[NJARC] Old Battery Question

[Scott Roberts]

Hi Jim(& all!)

Thanksgiving was fine- Thanks! Family came by, had a huge turkey, lots of good stuff :) Hope everyone else had as great a time!

I also rebuilt the two Burgess battery packs- the cases came off nicely, and so I copied the inside using nice, new Energizer AA cells- 5 of them each, wired together in series, with a tap at each for the take-out voltage. When I was done, I hot glued them all together, and refit them to the original case and terminals, and all give pretty good battery readings for the desired voltages. Already installed in the tester. And I finished building the adapter I needed, & checked its wiring. So, now I should have enough together to do a proper check out with it. Just have to get my specs together.(Now I have to build a 4.5v cell for the small MM for the set... lol...)

I took plenty of pics of the build procedure, so I will have to go through them and find the best representative ones, and prepare a slide show for a later show and tell meeting ;)

Scott


--- On Thu, 11/27/08, Jim Whartenby <antqradio at sbcglobal.net> wrote:

> From: Jim Whartenby <antqradio at sbcglobal.net>
> Subject: Re: [NJARC] Old Battery Question
> To: njarc at mailman.qth.net
> Date: Thursday, November 27, 2008, 9:00 PM
> Visit our web site - See http://www.njarc.org
> _______________________________________________
> Greetings Scott
> You express very well why most of us like to repair and
> collect old vacuum tube equipment.
> 
> There are several reasons why your "True RMS" DVM
> is giving you readings that may not seem right.  The first
> thing to remember is the difference between accuracy and
> precision.  A good tutorial is:
> 
> http://www.mathsisfun.com/accuracy-precision.html
> 
> The second item to consider is how much the meter loads the
> circuit under test.  A good explanation is found here:
> 
> http://www.allaboutcircuits.com/vol_1/chpt_8/3.html
> 
> As I understand the issues that you bring up; you prefer to
> use period test equipment to repair equipment of roughly the
> same age.  There is no problem with this as many of us have
> done just that.  You can also use modern test equipment and
> just ignore the numbers past the first two or three
> significant digits.  This should give you the same precision
> that reading an analog meter will give you.
> 
> The precision of the measurement when you are measuring B+
> with your DVM is most likely caused by the AC ripple in the
> power supply.  Your DVM is so good, it is trying to measure
> a fluctuating voltage.  An analog meter would average this
> fluctuation out because of the mass of the meter pointer.
> 
> An accuracy issue is caused by the meter loading the
> circuit.  This changes the actual voltage at the measuring
> point.  The old VOMs are 1000 ohms per volt, the better VOMs
> like the Simpson 260, Triplet and the like are 20,000 ohms
> per volt and some are 50,000 ohms per volt.  This means that
> the Simpson will read a bit higher then the former since it
> loads the circuit less.
> 
> Your DVM is most likely 10,000,000 ohms per volt with 4 1/2
> digits and is highly accurate.  The DVM is comparable to a
> VTVM but the precision is at least another digit or so
> because of the issue of reading an analog scale better then
> three significant digits.
> 
> You can do a test with the battery you are assembling. 
> Measure the cell voltage with as many meters as you have. 
> All should agree at the two most significant digits with the
> DVM able to tell the actual voltage to  better then four
> digits when measuring a single cell.  In other words, all of
> your meters should tell you that the cell voltage is 1.5
> volts.  The DVM should tell you that the cell is 1.5123
> volts.
> 
> Hope your Thanksgiving was as good as mine,
> Jim