Hello,
The trick to recoating the NPD is patience, if you try and rush it, you will fail.
There usually was an instruction piece of paper that described the technique so ever thing below is from what I remember 20 years ago, so if I miss something, hopefully someone else will pick up on it. or remember the points I have forgotten.
1) Turn down the NPD bead setting down on the bottom LHS of the GC
2) Put a sheet of Alfoil on the GC that you will work on, as the Al oxide will make a mess when you add it, and remove it from the NPD.
3) Make a small piece of paper, and fold it so it has a distinct crease, and then unfold it, as you be putting the Al oxide in this, and using it to sprinkle onto the bead, you should practice this beforehand so as to perfect the technique.
4) Remove the NPD and with the cable still connected, place it upside down on the Alfoil so that you can clearly see and have access to the ceramic bead.
5) Slowly turn up the bead setting until it just starts to glow.
6) Now here I don’t remember exactly, and if other people do please comment, I think you then increase the setting by 100
7) Whilst wearing nitrile gloves remove the Ru stick, it should be clear, if cloudy then water have absorbed into it.
8 ) Very very gently put the Ru stick close to where the wire is attached to the inside metal wall of the NPD, as this is the hottest point and the Ru stick should begin to melt.
9) Then in a SLOW wavy pattern with lifting the Ru stick , as if you were putting mustard on a hot dog, coat the bead , when you get to the end, coat the flat part at the end too, do not lift the Ru Stick and coat the bead again in a wavy pattern until you are back at the wire that is attaching the bead to the metal wall, as this is the hottest point.
10) Very slowly start to lift the Ru stick away, what will occur is the Ru stick will start to cool and start pulling off the molten Ru that is on the bead, stop and slowly push the Ru stick back towards the bead, but not all the way, and very slowly start to lift the Ru away again
11) Repeat the above until the tip of Ru stick becomes so narrow when pulling away, that it snaps off the thread of Ru.
12) I usually leave it for a minute to allow the Ru on the bead to melt and self-level
13) You now are ready to add the Al oxide to the NPD, the following is a technique that I found to work best, so it wasn’t written in the instruction sheet.
14) Using the folded piece of paper, add the Al oxide (I always added excess), then fold the paper closed again but loose, so that when you tap the end of the paper Al oxide will fall out the other end.
15) Now tap the paper and coat the ENTIRE length of the bead, any excess Al oxide will fall around it and pool in the chimney area, this is perfectly find, and is actually what you want for the next stages.
16) Now that it is cover, this is critical, wait at least 2 minutes for the Al oxide to absorb into the molten Ru
17) Now lift the NPD housing, and in a very quick fashion turn the NPD upright, the excess Al oxide that had pooled at the chimney will fall out onto the Sheet of Al foil, but importantly because you turned it quickly, it will coat the other side of the bead with Al oxide ( do not rest the NPD and hold it level),
18) Again wait at least 3 minutes for the Al oxide to absorb into the molten Ru on the other side of the bead lift, plus so that the melted Ru self-levels again.
19) Repeat the above Al oxide coating of the bottom and top of the bead using new Al oxide, not the Al oxide that fell out.
20) Very slowly whilst you are still holding NPD housing upright, dial down the NPD setting, as you don’t want the molten Ru to crack during cooling.
21) Once cooled, gently tap the housing to remove any Al oxide, and re-assemble.
22) Now you need to condition the NPD, and don’t remember the exact procedure
23) I think what you do is whilst looking down the chimney of the NPD, turn on all the gases, and very slowly turn up the NPD setting until there is a very faint glow, and I think the display panel reading is less then 5pA
24) You then wait until the system stabilises and slowly turn up the NPD setting until it is 5pA and wait again until it is 5 pA, and do this repeatly.
25) You then increase it until to another pA reading that I don’t remember etc…
26) Then hopefully you will be at the usual pA level (again that I don’t remember), and you start using the NPD.
Alex
