Author Archives: Randy

Low Cost Low Cost Molecular Sieve Trap

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Update 5-28-23 – We tried this on two different pump set ups and it seemed to work well at first. But on one of the set ups the fine mesh O-ring became clogged with small Zeolite pieces and plugged up the line. We removed the Zeolite from that set up, the other one is working fine so far. It could be a function of the mesh size, we will continue to monitor set up #2.

Molecular sieve traps use zeolite pellets to adsorb water vapor, oil vapor and other gas molecules. They are particularly useful in preventing the back-streaming of oil vapor from the rotary vane pump into the turbo pump and vacuum chamber.

Molecular sieve oil mist trap

Molecular sieve traps are placed between the rotary vane backing pump and the turbo pump. Most molecular sieve traps also have built in heaters that are used to regenerate the zeolite once it becomes saturated with contaminants.

If you have an oil filled rotary vane pump, a molecular sieve trap is a must.  However, molecular sieve traps can be relatively expensive running from $500.00 to $1,000.00 or more depending on the size.

This blog post shows a way to use existing flexible metal hose to make a molecular sieve trap for a lot less.

The general idea is to fill an existing flexible metal hose which is connected between the roughing pump and the turbo pump with zeolite pellets.

Step one is to remove the flexible metal hose.

Next, insert a mesh screen centering ring on the end of the flexible metal hose that is closest to the roughing pump. Connect the flexible metal hose to the roughing pump.

I used this one from LDS – https://www.ldsvacuumshopper.com/nwstceri.html

The next step is to fill the flexible metal hose with Zeolite pellets.  I used these Zeolite pellets (also from LDS) – https://www.ldsvacuumshopper.com/mositrb1zepe.html

Once the flexible metal hose is filled completely (leave a little bit of room at the top) connect the flexible metal hose back to the turbo pump roughing port using another mesh screen centering ring.

Finally, label the flexible metal hose to indicate that the flexible metal hose is filled with Zeolite pellets.  

That way when the flexible metal hose is removed at some point in the future who ever removes it will know that they need to remove the top of the flexible metal hose first and to empty the Zeolite pellets before removing the flexible metal hose from the roughing pump.   You can also hold the bottom centering ring as the flexible metal hose is removed from the roughing pump. (That is one advantage that a regular sieve tap has is that the Zeolite pellets are contained and will not spill out when the sieve trap is removed from the backing pump).

In the experiment where I tried this idea the turbo pump and backing pump worked normally, there was no noticeable increase in the pumping time.   At some point in the future when the Zeolite needs to be baked out heating tape could be used.  Or the Zeolite pellets could just be replaced with new ones. Note that the volume of Zeolite pellets in the flexible metal hose is much less than what a regular sieve trap has, so the amount of adsorption would be reduced proportionally.  Even so I think that this is worth considering, especially in situations where there is no existing molecular sieve trap installed.

Total cost for the two mesh screen centering rings and the Zeolite pellets was under $90.00 Finally, if you are not familiar with LDS you should check them out.  They have a wide selection of vacuum related hardware at good prices and fun categories like REALLY Cheap Stuff and Surplus items.  https://www.ldsvacuumshopper.com/drypumps1.html

XPS analyzer control non-linearity problem

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The 80-360 and 80-365/366 analyzer controls provide all the voltages to the 10-360 spherical capacitor analyzer that is used on many PHI (Physical Electronics) XPS systems.

80-365 boards Retard board is third from left

The retard voltage is used to slow down electrons and is essentially the sweep voltage.  In conjunction with the pass energy supply, the retard voltage controls the energy of the electrons that are being passed through the analyzer and into the electron multiplier and counting circuitry.

I recently had an interesting problem with a retard board on an 80-360 analyzer controller.  The issue was that the retard board output voltage was not linear.   Part of the calibration procedure for the retard board is to test the voltage at 11 specific voltages ranging from 253.6 volts to 1253.6 volts in increments of 100 volts.  This is a convenient way to confirm that the retard supply is linear.  The table below shows the hex commands that are used to set the voltages and the expected results.

In this particular case, the output voltages were close to correct at some points, but way off at other points as shown below. This non-linearity would present itself as inconsistent peak widths in the data as a function of kinetic energy.

These results at first glance look like a bit problem.  That is, the digital to analog convertor (DAC) voltages are likely off.  The DAC used in the retard board circuit is a 16 bit DAC and the output voltages should follow the voltages listed  below.

