Please note there are many variables that can change the pressure such as tubing length, temperature, solvent composition, and precolumn.  There is some back pressure expected  even without a column due to frictional forces in the tubing.  Below is a guideline of the expected pressure range under normal conditions using mobile phase.

• PP-ODS for DA and 5HT - 7.5 to 10 MPa
• AC-GEL for ACh - 4.5 to 6 MPa
• CA-5ODS for NE and E - 8.5 to 11 MPa
• GU-GEL for Glutamate - 5 to 7 MPa
• SC-5ODS for Monoamines - 6 to 9 MPa
• SC-3ODS for Monoamines - 9 to 13 MPa.
• AC-GEL for Acetylcholine- 4-6 MPa
• CAX for NE, DA, 5-HT- 7 to 9 MPa
• NOPAK (ENO-20) for Nitrite and Nitrate-1.2-1.8 MPa

Please also note that these values are just for reference. If the displayed pressure is outside of the range above, it's not necessarily a cause for concern as long as the pressure reading is stable.

Higher pressure:

Possible causes

• Time to repack the precolumn.
• Time to wash/replace the separation column.
• Clogging in the tubing/connection fittings.

Lower pressure 

Possible causes

• Flow problem.  Check the measured flow rate compared to the set value.
• Check all connecting tubing for leaks.
• Air bubbles trapped in the pump (recommend purging).