Antares Analytical Technical Bulletin for
October 2002

Subject: Continuation of Vacuum Systems started in Sept 2002 Antares Technical Bulletin.

Last month this Antares September Bulletin covered the basic components and theory of operation of the vacuum systems used on the various Baird spectrometers. Some basic problems, troubleshooting and repairs were also started. This month’s Bulletin will be a continuation of various problems and their solutions.

Spectrometers not using a panel mounted vacuum gauge (DV6 / 6E, later FSQ, F’Mate)

 Zero Reading on System Status

• Sometimes the “System Status” reading will show a zero for a vacuum pressure. This does not necessarily mean something is wrong with the vacuum system. The BAIRDAS software does not show a number less than 1 for a vacuum reading and if the cell vacuum pressure is less the 1 micron, a zero will be shown on the vacuum meter.
• If a problem is suspected try the following to be sure the vacuum reporting system is working.
  1. Turn off the high voltage
  2. Close the gate valve on the top of the vacuum pump.
  3. Take off the top cover of the spectrometer and find the vacuum release valve. This will be a rotary valve located near the grating on the top of the vacuum cell.
  4. SLOWLY turn the valve and watch the vacuum reading on the computer screen.
  5. The number displayed on the screen should rise quickly. If you are opening the valve slowly enough, this number should rise to about 20-30 microns. When you see this, stop opening the valve. Close the vacuum release valve but DO NOT FORCE IT TIGHTLY CLOSED ! Forcing the valve tightly closed could destroy the small Teflon ball in the valve which provides a tight a seal for the valve and this would cause a vacuum leak.
  6. Open the gate valve and be sure the vacuum reading starts to go back down to zero. This small test will tell you the vacuum system is working correctly.

Reading of any number over 5500

• A reading of over 5500 (could read as high as 58000) indicates either no vacuum pressure (as when the cell is at atmosphere) which will mean a vacuum problem OR a system problem, usually electronics.
• If high voltage comes on then the most likely problem will be the EB1 in the MC20. The reading for vacuum pressure (coming from the thermocouple) is routed through channel 2 of element board 1. The high voltage would not be affected because the high voltage is interlocked through a separate signal coming from the thermocouple.
• If the high voltage does NOT come on and the vacuum reading is over 5500 then the problem can be A) a vacuum problem OR B) a thermocouple problem OR C) a problem with voltages either going into or out of the thermocouple.
• For more help with vacuum system interlocks, please see the Antares Analytical Tech Bulletin from July 2002 titled “Loss of High Voltage – MC20”

Suspected Thermocouple Problems

• The Thermocouple mounted on the vacuum cell can be very difficult to test for the average Baird user and it’s testing should be left to qualified Service personnel. The exact schematics for this thermocouple are not included with the Baird supplied factory manuals.
• This thermocouple is powered by  a control voltage supplied by the low voltage power supplies located in the MC20.
• There are 2 voltage signals coming out of the thermocouple which are critical to the correct operation.
  • 1st is the low voltage DC signal which is used by channel 2 of EB1 in the MC20 to display the correct vacuum pressure on the “System Status” of the MC20. This is dependent on the internal vacuum pressure in the optical cell
  • 2^nd is the 24 VDC used to turn on / off the high voltage interlock.
  • Both these output signals are dependent on the constant input voltage from the MC20 (low voltage power supply board for the DV6 / FoundryMate) ….. loss of this input voltage will cause no voltage signal output from the thermocouple and give the impression of a vacuum system problem.
  • These output signals are directly controlled by the vacuum pressure in the vacuum gauge.
  • Vacuum problems OR failure of the thermocouple will give an inaccurate reading on the “System Status” and / or show no high voltage.

 Spectrometers using a panel mounted vacuum gauge

Earlier model Baird spectrometers ( DV2 to 4, early ICP and some FSQ models) used a panel mounted vacuum gauge, manufactured by either MKS / HPS or Varian, mated to the equivalent thermocouple. The function & method, as they relate to the vacuum system operation, is the same as units not using a separate vacuum gauge.

An earlier model vacuum gauge will use mechanical contacts for the vacuum pump cycle time (see below) and the high voltage interlock. The later vacuum gauges use internal electrical circuitry to take the place of mechanical contacts.

On the earlier Baird models the vacuum pump would turn off when a certain vacuum pressure was reached and then turned back when the vacuum pressure “creep” up to a preset level. This level was usually 20 microns and was set using the 1^st black needle on the vacuum gauge.

When the red vacuum indicator needle went goes the 1^st setpoint, a voltage is taken off the vacuum pump motor controller (box the vacuum pump is plugged into), which starts a timed voltage which lets the vacuum pump run for another 2 minutes.

After 2 minutes, a relay in the vacuum pump motor controller opens and power is taken off the vacuum pump. By this time the vacuum pressure should be down to less than 5 microns. The pump will stay off until the vacuum pressure “creeps” back up to the 1^st setpoint, when the motor controller turns the pump back on to bring the vacuum pressure back below 20. This pump on-off-on, in the Baird manuals, is the “cycle time” and in a correctly maintained vacuum  system will vary between 45 minutes to as much as 4 hours.

The 2^nd black setpoint on the vacuum gauge is normally set at 100 and is used for the high voltage interlock circuits.

On later DV4 and some DV5 models this vacuum pump motor controller was eliminated and the vacuum pump was left to run continuously. There is no advantage or disadvantage to this and the vacuum pump motor controller was simply eliminated as a manufacturing cost reduction.

No high voltage, even though the red indicator needle is below setpoint 1 (1^st black needle.)

• This can be caused by 1 of 2 things. The contacts in the vacuum gauge may be stuck open. There is no repair for this except to replace the vacuum gauge. The gauge can try and be repaired but in 25 years I have never seen it succesfully done.
• There can be a problem with the 12 VDC which goes through the high voltage interlock contacts. The problem here could be in the low voltage power supply located in the MC20 / CMU16-18

Vacuum Pump doesn’t turn off (for DV spectrometers using a vacuum pump motor controller)

• Most likely cause for this will be either a failure of the motor controller circuit board or the optically coupled relay (both located within the vacuum pump motor controller)
• The contacts within the vacuum gauge have closed shut, not allowing the timer signal to get to the motor controller. The voltage signal, which normally flows though the setpoint 1 contacts, doesn’t stop (the contacts don’t open) so the timer circuit isn’t triggered.

Vacuum Pump doesn’t on

• This can be cause by a failure of the vacuum pump motor controller (circuit board or relay)
• Failure of the vacuum gauge contacts
• Failure of the vacuum pump.
• Failure of the CMU / MC20 low voltage power supplies.

Most answers to vacuum system questions and problems can be found in this and the preceding September 2002 Antares Technical Bulletin. We can supply a complete range of products from Zeolite to Vacuum Gauges to Vacuum Pumps …. Whatever you need to maintain, troubleshoot and repair your vacuum systems problems. If you do not find an answer to your problems or questions within this The Bulletin, please send an email to the Antares Technical Services Group for further help.

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This Technical Bulletin is written by Gerry Smith, Technical Services Manager of Antares Analytical. Having spent 22 years with Baird in various capacities, including many years as Final Test / Calibration Manager and Senior Service Engineer, International Operations, he or any of our Technical Support Staff, are well qualified to answer any Technical or Applications problems you may have.

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