Why does the pressure hold button on the STH pump controller stop the pump instead of adapting the flow?

The STH Pump Controller (IN175) allows to run your Langendorff system in either constant flow or constant pressure mode using the Pressure Hold Button. When the button is not lit, the controller is in constant flow mode and the flow rate of the pump can be adjusted with the Set Flow knob. When the button is depressed and illuminated, the unit is in constant pressure mode. In this mode the STH Pump Controller holds the perfusion pressure at the value that was present when constant pressure mode was selected.
You might run into a situation where you can control the roller pump by the STH Pump Controller (change speed and direction and turn it on/off) with no problem, but when changing into constant pressure mode the button is lit and the pump stops. Going back into constant flow mode the pump starts again. In this situation make sure the Pressure Adjust Knob is in the middle (12 o'clock) position. If the knob is close to the utmost clockwise (5 o'clock) or anti-clockwise (7 o'clock) position the Hold button behaves as an on/off button.
Note: When the controller is in constant pressure mode, the Pressure Adjust knob allows you to trim the pressure by up to ± 5% of the initial pressure signal. For further information please refer to the STH Pump Controller Owner's guide.