As previously mentioned, miswired, misplaced, or malfunctioning limit switches have caused ships to be loaded with the wrong amount of grain. One such mistake is shown here. The indications that the shipping and return gates are closed are coming not from the fully-closed limit switches, but from the fully-open limit switches! If a gate is partly open, but has not hit the fully-open limit switch, grain can go the wrong way without the monitoring system knowing anything is wrong! Try opening the gates on the VRML model and see.

Because an elevator control system is usually designed to close one gate of a diversion point automatically when it opens the other, this error may exist for years without causing any weight complaints. Then a malfunction causes a gate to stick open, and several ships are loaded with too much or (more likely) too little grain before the problem is found and corrected.

Notice that there is no indicator light labeled "FO" for the shipping gate or the reject gate. in this example. GIPSA does not require fully-open indicators for most gates, because the grain goes to the same destination whether the gate is fully open or partly open. In this case, however, a fully-open indicator would probably have allowed the problem to be detected quickly because somebody would have noticed that the "OP" and "FO" lights always came on together.

Detecting this error by a gate performance test involves constructing a truth table, as shown below, and putting the gate pair through all possible combinations of fully closed, open, and fully open. When the lights do not indicate the way the truth table says they should, the inspector notes the error and orders a repair. This test requires at least three people, communicating by radio: one elevator employee moving the gate to the specified position; one GIPSA employee at the gate, physically verifying its position; and one GIPSA employee at the indicator lights, checking their On/Off condition. In the following table, the incorrect indications are in red and marked with an "X". You can move the gates in the VRML model and compare the indicator lights to the table yourself. You can also open the shipping bin when the indicators are not lit properly and send grain both ways.

Truth Table for Gate Indicators