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PPCH-G with PG7202 Autofloat Control Issues

The ability of the PPCH-G to float a piston is a difficult task due to the extreme thermal dynamic influences of compressed gas.  To simplify the problem, almost everything has to be perfect for the float process to be successful.  This includes the following key areas: 

  1. have the required high pressure gas supply.  
  2. minimize all leaks
  3. Have a valid valve opening point determination
  4. have a clean piston-cylinder

 

  1. "Have the required high pressure gas supply":  The PPCH-G must be operated at all times with a known high pressure supply.  A PPCH-G from the factory will have the valves configured for operation with a supply gas pressure of 105% to 110% of controller full scale pressure.  Even if the PPCH-G is used to test a DUT with a smaller full scale value, the PPCH-G still needs the full scale supply pressure.  The GB-H gas booster should be set and kept at the pressure.  Occasionally check the output regulator as it tends to relax with time.  The inlet (and outlet) servo controlled needle valves are mapped to open and close based on a prediction with a known supply pressure.  When the supply pressure changes, the opening points are negatively influenced, and a valve configuration is required.  (see point #3)

 

  1. Minimize all Leaks:  The PPCH-G uses the thermal pressure control unit ("TPCU") to bring the the piston into float.  If there are pressure leaks, then the effectiveness of the thermal control is diminished.  If the leak is too large, then the effects can appear as if the PPCH-G has momentarily stalled only to rapidly change pressure a short time later.  This rapid change in pressure is due to a TPCU routine and the main control valves re-engaging to compensate for the leak.  The goal is to reduce leaks to a rate of 0.001% Indicated Value / second.  A slightly larger leak rate of 0.002%/sec can be acceptable in some situations.  See the Fluke Calibration Help Center linked article: "PPCH-G Leak Check with PG7202"

 

  1. Have a valid valve opening point determination:  The opening point is unique to each valve and to a given supply pressure.  If the supply pressure changes by more than 10% then the opening point should be re-determined to optimize control.  When the opening point results don't match the supply pressure, the common symptom is to have pressure overshoot and undershoot.   Read section 5.6 of the PPCH-G Operations manual for instructions on how to run the automated opening point configuration.

 

  1. Have a clean piston-cylinder:  when using a PC-7200 piston-cylinder, it's not the polished surfaces which require cleaning, but rather the end of travel stops.  The end of travel stops need to be clean and dry.  It is common for oil mist to accumulate here.  The surface tension of the oil film prevents the piston from freely moving off from the end of travel stop.  See the Fluke Calibration Help Center linked article for details: "Cleaning Splashed Oil from PC-7200 End of Travel Stops"

 

Following these four steps will resolve most of the PPCH-G / PG7202 autofloat problems.

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