of the wind. On the sides of the tank, the forces are equal in magnitude but opposite in direction, resulting in no effect on the overall stability of the vessel.

Air velocities of wind

The maximum air velocity of wind depends on the geographical place and height and the local situations (buildings, open field, sea, etc.). A national meteorological institute can provide more statistical data to determine how air velocity should be taken into account.

Calculation of wind force

The installation is mostly affected by horizontal forces, acting in the direction of the wind. These forces can be calculated by:

F = 0.63 * cd * A * v2

where:

cd =  drag coefficient, for an upright circle cylinder this coefficient equals 0.8

A = exposed cross-section, equals the height of the vessel* bore of the vessel (m2)

h = height of the vessel (m)

d = bore of the vessel (m)

v = air velocity of wind (m/s)

F = force generated by the wind (N)

Hence, for upright cylindrical vessels, the following formula can be used:

F = 0.5 * A * v2 = 0.5 * h * d * v2

• The installation should be protected against capsizing.
• Wind force should be taken into account during load cell capacity selection.
• As the wind does not always blow exactly horizontally, a vertical component could cause a measuring error, by an arbitrary zero shift. Errors bigger than 1% of the net weight can only be expected with very strong wind >7 Beaufort.

The effect on load cell capacity and mounts

The effect of the wind force on the load cells is different from that to the vessel. The wind force causes a capsizing moment, which will be counteracted by the reactive moment of the load cells.

F1 = force on the load cell

Fw = force caused by wind force

a = distance between load cells

F * b = Fw * a
F =(F*b)∕a

which will cause an increase in Fw of Fl1 and a decrease in Fw of F2.

Using a calculated wind force of 13500 N and a value for b which is approximately half the height of the vessel, Fw can be calculated by:

Fw = (F * b)∕a = (13500 * 5)∕3 = 22500 N.

Conclusion

In the case of an empty vessel, uplift protection should be considered if the tare weight of the vessel on each load cell is smaller than or equal to 2 250 kg. In the case of a fully loaded vessel, 2250 kg should be added to the calculated load cell capacity to prevent the load cells from being overloaded. It is common practice to multiply the calculated wind force on a load cell with a safety factor of two (2) or three (3).

If the mount does not offer uplift protection, one may be established externally as per the drawing opposite. The uplift protection should be adjusted with a clearance of at least one mm and requires regular inspection of this clearance.

Load cell capacity selection

The selection of which capacity to use in a weighing application should be based on the following factors:

• determine the maximum weight of the applied load, or “Live Load”.
• calculate the weight, “tare”, of the construction, or “Dead Load”.
• determine the number of load cells to be used in the structure (N).
• check the possible presence of unequal loading conditions (factor fa). This factor is an allowance for low tare estimates and unequal load distribution. Standard: fa = 1.3.
• check on extra factors like vibration, shock, etc. (factor fb). This factor is a dynamic load factor; for static weighing fb = 1.
• calculate Fw (in Newton, not in kg). The individual minimum load cell capacity can be calculated by:

Fw + ( fa * fb * (LiveLoad + DeadLoad) ∕ N)

Accuracy

The specifications illustrated on the load cell data sheet should be the basis for calculating the accuracy of a non-approved weighing system.

The accuracy of a weighing system which must be approved by the “Weights and Measures” has to meet a relevant OIML recommendation. For example, non-automatic weighing systems have to meet OIML R 76-1 edition 1992 (E).

INSTROTECH represents VPG (Vishay) Transducers, combining fifty years of load cell manufacturing and application know-how. Contact INSTROTECH for more information on 010 595 1831 or sales@instrotech.co.za