We have the turbo on our SX100, but to quote the person I learnt from: "Never trust the gauge!".
Yes, I started to understand such narrative. But is the situation still the same as decades ago and can't we have anything better?
At first, Alcatel Penning gauge (gauge installed on early models of SX100) is indeed hard to trust - there is whole pipeline of things where stuff can go wrong... I dig up through piles of old emails (my late predecessor had printed all communications and emails, which I could look through and learn the whole history of repairs of our SX100), and learned that indeed the problems with vacuum measurements were re-emerging again and again and again... Not only gauge but also cable was changed many times. Investigation of complete construction had revealed major weaknesses of this type of gauge.
1) High Voltage is sent through coaxial cable. That is about 4kV which is constant. If dielectric material weakens somewhere in the cable it start to leak and generates some current at HV source side. The whole vacuum measuring is based on precise current measurement through shunting resistor at returning path of HV generator, so such additional cable leakage current will sum up with current generated at gauge and worsen the displayed value of vacuum. But, in case of a huge leak in cable there will be no possibility to start penning gauge at all, as there will be significant voltage drop at gauge (leakage in cable will form something like voltage divider). Even if it will be clean gauge - bad cable will hinder its functioning. As for alcatel gauge cables - these are no more produced - all available at second hand shops are of questionable quality! Making new cable (DIY) from scratch is also hindered by requirement of HV reliable cable terminations (tools, expertise), and proper cable availability on the market (There are cables more think, and expensive).
2) Penning gauge, while being in simple construction, can give easily wrong idea about vacuum when not striking. Its lack of current (no strike event) could be compensated with small leak and final reading would look as good properly working gauge value. Experienced user knows that reading is wrong - but only if user tracks vacuum readings closely after the moment the vacuum valve is opened, and tracks the vacuum value response to that event. If vacuum suddenly just goes to 5E-5Pa from very rough vacuum - that is clearly unrealistic. However, if user leaves pumping unattended, and gets back to instrument after 10 min (and cable is leaky a bit), he can see the i.e. 4E-4Pa, even if gauge had not strike. Also Penning gauge needs constant high voltage, that have tendency to deteriorate the weakened isolation material in the cable and progress the problem(s).
Thus I came up with initiative, to make a replacement card which would interface modern gauges to SX100 (giving-up alcatel penning). That would be not jumping one tier in technology (like SXFive's used Agilent IMG) but two tiers. The next technological tier, IMG (inversed magnetron gauge), is more advantageous compared to Penning, but still requires the high voltage cabling and its correct termination and custom made current sensing circuits are difficult to do properly. Interfacing stock controlers of gauges is also hard as it would require changing firmware and finding some serial port on Cameca SX100 vacuum logical board. (That is rather completely out of equation).
Inside SX100 vacuum supply box, there is small board interfacing Alcatel gauge. It produces HV supply, measures the return current and sends the calculated vacuum as log10(P) as DC signal from 0 to 10V (10V roughting, 0V -ultimate vac) through coaxial cable to Vacuum logical board.
Thus naturally my attention got stolen by active gauges, which is higher tier than normal IMG. Active gauges work with low voltage cables, and produce all required HV for integrated IMG inside the gauges integrated electronics and send out only low voltage 0-10V signals (see the pattern?). Such gauges are also flexible as they can be powered from 14.5 to 40V, and such supply is already present in the Vacuum supply box. Also it looks that few different vendors have very similar connection based on 8C8P (or FCC68, or well known RJ45 ethernet cable/sockets), with same basic pin configuration: at least I found out few such gauges from Edwards, Curt J. Lesker and Inficon with such connections.
But what are advantages?
1) No HV cable. Basically simple Ethernet cable will do the job, as only 2W +15V is required to power such gauge, and signal is 0 to +10V. Replacement cable can be easily made or bought. (but it should not age at all, differently to high voltage cables).
2) These active gauges have builtin microcontrollers and LED(s) which show(s) status of gauge (i.e. if strike was successful) - the meaning of vacuum reading is clear. Situation then strike failed, and we have low current and very low vacuum reading is easily to catch. Also Active Gauges reduce high voltage to minimum after strike - this means less contamination of gauge - longer exploitation time between need of cleaning.
3) Edwards gauges (i.e. AIM200) has special multi-strike geometries which guaranties striking even in dirty environments.
How it will work?
My idea is to translate the voltage from such modern gauge to corresponding expected DC voltage by logical vacuum board of SX100. Basically my design is few OPAMPS for DC offset, scale and clip (values out of expected range). Two potentiometers will allow to easily recalibrate board to different kind of gauges from different vendor. It would be possible even to use WRG from Edwards (albeit pirani range would not be seen). The idea is to have seamless replacement (no firmware or software changes).
So how many people would be interested in such design?
I have schematics
alreadynearly finished (see the attachement), and now I am at stage of PCB design.
I won't produce and sell the boards, the EU laws and handicaps are too enormous for me (Basically RoHS3 forbids me to make and sell reliable electronics). But I could share the design, gerber files, bill of materials and notes for proper assemblage at site overcoming RoHS hindrances. PCB when having these is possible to order in most of the world. Buying parts and assembling them on PCB should be not too difficult, board is designed to be hand solder-able (no surface mounted parts, everything "though hole" parts). Albeit design could be updated with surface mounted parts if someone would decide to produce and sell larger batch of such boards.
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). I fed coaxial cable of secondary gauage with different voltages while watching vacuum values shown in GUI. The setup was simple 20k potentiometer, two 9V batteries (two connected in series, so I could cover with scanning voltages up to 10V), SMB coaxial connector, small breadboard to make some T junctions to attach the voltmeter for V measurement. By changing potentiometer I scanned from 0 to 10V and wrote down corresponding vacuum values. It seems that up to 9V the relation between log(P) and U is pretty linear. Actually in normal circumstances the gauge is engaged with better vacuum so below 9V. Thus that low vacuum range (9V-10V) can be ignored, and then looking for offset and scaling values I was looking that it would cover 0.5-9V range of expected values. The attached spreadsheet contains these recalculations with estimated gain and offset values for DC. tables has two offset values, mathematical and InAmp offset (InAmp - instrumentation amplifier). Mathematically we see no difference between these two equations:
