Few questions to Luna Pro / Lunasix 3 users

Fellow photographers, I'm trying to calculate how much time a given battery powering Luna Pro will last. I know
everything I need to know circuit wise, but one critical parameter - typical energy consumption over some usage
time is missing as it depends on very diverse pool of individual people - pro, amateur and casual users. I can use
great average or group the users, will have to see the data first.

So my questions are simple - please guestimate:

- how many times per day you take a measurement (depress the side rocker switch)? If you can tell what approx.
percentage of that is on high scale (e.g. mostly outdoors), even better. There is slight difference in power
consumption on each scale, so I can take this into account.
- how long do you keep the rocker switch depressed for a single measurement? In my own experience
it takes 5 sec on average, but I'd want to know what people do, esp. frequent users.
- how often do you check battery condition using sliding switch on the back? Like, once per day of shooting?
before every [or so many] measurements? The power consumption during batt. checking is the greatest, so
how long do you usually keep the switch on? I notice the needle in my meter settles within 3 sec, and I usually
do check once in the morning of a field trip day, and may be once more during that day.
- finally - once you cite average usage pattern on a typical shooting day, how many days per year you'd say
you do it in average?

Once I collect enough opinions and numbers, I will present calculations here and we can figure out which
battery for the Luna Pro or Lunasix 3 (or any meter that use to take two mercury cells) is the best overall
option these days. I'm making the battery adapter that takes few common batteries, but want to optimize
it for just one type - more convenient in case some of you might be interested in getting such adapter.

Thanks in advance!

Leakage currents, self discharge and environmental conditions will have a profound influence, quite likely much more so than actual use. Put simply, the batteries in a meter like this die due to sitting around more so than due to actual use.

koraks said:

Leakage currents, self discharge and environmental conditions will have a profound influence, quite likely much more so than actual use. Put simply, the batteries in a meter like this die due to sitting around more so than due to actual use.

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Yes, I'm well aware of self-discharge and slow discharge through the regulator circuit, but to make a judgement whether those are negligible compared to even sporadic use I need numbers. Once I know how much energy just metering itself takes, say, in a year, I can figure how significant is contribution of the factors you've mentioned. For someone pro reporters using these meters may be 50 times/day every day, known [decent quality] battery self-discharge rate may not be relevant as most of battery energy is put to actual good use. Sorry, I can't plug "quite likely much more" qualifier into excel, I need actual numbers if I want more-less accurate estimation.

If the regular is just a diode in series then it won't drain the battery but it's not a good regulator as it requires the battery voltage to be steady. But I wouldn't worry much about battery life as I woudn't use such a meter for real. There are many better ones and they are not that expensive.

Seems like an awful lot of effort for a meter that few probably still use. And one where manufacturers battery adapter solutions exist already. (But you’re a EE and I know a thing or three about EEs. LOL) Certainly no pros are using it anymore, I’d venture a guess.

4 times per day; 100% high. About 33 % of the time I click it the wrong way first, tho
4 seconds duration
Battery check once per day/session
12 days per year.

I use the Gossen adapter with silver cell and replace batteries annually, approximately, whether I need to or not. I don’t ever recall wearing out batteries even when I used the meter a lot more.

336v said:

Sorry, I can't plug "quite likely much more" qualifier into excel, I need actual numbers if I want more-less accurate estimation.

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In your place, I'd do some worst-case scenarios with assumptions of high use. Take a use case where a photographer uses the meter 5 days/week, for a 3-hour session every day, using the meter X times an hour, etc. That'll give you some numbers and a feeling for the orders of magnitude you're up against.

Leakage currents etc. you'll also have to make assumptions for, so whatever you plug into Excel, it's going to yield a very, very crude approximation anyway.

