There are six effects which cause variation of a measured weight at a fixed location:
- Buoyancy Large objects weigh less than small objects of equal mass, because of the buoyancy of an object as it floats in a sea of air. This effect is much more significant than most engineers realize. Buoyancy varies due to change in air density, which is affected by the weather and is a function of the atmospheric pressure, relative humidity and temperature. You can compensate for this effect by calculating the air density and determining the buoyancy effect for the volume of the object being weighed. This is discussed in more detail in the following section.
- Tidal Variation As discussed previously, the gravitational mass attraction to the sun and moon varies during the year. It may reach 0.003% of the acceleration of earth gravity at certain dates during the year when the sun and moon align.
- Condensation In areas with high humidity, sudden changes in temperature will produce condensation on the object being measured, which adds to the weight of the object. This most commonly occurs when an object is brought from an air conditioned space to a non air conditioned one. Condensation can be minimized by allowing an object’s temperature to equalize before weighing it.
- Electrostatic attraction It’s remarkable how dramatic this effect can be if you are weighing a large lightweight object such as a mylar decoy reentry cone. If the object is covered by a clear plastic draft shield during measurement, the attraction between object and shield can be as much as 2% of the weight of the object.
- Magnetic attraction If the object you are weighing contains permanent magnets, there will be an attraction to any magnetic material near or on the scale. There will also be a small attraction force to the magnetic north of the earth. Often an object has been magnetized by the magnetic chuck used in the machining of the object. It may be necessary to demagnetize the object before weighing it. Generally you can detect magnetic errors by repositioning the object on the weighing pan of the scale. If the readings are very sensitive to position on the pan, then the problem may be magnetic attraction (but it might also be the corner loading error of the scale). A Boy Scout compass is also a good way to test for magnetic attraction.
- Drafts or air currents Generally, this effect will be quite obvious, since drafts will introduce a random variation in the readings. However, there are instances where drafts can produce a relatively steady downward or upward force. For example, if sunlight heats an object, then the updraft from the surface will produce an upward force, reducing the measured weight of the object. Conversely, if an object is brought in from an unheated storage area to be weighed, the cooler object will cause a downdraft, increasing the measured weight. These effects can be minimized by making sure that the object is at the same temperature as the surrounding air and by avoiding direct sunlight or bright lights.