Well, with 5 V and 1E14 ohms, the current would be down to 50 fA, which
ain't too goodly a number of electrons, considering I designed IRMA-1 in
1986. I discovered all sorts of peculiar ionic effects in FR-4 with it
and published a few papers on the subject. One polymer science student
at the Swiss Federal Institute of Technology, Lausanne, used it for his
dissertation on the electrical characteristics of epoxy resins. I was
named the external expert adjudicator for his Dipl.Ing. He did some
really outstanding work and his prof told me that his dissertation would
have been good enough for a MSc thesis. He won the annual prize for the
most outstanding graduate. I tried to persuade him to go for his
doctorate, but he exported himself to a US industry unrelated to his
undergrad work.
I really was disappointed that the IRMA-2 project was dropped because I
spent >2 years on its development and it really did work a treat. I
guestimate it would have cost c. $60k for 96 measurement points,
complete with custom computer with high data security and UPS for 12 h
power failure (it would have been really annoying to do a 3-month SIR
test on 96 samples for the power to fail after 89 days!).
I suppose it's flattering that Keithley copied my idea: I thought about
applying for a patent in 1985, when I first had the idea, but my
attorney said that it would be a thorny one proving there was no prior
art: someone could argue that the controlled discharge of a capacitor
through a xenon tube in a photographic flash to produce a given number
of photons was partial prior art, even though the purpose was different.
The RC discharge formula has been known for over a century!
My trusty AVOmeter is still in the land of the living, covered in dust.
When I retired and came out to this island, I brought my oscilloscope
and some other things with me but, believe it or not, I don't think I've
even switched it on since, and that was 10 years ago!!!! So, no! I'm
busy with all sorts of non-electronic projects!!!
Brian
Hfjord wrote:
> Brian, it looks like you've been involved in myriades of projects! Anyway,
> your principle is used by Keithley. They make an instrument that can measure
> attoamperes! 1 attoampere = 6 electrons per second!! They use a
> Voltage/Current source same way as you did, i.e. a kind of capacitor that is
> discharged through TeraOhm resistors. The discharge current is plotted vs.
> time and a number of such events are integrated and computerized to give a
> readout. Dynamic principal with other words. I have their instrument P/N
> somewhere, if you are interested, or you will find it yourself. I believe
> it's a 100K dollar instrument. Instruments have undergone a remarkable
> evolution. My private and cheap and simple handhold Fluke is capable of
> measuring microamperes with a resolution of 0.1 uA. Price : 350 USD.
> Are you still mixing with electronics at home?
> Can't help admiring you, oldie!
> /Inge
>
> -----Ursprungligt meddelande-----
> Från: TechNet [mailto:[log in to unmask]] För Brian Ellis
> Skickat: den 1 december 2007 11:03
> Till: [log in to unmask]
> Ämne: Re: [TN] SV: [TN] SV: [TN] Low Noise Coaxial Cable Question
>
> AAAAAH! Now I understand the question. In the dim and distant past, I
> designed and built a prototype Insulohmeter IRMA-2, a basic improvement
> on IRMA-1. Unfortunately, for political reasons imposed by the company I
> was working with, it was never put into production. Believe it or not,
> it could measure up to 1E14 ohms ± 30% at 5-100 V or 1E12 ohms ± 2.5% at
> 5-100 V, without screening/shielding of the PTFE cables. How could this
> be? Because it did the measurement dynamically, not statically. A PS
> capacitor was charged to the required volts and allowed to partially
> discharge through whatever was required to be measured. The voltage
> across the capacitor was monitored with respect to time and the SIR was
> calculated from the discharge curve. It worked beautifully. Why was it
> so noise free? The capacitor effectively short-circuited the measurement
> circuit and its reactance, even down to <50 Hz, was several orders of
> magnitude less than the resistance being measured, so any induced
> voltages were negligible and did not affect the measurement.
>
> Unfortunately, this is academic, because IRMA-2 never got beyond a damn
> good working prototype.
>
> Brian
>
> Douglas O. Pauls wrote:
>> I will chime in on Joe's behalf. This actually started with a
>> conversation between he and I.
