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Pressure Measurement

Current to Pressure Conversions

Gamma Vacuum ion pump controllers determine pressure in the ion pump 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 calculating pressure using the current of the controller from the front panel, analog outputs, or RS232 port, the following equation can be used:

Pressure =
(0.066 * Amps * (5600 / volts) * Units * Cal Factor)
 
Pump Size

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 ion pump. These curves are based on the equation above.

See below for Perkin Elmer controllers.

Leakage Current

High leakage current does not appreciably affect the operation of the pump, but it does render the pump incapable of giving accurate pressure readings. Pressure can still be read, however, by determining the total leakage current after removing the magnets and then 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 7000 VDC UNDER OPEN CIRCUIT CONDITIONS. 4. OBSERVE SAFETY PRECAUTIONS IN THE USER'S MANUAL. 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.

Historical Ultek, Perkin Elmer and PHI controller equation

Gamma Vacuum maintains historical information on older controllers such as the DIGITEL 100, 500, and 1000. Contact us directly for available information including manuals and line drawings. For determining the pressure given by the front panel or serial outputs of controllers built before 1995, the following equation can be used:

Pressure (torr) = 
       (0.005 * Amps)       
 
Number of Elements

Amps = Current reading in Amps
Number of Elements = 25L: N = 1, 60: N = 2, 120: N = 4, 220: N = 8

Individual Curves

Individual pump current vs. pressure curves are still available for Ultek, Perkin Elmer, and PHI ion pumps. Please contact Gamma Vacuum for availability.


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