[volt-nuts] 732A and Prologix received
Don@True-Cal
truecalservices at gmail.com
Sun Aug 24 16:46:24 EDT 2014
Randy & all,
You have correctly concluded that some (maybe not all) of your measurement
problem is thermal EMF being added or subtracted in series within your
measurement interconnect. This thermal EMF is generated at the junction of
dissimilar metals when accompanied with thermal gradients between the test
lead and device terminals. You have to eliminate both the dissimilarity of
the metal junctions as well as minimize the thermal differences. The
terminals of the 3458A as well as the 732A are Beryllium Copper so you want
to use the same test lead terminals. Forget the typical Tin plated lugs or
even Gold plated as both are not Beryllium Copper and constitute dissimilar
metals. The best solution (as usually the most expensive) is to use a set of
Fluke 5440A-7005 (48") cables. I also have just as good results using the
much more flexible Pomona 11174A (lugs end always stay connected to the
732A) or 11058A with more convenient shielded banana plugs. The Fluke cable
has the added Guard built in but be sure to also use a Guard lead with the
Pomona cabled. The Guard lead does not need to be low thermal EMF. DIY
cables is usually not a good idea because the lead wire to terminal also
constitutes just as critical of junction. The above cables use Tellurium
Copper wire which is usually hard to find and hard to crimp properly and
NEVER solder.
As you observed, merely the friction of plugging in the spring banana plug
and heat transfer from your fingers will require ~3-5 minutes to stabilize.
Low mass terminals help with the time to initially stabilize but is not best
if dealing with local air current drafts that upset the instantaneous
thermal gradients between the higher mass device terminal and the lower mass
lead terminal. This issue is a trade-off and is specifically considered in
the Fluke cables but being careful with localized drafts around the
terminals during critical measurements can virtually eliminate this error
source.
I have attached an ~30-hour measurement run that I consistently do that
should give you some idea of expected measurement drift over time. Virtually
all of the measurement drift is due to the 3458A internal temperature
differences around the 7V reference caused by ambient temperature change.
The drift associated with the 732A is probably about 2-magnitudes less at
this ambient temp drift. In my situation, the inferred ambient temperature
is cycling with the home air-conditioning. Note the initial calibration
temperatures as well as the ACal temperatures and where the 3458A is
measuring exactly 10V. My primary 732A has been powered without loss for 4
years and >5 years before that. The 3458A is Agilent (Loveland) Cal'ed
yearly and is powered 24-7-365 except for the occasional mains power loss.
The graphical measurements is using a homegrown Agilent VEE program.
Don Johnson
-----Original Message-----
From: volt-nuts-bounces at febo.com [mailto:volt-nuts-bounces at febo.com] On
Behalf Of Randy Evans
Sent: Sunday, August 24, 2014 9:22 AM
To: Discussion of precise voltage measurement
Subject: Re: [volt-nuts] 732A and Prologix received
Bill,
I have convinced myself that the problem I an seeing is due to thermals.
If I move the cables (with gold-plated banana plugs) using a small towel
rather than letting my hand touch the plugs, it is much more stable. If I
then hold the banana plug with my hand after the reading has stabilized, the
reading drifts rapidly upward. I am trying to check the stability of the
reading but I haven't figured out the MATH function yet. I assume this is a
programmed function using GPIB only?
The stability I am seeing by manually recording readings (using NLPC of 100
and 1000) is much greater than what you are measuring on your system. Not
sure how to ascertain if it's the 3458A or the 732. The value of the
readings are very different between the two - the 3458 reads about 50 uV
high on the 10 V output and about 12 uV low on the 1V output. Rather large
differences (this is after an ACAL). I need to find some better cables to
make sure the errors are not due to thermals again.
Randy
On Sat, Aug 23, 2014 at 10:21 AM, Bill Gold <wpgold3637 at att.net> wrote:
> Randy:
>
> I get 6 volt 4 amp/hr (or 4.5 amp/hr) batteries and they will fit
> perfect. Power Sonic PS-640, Genesis NP4-6, Panasonic LC-R064R5C and
> others that are in this size and package. Order from one of the usual
> electronics distributors like Allied, Mouser, Digikey. This is a very
> common battery as it is used in a lot of "EXIT" signs so they are
> lighted when the power goes out. I don't see how the 12V 7AH will fit
> as they are too large. I guess you could use a 12V 5AH (PS1250) as it
> is the same size as 2 x 6 volt 4 AH but the terminals are in the wrong
> place so you will have to "nibble" out the aluminum plate that holds
> them in the 732A battery pack. You have to be careful if you use the
> 12v 5AH as you will have 4 extra battery connection leads to deal with
> and connect correctly. I would stick with the 6V 4AH.
> New batteries will last around 12 to 14 hours before the "CAL" light
> goes out when AC power is not applied. So shipping to Cal Lab can be
> a problem if it is a distance away, or you have to use a shipper like
> UPS or FEDEX and you ship the night before and then use their "Morning
> delivery" and the Cal Lab is expecting your 732A. Same on the way
> back to you. Of course you could always strap another battery on the
> 732A and hook it up to the "ext power" plug to last longer. I have
> seen it done. The issue is to get the Cal Lab to charge the extra
> battery before they ship the 732A back to you.
