The blaat of my phone roused me from my slumbers where I had been dreaming of packaging machines. Ralph couldn't control his filling volumes and needed help now.

"On my way," I told him.

On the floor, he explained the problem. "We set the fill volumes and it seems fine. As we run, volumes gradually decline. When they go out of limits, we stop to check and adjust the settings and they are OK. When we restart, fill volumes are normal then start decreasing again.

"Fix it, please. It's making me crazy."

I watched the machine run, taking frequent samples and charting the volumes. Stopping the filler for short periods showed volumes returning to normal after about 30 seconds.

"Fiddlesticks on faulty fillers," I told him. "Your tank is the problem. Bring a manometer and I'll show you what's wrong."

The tank vented through a filter to keep it at atmospheric pressure. The manometer showed that the tank went into vacuum with the filler running. As soon as the filler stopped, it went back to ambient pressure. 

"Your problem is the vent filter," I told Ralph. "When the filler is pulling product from the tank, it doesn't allow enough air flow, resulting in vacuum in the tank. This vacuum restricts the flow of liquid to the filler infeed, causing underfills. The longer you run without a pause, the greater the vacuum, and restriction, becomes.

"Fix the filter and the filler will be fine."