>>
>> We do SIR testing. As such it is not unusual to be measuring resistances
>> on the order of 1E11 or 1E12 ohms. In general, a voltage is applied to an
>
>> unknown resistance and a resulting current is induced. We are both doing
>> testing at DC, not AC domains. A very good text on low level measurements
>
>> comes from Keithley Instruments, originally authored by John Yeager. In
>> John's experience, the measurement world changes a great deal between 1E11
>
>> and higher decades of resistance measurement. Above 1E12 ohms, you have
>> to start watching other sources of current generation that can erroneously
>
>> add to your desired signal. This includes things like EMI coupling and
>> triboelectric currents from moving cables. EMI coupling and guarding is
>> why co-ax and other shielded cables are used.
>>
>> Keithley, in their very low current measuring instruments and switching
>> cards, makes use of "low noise cables", which essentially have graphite
>> coating the outer surface of the inner dielectric, to reduce triboelectric
>
>> charges from the braided shield whenever the coax is flexed. Of course,
>> such specialty coax is pricey and led to Joe's more generic questions
>> regarding specifications or measurements of "low noise" and how low a
>> noise is "low noise".
>>
>> That Joe is a demon when it comes to trying to exactly understand
>> measurement signals. That's a good thing.
>>
>> Doug Pauls
>>
>>
>>
>>
>> Hfjord <[log in to unmask]>
>> Sent by: TechNet <[log in to unmask]>
>> 11/30/2007 12:27 PM
>> Please respond to
>> TechNet E-Mail Forum <[log in to unmask]>; Please respond to
>> Hfjord <[log in to unmask]>
>>
>>
>> To
>> [log in to unmask]
>> cc
>>
>> Subject
>> [TN] SV: [TN] SV: [TN] Low Noise Coaxial Cable Question
>>
>>
>>
>>
>>
>>
>> Exellent! However, Joe has still not told us WHAT he wants to do with the
>> cable. I doubt he is on a level, that dielectric intrinsic noise will play
>
>> a
>> role. Few test engineers work with such problems. If he isn't a Nobel
>> Prize
>> aspirant of some kind. I suggest someone finds remedy against Tinnitus.
>> He/she will get the prize, I'm sure. And the inventor will create a lot of
>> noise...he-he
>> /Inge
>>
>> -----Ursprungligt meddelande-----
>> Från: TechNet [mailto:[log in to unmask]] För Seth Goodman
>> Skickat: den 30 november 2007 18:55
>> Till: [log in to unmask]
>> Ämne: Re: [TN] SV: [TN] Low Noise Coaxial Cable Question
>>
>> Joe,
>>
>> Some of what has been mentioned is really part of other cable
>> specifications and has nothing to do with noise created in the cable
>> itself. For instance, penetration of electromagnetic fields inside a
>> cable shield is often described by the manufacturer's specification for
>> shielding effectiveness. To the extent that the cable has in imperfect
>> coaxial shield, it can convert ambient electric or magnetic fields into
>> a loop voltage or flowing current. If external fields are the source of
>> your problem, you want cable with high shielding effectiveness as
>> opposed to low-noise cable. For electric fields, improving the shield
>> means thicker braid, finer wire in the braid, better conductivity of
>> braid wire and plating, more than one braid and/or a foil shield. For
>> magnetic fields, a shielded twisted pair may do better.
>>
>> The traditional electronic noise sources, which are Johnson (thermal)
>> noise, shot noise and a group of unrelated mechanisms that produce 1/f
>> noise, do not produce appreciable noise in cables. When cable
>> manufacturers list a cable as low-noise, they usually mean triboelectric
>> noise. If the connected circuit puts a dc potential on the cable, then
>> mechanical flexing can change the capacitance between conductors, which
>> will cause current to flow in the external circuit. The triboelectric
>> and capacitance change with motion can together be called microphonics,
>> as they are both motion-related. Another source or noise in
>> high-voltage cables is leakage.
>>
>> Regards,
>>
>> Seth Goodman
>>
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