>
> When you remove and work on the battery pack always have the AC
> power plugged in. The "CAL" led will stay on because the 18.6 v
> regulated supply is working.
>
> The "CAL" light is to indicate that power has not been lost to the
> Reference Amp or other associated circuits. When the raw supply
> (battery) voltage drops below about 21 volts the "CAL" light will go
> out. Below that voltage the heater circuits will not work correctly
> and the 18.6 volt regulated supply will not regulate. The requirement
> is that the Reference Amp be kept "alive" at all times to maintain the
> output voltage that was measured at the time of the most recent
> Calibration or Certification. When the semiconductor junctions are
> unbiased and cool off when power is lost, and then power is restored
> the result will be a different 10 volts than before the power failure.
> My experience is that after all of the years that these units have
> been powered up, this won't happen and when power is lost and then
> restored, even months later, the 732A will come back to almost exactly
> the same 10 volts as when they lost power, usually with in 0.2 PPM
> after 24 hours of "warm up".
>
> What type of hookup leads are you using when measuring the 1 volt
> output? What is the PLC set to? I always use 100 PLC to measure
> this. If you don't have "low thermal" connection leads you can
> experience uV changes for a minute or more after plugging in the leads due
to the "thermals"
> generated because of the difference in temperature between the banana
> jacks on the 732A and the banana plugs of the leads. I have found
> that even just plugging in the lead will generate a thermal difference
> because of difference of temps and some heating due to the physical
> act of just inserting the plug because of friction between the jack
> and plug (my theory at any rate). You have to allow at least a minute
> or more before being able to make a measurement after plugging in the
> leads. I just measured the variation of the 1 volt output of my 732A
> and using my 3458A and I got a total difference of 0.159 uV over 40
> measurements using 100 PLC on the 1 volt range of the 3458A. Using
> the MATH function and all of the data you can collect. That was after
> waiting for several minutes after plugging in the leads.
>
> I hope all of this helps.
>
> Bill
>
>
>
> ----- Original Message -----
> From: "Randy Evans" <randyevans2688 at gmail.com>
> To: "Discussion of precise voltage measurement" <volt-nuts at febo.com>
> Sent: Friday, August 22, 2014 9:03 PM
> Subject: Re: [volt-nuts] 732A and Prologix received
>
>
> > Todd,
> >
> > Thanks for the info. I have several Panasonic 12V 7 AH batteries
> > that I keep topped off and they have very low current draw (~2 to 3
> > mA at 13.5
> > VDC) when charged and at their float voltage, so I am pretty sure
> > they
> are
> > in good condition. I will look at getting those in the units after
> > I ascertain the condition of the 732.
> >
> > So now I have a what appears to be a functioning 3458A and a 732A
> > but
> they
> > slightly disagree. I am like the man with two watches that disagree
> > on
> the
> > time - which is correct? For the moment, i am only concerned with
> > stability. The need for absolute accuracy will come later.
> >
> > Randy
> >
> >
> > On Fri, Aug 22, 2014 at 8:06 PM, Todd Micallef <tmicallef at gmail.com>
> wrote:
> >
> > > Randy,
> > >
> > > You have two possible choices. It can be configured with 4 x 6v
> > > 4Ah batteries or 2 x 12v 7Ah batteries. Hopefully the previous
> > > owner has modified the battery pack already. A couple of mine
> > > needed a nibbler
> tool
> > > to remove enough of the aluminum cover that fits over the tops of
> > > the batteries. The original cover will short out to the battery
> > > tabs
> regardless
> > > of the battery configuration if this is not done.
> > >
> > > You can find larger capacity batteries that will give you slightly
> > > more battery life. I lost a couple sets of mail-order batteries
> > > after a few extended outages. I would recommend going with locally
> > > bought batteries instead of the cheaper mail order. My local
> > > Batteries Plus will
> typically
> > > have some warranty if I remember correctly. Moving forward I will
> > > only
> use
> > > 2 12v batteries and pre-charge them on a battery charger to
> > > equalize
> them
> > > before putting them in the 732A. I think the cheap batteries did
> > > not discharge equally, and would not recover when power was applied.
> > >
> > > Inspect the back plane for damaged traces and look at the
> > > capacitors. I
> had
> > > a few that looked questionable. So far, I have replaced all the
> > > big
> caps
> on
> > > the pre-regulator and regulator boards. My feeling is that once
> > > these
> go
> > > online, they should run as long as possible between repairs.
> > >
> > > The battery charger circuit may need adjusting. I tweaked mine and
> > > it seemed to work fine.
> > >
> > > Todd
> > >
> > >
> > > On Fri, Aug 22, 2014 at 10:42 PM, Randy Evans <
> randyevans2688 at gmail.com>
> > > wrote:
> > >
> > > > I received my Fluke 732A today. Just powered it up but it needs
> > > > new batteries. Any suggestions for sources (I haven't opened up
> > > > the unit
> > > yet -
> > > > I want to make sure it works before doing that). Also received
> > > > the ProLogix USB-GPIB adapter. I plan on using Mark Sims' CAL
> > > > ran data
> > > dumper
> > > > program to get the CAL data from my 3458A. Should be a busy
weekend.
> > > >
> > > > Randy
> > > > _______________________________________________
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> > > >
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