DAC Voltages

However, the DAC voltages were fine.  The retard voltage circuit comprises the DAC which drives a precision operational amplifier that in turn drives a high voltage switching supply.  Some precision high voltage resistors are  used to provide feedback.  The next most likely component that might be non-linear was the OP07 ultra-low offset operational amplifier (op amp).  The OP07 was replaced but did not solve the problem.

The next most likely cause of the non-linearity problem was the feedback resistors.  There is a total of 5 of these SX3730 5 watt wire wound high precision axial resistors in series.  To accurately measure those resistors, you need to lift one end off the circuit board.  Using my Fluke multimeter, I tested the resistance of each resistor and they all checked out as OK.  Ideally you will see very close to 1.0 megohms, but it might be off by .05 megohms. When a resistor is bad it will be open or be off by as much as .5 megohms. So, it seemed that the feedback resistors were OK as well.

That left not much in the circuit other than a few potentiometers. After spending some more time retesting all the components, I came to the conclusion that it had to be one of those feedback resistors. 

To test that theory I removed all 5 of the SX3730 1 megohm feedback resistors and replaced them with a single 5 meg ohm resistor.  And that worked!   So now I knew for sure that one of those 5 feedback resistors was the problem.  I measured the resistance of each resistor, and they looked OK.  But then I realized that the non-linearity is a function of the voltage applied to the resistor. At some voltages the resistance was OK, but at other voltages the resistance was off. 

I then decided to measure the resistance value again using a megohmmeter.  The model that I used was a Protek DI-2000M.  This megohmmeter (also called an insulation tester) puts 500 V across the resistor when measuring the resistance. I hoped that by putting 500V across the resistor that I would be able to see a greater difference in the resistance values. And that worked out as expected.  One of the resistors showed only 965 Kohms with the megohmmeter and 995 Kohms with the Fluke. I replaced that resistor and the calibration was perfect.  😊

In hindsight, duh. Since the gain of the circuit was changing as a function of the non-linearity of one resistor, the lesson is that when checking the resistance of suspected non-linear resistors, always use a megohmmeter since that will put much more voltage across the resistor than what a normal DVM will put out. Better yet, if possible, use the highest voltage that the resistor is rated for.

If your 80-360 or 80-365/366 analyzer control is not functioning properly and you need some help, please contact RBD Instruments for assistance as needed.

DGC III Filament Select Relay

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The DGC III ion gauge controller (also called the DIG 3) used on many of the older PHI surface analysis systems can operate 2 ion gauges (only one at a time). There is a relay inside the DGC III that puts the filament current output to the ion gauge connectors on the back of the DGC III. Normally there is only one ion gauge on the vacuum chamber and it is plugged into the Ion Gauge 1 connector (normally closed) on the back of the DGC III. The relay is shown in the photo below.

DGC III filament select relay

If your DGC III does not read correctly, you can first check the +/-12V and +5V power supplies. Here is a link to some information on how to do that – DGC III power supplies test. If any of the power supplies are low and have a high AC component, then usually that issue is caused by a leaky capacitor on the power supply board. ** CAUTION! Make sure that someone who is trained on working safely with voltages up to 500V performs the voltage measurements. There are potentially lethal voltages inside the DGC III.**

If the power supplies check out OK then it is possible that the filament select relay is dirty. To test that, make sure that the DGC III is OFF and then move the black filament cable on the back of the DGC III from filament 1 to filament 2. You do not need to move the COL BNC cable as those are both tied together.

Turn the DGC III back on and press the 2 button. That will select ion gauge 2. Press the I/T 3 button to measure the vacuum and see if the DGC III works normally. If it does, then the filament select relay is dirty. You can just keep the ion gauge connected to ion gauge 2, or, you can clean the relay and connect the ion gauge back to ion gauge 1.

To clean the relay, make sure that the power is OFF to the DGC III and if not already down, pull the unit out of the rack and remove the cover.

Pull the relay out and remove the 4 screws on the bottom of the relay.

The contacts that are touching are the normally closed ion gauge 1 contacts. Use a small strip of some very fine emery cloth or sandpaper to gently clean the contacts on both sides. Then, use a small strip of paper with some isopropyl alcohol on it to remove the leftover grit. Replace the cover on the relay and reinstall the relay.

Ion gauge 1 should work properly now but if not you can order a new relay from companies like Grainger or Mouser. Just make sure that the pins match schematic that is on the side of the relay. Google 105 3PDT 10A relay and you will find it.

If your DGC III does not work and you need some help or a loaner, please contact RBD Instruments for assistance.