Fluke 79III battery indicator - true meaning discovered

[LabSpokane]

[This is very old news to old timers, but I was spoiled at my last job and had mains-powered DMMs for day to day use.  So, this is news to me...]

From the "I know better, but I'm too lazy to crack open my meter and do the right thing now department..."

I have a Fluke 79III that was given to me by a coworker a few years back.  It works fine - or at least I thought I knew how it worked.  It calibrates out perfectly.  I've been using it strapped to my pegboard as a surrogate bench meter for when I'm too lazy to boot up my real bench meter. 

A couple of days ago, I was getting all sorts of crazy readings off a voltage regulator.  I assumed that I had screwed up the regulator somehow.  The battery indicator on the Fluke was on, so I figured that it meant: "I will die soon, but will give you good readings for a little while longer."  Nope. 

What that indicator means is "my battery is at 6V, very very soon I'm going to give you complete shit readings until my display fades out completely."  With the battery flag on, that meter will display readings up to 2X of their actual value. For the 79III,  Vbat=5V is when the readings go south.  I honestly expected the meter to have internal logic that would shut it off if it didn't have enough battery voltage to give correct readings.  Not so.

 If you have a DMM and a variable power supply, it might be worth your time knowing what it means when the battery low flag comes on.  This isn't meant to put down Fluke at all.  They did tell me to change my battery.  And I didn't.  I simply assumed that the meter had more smarts than I did.   

[retiredcaps]

I honestly expected the meter to have internal logic that would shut it off if it didn't have enough battery voltage to give correct readings.

The Fluke 77 I, II and III will run for about 2000 hours on a fresh 9V battery.

The newest generation of Flukes including the 77IV will display bAtt on the lcd and not let you take readings when the battery runs low.  The 77IV, however, will only run around 400 hours on a fresh 9V battery.

Some of the cheap 830 like multimeters won't even show the low battery indicator and give higher than expected readings.

PS. I put "weak" 9V batteries into the Fluke 70 series I, II and III series as they can use up every last drop of juice until the low battery icon shows up.

[SL4P]

I simply assumed that the meter had more smarts than I did.   

And you were right!
It did exactly what it was meant to --- but I thinbk we all agree, it should have stopped reading at the point of no return.

[Psi]

but I think we all agree, it should have stopped reading at the point of no return.

..but.. how else can they pad out their battery life estimates.

[retiredcaps]

Saw an ebay listing for a Fluke 70 with low battery indicator on.  1251V AC!  Or is the decimal point missing due to low battery?

[LabSpokane]

I'll check mine tomorrow night. I have that meter's red headed step brother.

[LabSpokane]

I just checked my 79II, which is the same vintage meter as the one you found.  It has the exact same behavior as my 79III.  t think the decimal place is just missing on that eBay meter for whatever reason.  The LCDs on those meters were never very good.  My LCD developed issues early on, but has stayed about the same for 20+ years. 

[xavier60]

My friend says his Fluke 79III is discharging its battery too quickly. I have just measured the current draw at 1ma. Has anyone else measured the current draw of a Fluke 79III ?

[retiredcaps]

My friend says his Fluke 79III is discharging its battery too quickly. I have just measured the current draw at 1ma. Has anyone else measured the current draw of a Fluke 79III ?

User manual

http://assets.fluke.com/manuals/7926____iseng0200.pdf

says 500 hours typical with alkaline 9V battery.  Assuming 9V battery is 500mAh, 1 mA current draw is the expected value on most of the functions.

[ModemHead]

I have a 26-III here which is the same as the 79-III.  It draws 1.05mA in VAC mode, 0.98mA in VDC.

[xavier60]

Thanks for the info. It was intermittently drawing 3ma, even in sleep. I noticed that I could make it come good by jiggling the mode switch. It now seems to have permanently come good after oiling the switch, drawing close to 1ma for most modes.
It now draws 360ua in sleep which seems too high. I found two 7555 timer ICs that have 3.2v still applied to them in sleep which would account for the current draw.
So far I have only found the service manual for the 79 II which doesn't seem to show the 7555 ICs.

[ModemHead]

It now draws 360ua in sleep which seems too high.

In sleep mode, this 26-III varies a lot then settles at 390uA.  It draws 6.5uA with the selector in the OFF position.

So far I have only found the service manual for the 79 II which doesn't seem to show the 7555 ICs.

The 79/29-II and 79/26-III are quite different, at least on the outside.  Never compared the circuitry but I would assume big differences.

[xavier60]

Thanks for taking the time to do the measurements. I'm going to assume the 360uA sleep draw is normal for the 79III for now.
It draws 4.5uA while set to Off.