Thanks Brian, sure not a lot of use. It may be a lot of effort, but as any hobby (like photography) it is an effort for us to prepare cameras, take photos,
mix chemicals, develop film and paper, etc, but we still do it because we enjoy it. Same here, I enjoy messing with electronics, so pretty much don't care
what it takes - I just want to accomplish my goal to make stable power source I can install and forget about it. Actually, it won't take a lot

FWIW, I'll be using a single 170mAh CR1/3N lithium cell and the quiescent current of the 2.7V voltage regulator is 250nA (actually measured).
Continuously draining that cell at that rate theoretically amounts to 170,000 uAh/0.25uA=680,000 hours or 28,333 days or 77 years before it goes dead
due to this quiescent draw. Obviously it will loose charge on its own far before, so for practical purposes regulator's parasitic current draw can be assumed
zero and so no power switches needed.

A single measurement consumes around 200 uA in average on high scale, same for battery test. In 4 seconds that's 0.2mA*4=0.8mA*s. 5 times a day
(4 measurements + batt check once) then is 0.8*5=40mA*s, 12 days a year is is with worth of energy. 4 times a day 12 days/year is 480 mA*s worth of
energy, which is 480/3600=0.13mAh/ year. With that energy use a 170mAh battery cell would last 170/0.13=1,307 years energy wise, so certainly it will die
on its own from natural causes far before then. Like a far smaller coin cell in my wrist watch lasts at least 10 years running 24/7, 10 or more years would be good enough working time for Luna Pro, esp. considering it's just $1.63 per cell on Amazon today ($9.79 for a pack of 6, which will certainly outlive me by a long shot...).

Anyway, while above is just small fun math exercise, it's proof that you just cannot run out of energy by using the meter - the energy use even 100 times heavier than you cited is still negligible compared to available battery capacity. Also, it's the proof to purists that using a diode in series, which makes idle consumption true zero, doesn't buy you anything: theoretical 77 years of continuous drain by the proper 2.7V regulator would just gets stretched beyond 77 years if such a reg is replaced with a diode. We know that no cell will last even 1/10 of that anyway, so a diode is not electrically more superior solution at all. People do it just because it is trivial to implement and nothing better is available, but we know diode's shortcomings.

Anyway, thanks for your input, Brian. Once I'll put few adapters together, I'll report the outcome of the project here - in case someone would be curious...

336v said:

it's proof that you just cannot run out of energy by using the meter

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And therefore, the outcome falls away against the more relevant factors like battery chemistry, manufacturing consistency/quality, storage conditions etc. Many of the more relevant factors are far beyond the control of the consumer so we can only take our best guess at it and buy batteries of a 'reputable brand' (insofar as that has any meaning left today).

I completely understand you, generally speaking… I spent most of yesterday trying to source a 3BA screw (really hard to find in USA) because the one I have is cross-head and I’d prefer slotted.

Not intending to distract or discourage you, but have you considered a different meter? The LunaPro served me well since the 1980’s yet has some drawbacks. Battery is just one of them. The CdS memory effect led me to recently replace it with a LunaPro SBC… no memory effect and a standard 9v battery. The drawback is size. The dial is very similar and the null metering scheme is very fast. Cured a lot of extremely minor annoyances…

336v said:

Anyway, thanks for your input, Brian. Once I'll put few adapters together, I'll report the outcome of the project here - in case someone would be curious...

Click to expand...

I mean, I have a Lunasix and tinker in electronics. If you found a design that worked and put it up here, I'd probably give it a go implementing it myself. It's a "why not" kind of deal. It's not a meter I use a lot, but if it's a fun little project to get it back in spec, why not? After that the cost to keep it running is negliable.

These meters can be adjusted to read correctly with a 1.5 V cell. Is the discharge curve of a silver oxide cell too steep to give correct readings for significant time, in your opinion as an electrical engineer? For my needs, an accuracy of +- 0.3 stops is plenty.
Another reason why I would be hesitant to invest much in these meters is that the bearing of the rocker switch is brittle and the spring quite strong - the bearing has cracked on mine.