Electrostatic spray coating apparatuses of the type for applying liquid coating material onto a workpart are frequently automated to allow for sequential workpiece coating such as for motor vehicle bodies. Such coating is typically conducted in an isolated internal spray zone in a spray booth for safety. A conduit inside the spray booth defines numerous internal flow passages through which alternating flows of liquid coating material, liquid solvent and air are conducted.
It is frequently the case that liquid coating material or liquid solvent inadvertently enters a portion of the conduit restricted solely for pneumatic flow. When this occurs, sensitive and expensive electronic equipment become susceptible to damage upon contact with the intruding liquid.
The prior art teaches the placement of a fluid barrier upstream of the sensitive and expensive electronic equipment in order to prevent the passage of liquid therepast and thus to protect the electronic equipment. Such fluid barriers, however, utilize porous material sheets which only block highly viscous fluids while allowing fluids having very low viscosity, e.g., paint solvent, to pass through.
It is frequently desirable in spray coating apparatuses to determine when the head of liquid flow reaches a predetermined location in an internal flow passage. For example, in an automated spray coating apparatus, the leading portion of liquid paint moved through an internal flow passage is unusable as being adulterated from residual paint solvent in the recently cleaned internal flow passages leading to the discharge spraying device. Therefore, instead of being directly conducted to the discharge sprayer, the leading portion of liquid flow, or head, must be conducted away from the discharge sprayer so that the clean, unadulterated paint behind the head can be moved to the discharge sprayer.
It is old and well known to measure the time required for the paint to travel a predetermined distance inside of the internal flow passage. The time lapse is measured so that the adulterated head of liquid may bypass the discharge sprayer and then the clean portion of liquid behind the head can be directed into the sprayer. However, this method requires labor intensive calculations and experimentation to accurately define the time requirements, and fails to take into account the possibility that different coating materials have different viscosities and therefore travel through the internal flow passages at different rates.