Gamma Vacuum ion pump controllers utilize an equation to determine pressure from the current in the ion pump. That current is subject to several variables, including different types of leakage . Provided all leakage currents are minimized and the controller is properly calibrated, the equation used in the controller provides an accurate representation of the pressure in the system. When manually calculating pressure from the current of the controller from the front panel, analog outputs, or RS232 port, the following equation can be used:

Amps = Current reading in Amps
volts = Voltage reading in volts (MPC is constant 7000 or 5600, SPC is variable)
Units = Conversion factor (Torr = 1, mbar = 1.33 & Pascal = 133)
Cal Factor = MPC/LPC programmed calibration factor (typically set to 1)
Pump Size = The size of the ion pump is liters per second (l/s)

Individual Curves
Individual pump current vs. pressure curves are provided in the download information for any given ion pump. These curves are based on the equation above.

Click here for the equation used by Perkin Elmer controllers.

High leakage current does not appreciably affect the operation of the pump, but it does render the pump incapable of giving accurate pressure readings. However, pressure can be read by determining the total leakage current after removing the magnets, and subtracting this value from the total current drawn by the pump. To reduce electrical leakage:

1. Connect the Gamma Vacuum control unit to the ion pump.
2. With the pump pressure in the low micron range (10-2torr), turn on the control unit.
3. Several applications of this technique may by necessary. Be sure to allow the control unit to discharge completely between each application.
4. If this procedure does not eliminate the leakage current, disconnect the high voltage feedthrough from the connector straps inside the pump body. Check the feedthrough for leakage as described. CAUTION: THESE CONTROL UNITS ARE CAPABLE OF DELIVERING 7000VDC UNDER OPEN CIRCUIT CONDITIONS. OBSERVE SAFETY PRECAUTIONS DISCUSSED PREVIOUSLY. If leakage is not eliminated, replace the feedthrough.
5. If the feedthrough is not the cause of the leakage current, the pumping element ceramic insulators are the conduction path and should be replaced.