Mechanical side, so bear with me. The Tank 3 level control valve on the mixer line has been chattering for a week now. You can stand next to it and watch the stem go up and down every few seconds. The pump bearings downstream are taking it too.

So I asked the controls guy to pull the trend, Ignition 8.1, and the level and valve output come up as a smooth line, near enough dead straight. He reckons the loop is fine because the trend says so. I showed him a video on my phone and he went and pulled the tag history for that same minute and it still shows nothing. The positioner was replaced last year, so I don't think that's it, though he does.

Is the trend lying or am I? Any ideas what to get him to check before the positioner carries the blame again?