Heat exchanger cooling fan problem

16-025 and 16-050 radiator cooling fan

16-025 and 16-050 radiator cooling fan

 

 

 

 

 

 

 

 

 

 

 

The heat exchanger cooling fan used on the Physical electronics 16-025 and 16-050 heat exchangers which are used to cool the x-ray sources on an XPS system can run slowly as the fan motor ages once the bearing lubricant begins to dry up. The result of a slow fan is that the anode will run warmer than normal and possibly melt.  At a minimum reduced cooling will shorten the anode lifetime.  The 16-020 heat exchanger has a fan that is attached to the pump motor, so it will not have this problem unless the pump motor fails.

The symptom for a slow heat exchanger fan is that with the source at full power after 20 minutes of operation the water lines to the x-ray source will be very warm or even hot  rather than slightly warm.

To test the fan you can feel how much air is coming out the back side of the radiator when the 16-025 or 16-050 heat exchanger is on.  The radiator will slow down the airflow, but it should still be a reasonable amount of air that escapes out the back side of the radiator.  If the fan looks like it is spinning slowly and there is minimal airflow then the fan should be replaced.

If you want to test the actual RPMs of the fan you can use a non-contact laser tachometer (available for as little as $15.00).  Or you can use an audio method as shown in this video on YouTube that uses Audacity:

https://www.youtube.com/watch?v=PZby_Y3iuuk

https://www.audacityteam.org/

The fan motor should spin at 1650 RPMs.

If you determine that the fan should be replaced (or if you want to as a preventive measure if your heat exchanger is 20 years old), here is the model number information –

Manufacturer – Orion

Fan, AC, 230V, 254 X 89mm, Round, 547CFM, 35W, 55dBA, 1650RPM

Manufacturer part number: OA254AP-22-1TB

Allied Stock # 70103674

It is held in place with some radiator mounting rods that are available at most auto parts stores.  Look for Hayden Nylon mounting rods with vibration pads.

Nylon mounting rods

Nylon mounting rods

 

 

 

 

 

 

 

 

 

 

 

 

 

In addition to the cooling fan, if the pressure indicator on your 16-050 heat exchanger bounces during operation you should consider replacing the water pump as well.  RBD Instruments provides the water pumps.   The 16-020 and most 16-025 heat exchangers do not have a pressure gauge.

And of course, make sure that the radiator is free from dust as being covered with dust will also reduce the cooling efficiency of the 16-020, 16-025 or 16-050 heat exchangers.   Refer to the RBD TechSpot post Heat Exchanger Preventive Maintenance for more information on dust and the radiator.

Heat exchanger preventive maintenance will prolong the anode lifetime on your 10-610 mono chromator anode and prevent a costly and time consuming anode meltdown.

How an electron multiplier works

This post will explain the basic concept of how an electron multiplier works.

Electron multipliers are used in surface analysis instruments to boost the detected signal to a level where it can be amplified and processed into data. For Auger Electron spectrometers and X-ray photo electron analyzers the detected signal are electrons. Secondary ion spectrometers detect ions.

In the 1960s electron multipliers were made out of a series of Oxygen treated copper beryllium (CuBe) plates.  Copper with 3 to 4% beryllium that is heat treated with oxygen has a secondary electron yield of approximately 3 (varies slightly for kinetic energies between 100 up to 1500V)

The drawing below shows the basic concept.  One electron impacts the first plate and then a few more secondary electrons are generated.  A positive voltage is applied across the multiplier array which is divided by a series of vacuum compatible resistors.  Each plate is progressively more positive and so emitted electrons are attracted to the next plate.  The resulting avalanche of electrons is attracted to the final collector plate where the signal is decoupled from the electron multiplier.  The total number of plates determines the gain of the multiplier. Most of the CuBe electron multipliers used on Auger spectrometers had a gain of 2 X 10E6discrete dynode electron multiplier gain

 

 

 

 

 

 

 

discrete dynode electron multiplier

 

 

When X-ray Electron spectrometers were first developed electron multipliers with higher gains were required in order to achieve better signal to noise.  During that time continuous dynode electron multipliers (Channeltrons) were developed.  Instead of a series of discrete plates, a Channeltron electron multiplier uses a high resistance semiconductor material that also has high secondary electron emissivity.  Gains of a Channeltron are typically 2 X 10E7 to 2 X 10E8. The drawing below shows the gain concept.  Many Channeltrons today are spiral instead of horn shaped to provide an even higher gain.continuous dynode electron multiplier gain

 

 

 

 

 

 

 

 

Channeltron multilplier

 

 

 

 

A third type of electron multiplier, the Micro Channel plate, was developed in order to obtain a larger detector surface area in conjunction with multi-channel detectors. Channel plates are essentially a lot of tiny Channeltron multipliers in parallel. Two plates are stacked on top of each other to increase the gain.  The drawing below shows the gain concept. Channel plate electron multipliers are commonly used on X-ray Photo electron spectrometers.MCD channel plates gain

 

 

 

 

 

 

 

Micro Channel plates

 

 

 

 

 

 

 

 

Electron multipliers typically last for several years with normal usage. With just occasional use they can last for decades.  Eventually the high secondary electron emissivity materials in the multiplier are depleted or the multiplier becomes contaminated and then the signal to noise degrades at which time the multiplier needs to be replaced.

Some additional reference links are listed below.   Most of these refer to ions and mass spectroscopy but it is the same principle for electron based detectors used in Auger Electron and X-ray photo electron spectrometers.

cires.colorado.edu/jimenez/CHEM-5181/Lect/MS_Detectors_AD_SNR.pdf

http://www.irjponline.com/admin/php/uploads/1414_pdf.pdf

http://www.chm.bris.ac.uk/ms/detectors.xhtml

9103 Floating Ground Reference

The term “floating ground reference” in the title of this post refers to an electrical circuit that does not have a ground connected electrically to earth. (This type of connection is also referred to as “floating input”.)

The 9103 USB data logging picoammeter has the ability to float higher than earth ground by up to 1,500V DC. A new 9103 version coming out in early 2018 will be able to float up to 5,000V DC.

This post explains how the connections to the 9103 USB picoammeter are made and how the floating capability works.

The HV option for the 9103 uses SHV and MHV 5kV connectors instead of a BNC.  The center pins on the INPUT and HV connectors are the signal input and ground reference input, respectively, to the 9103.

9103 HV option

Step one is to build an isolated (floating) power source.  A very easy way to do this is to use a 9 volt transistor battery and a resistor. For this example, I used a 9V battery with a 10 Meg ohm resistor to get about 900nA of current. Using Ohms law you can create any current that you would like to use for this test as long as it is in the range of 1nA to about 1mA.  A 9 volt battery works well because it is small and a very clean source of DC voltage and current.

simple isolated current source

Next, I wrapped the battery in electrical tape and mounted the battery in an enclosure.

9103 float test box

9103 float test box

One end of the resistor is connected to the INPUT on the 9103 and the other end of the resistor is connected to the battery. The battery is referenced to the HV input ground on the 9103.

Finally, I connected a high voltage power supply to the ground reference via a 1 Meg ohm current limit resistor as show in the schematic below.  The current limit resistor helps to reduce noise and current surges from the high voltage supply.

Current limit resistor.jpg

After connecting the INPUT and HV leads to the 9103, I am ready to measure current. It is important to note that the 9103 Input should be set to Normal and not Grounded. (The “Grounded” Input is used to short the specimen stage to ground when not measuring current. This is useful when measuring electron or ion currents in vacuum, but when floating the 9103 you do not want to short out the input or you may damage the 9103 or your high voltage supply.)

normal not grounded

We can now measure current and can see that we are getting 913.2 nA of current.

Using the Data recorder we can monitor the current vs. time to see a graphical representation of the current.

The signal ground reference on the 9103 is tied to the high voltage supply.  As I increase the high voltage supply from 0 to 1500V DC in increments of 500V (a limitation of the high voltage supply I am using) you can see some small instabilities in the data.  This is normal; there is some capacitive coupling as the ground reference voltage is changed.  It looks like a lot of noise but in fact is only about 20 pA.

9103 graph spikes when increasing voltage

1500V DC voltage source

If you look at the data referenced to zero you can see that the instabilities are very minor and also that the output is very stable once the high voltage supply stabilizes.  If you were to measure the voltage on the 9103 HV reference to earth ground you would measure 1,500V DC.  So for this example the 9103 ground is floating by 1,500V DC.

9103 graph zero base line

In this test I changed the high voltage supply from +1500V to -1500V DC with no change in my current reading which demonstrates how well isolated the 9103 input is from earth ground.

Applications for a floating input picoammeter include measuring the output of an electron multiplier directly, as well as bias experiments with electron and ion beams.  For more information on our new 5kV floating 9103 please contact us by emailing us at sales@rbdinstruments dot com or visiting our website at www.rbdinstruments dot com.