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...

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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.

grain elevator said:

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.

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Yes, they can, I' assuming you meant work with two 1.5V cells in series (3V total). The discharge curve of a silver oxide battery is opposite - very flat as oppose to too steep (e.g. sloped) if that's what you meant, and that's a very desired characteristic of it - it's not as almost perfectly flat as for mercury cells, but flatter than any other coin cells out there. So yes, the voltage itself is not an issue, the issue is declining voltage over the course of discharge, and for silver oxide it is minimal. As you notices, until near the end of discharge, the error could be no more than +/- half EV (half stop).
Out of curiosity, I've simulated the Luna Pro performance in LT Spice circuit simulator with the total battery voltage down to 1.2V instead of nominal 2.7V. Depending on the CdS photoresistor characteristics you may be able to recalibrate Luna Pro to work off even one 1.2V cell; almost certainly off of 1.5V cell. Again, voltage itself is not as much of a factor - "wrong" initial voltage induced error can be calibrated out and the meter will remain very accurate. The main source of "long term" error is voltage [in]consistency over time. That error is proportional to the ratio of present voltage to the voltage the unit was calibrated at.

My batteries were installed before Covid. Still going strong.

beemermark said:

My batteries were installed before Covid. Still going strong.

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I thought that my Gossen Luna SBC was eating batteries until I figured out that the meter switch was being poked and bumped in the camera bag. I started packing the bag so that nothing could hit the switch when laying still or being carried, at the light meter stopped eating batteries.

Luna pro is funny because there are three switches, low high and battery check.

I put a micro switch in the path of the needle release mechanism and routed to the regulator.

Sirius Glass said:

I thought that my Gossen Luna SBC was eating batteries until I figured out that the meter switch was being poked and bumped in the camera bag. I started packing the bag so that nothing could hit the switch when laying still or being carried, at the light meter stopped eating batteries.

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Strange testimonial. I once had an SBC that sucked batteries empty in no time. Got rid of it. 9V batteries are not cheap, Not at the rate it was eating them. Besides it's too big as a pocket meter. I saw no redeeming quality of that piece of crap over the ordinary gray Luna Pro I now use. Funny how production line manufacturing can turn out one unit, and the buyer swears by it as a cherished and valued possession, and the buyer of another unit made 20 units earlier or later that day can curse it as bitterly as mine.

F4U said:

Strange testimonial. I once had an SBC that sucked batteries empty in no time. Got rid of it. 9V batteries are not cheap, Not at the rate it was eating them. Besides it's too big as a pocket meter. I saw no redeeming quality of that piece of crap over the ordinary gray Luna Pro I now use. Funny how production line manufacturing can turn out one unit, and the buyer swears by it as a cherished and valued possession, and the buyer of another unit made 20 units earlier or later that day can curse it as bitterly as mine.

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The weakness of the Profisix/SBC is the 9 volt connector used. Over the years, the relatively fine wires can be prone to damage. Yours probably was suffering from that.
Otherwise, if you can live with the size, it is/was the best meter I've ever used - distinctly better in use than the old mercury battery powered LunaPro meters.
I also had the flash metering accessory for my Profisix - the combination was great for weddings!

MattKing said:

The weakness of the Profisix/SBC is the 9 volt connector used. Over the years, the relatively fine wires can be prone to damage. Yours probably was suffering from that.
Otherwise, if you can live with the size, it is/was the best meter I've ever used - distinctly better in use than the old mercury battery powered LunaPro meters.
I also had the flash metering accessory for my Profisix - the combination was great for weddings!

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This is very true. Although it has been said in another thread that the “black wire syndrome “ probably doesn’t contribute to battery drainage, this hidden condition seems fairly prevalent in meter failures. It’s not clear in my mind our that (and out her late-in-life failures) are a design flaw or just a reality of electronics. I tend toward the latter since the product certainly has proven itself over the years.

BrianShaw said:

This is very true. Although it has been said in another thread that the “black wire syndrome “ probably doesn’t contribute to battery drainage, this hidden condition seems fairly prevalent in meter failures. It’s not clear in my mind our that (and out her late-in-life failures) are a design flaw or just a reality of electronics. I tend toward the latter since the product certainly has proven itself over the years.

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I had replaced the 9V battery connector and wiring. Also had the thing professionally serviced. It was just a battery eater, plain and simple. But that's old history, long gone. Even had the fancy spot metering attachment. As for my trusty gray Luna Pro, I fabricated a ring to use the smaller 357 silver batteries plus a small folded up square of aluminum foil to add the needed thickness, and recalibrated the meter for the slightly higher voltage. The same 357's have been in it for years. I've used it a million times and the batteries are STILL perfect. To answer the OP's main question as to how many time you can press the switch for a reading before the batteries get used up. I'd say off-hand, I don't know. At this rate, it looks like they will out-live me.

Regarding Luna Pro meters, when the rocker switch stays in the middle (not pressed for high or low range measurement), they draw no current at all - the battery is electrically disconnected. If the unit still "drains" the battery while mostly sitting unused, that can ONLY mean you got crappy battery batch - leaky battery [electrically, e.g. high self-discharge rate]. Does not reflect on the meter quality itself, but I can see how a user turning it on or checking the batt level in the field and seeing it dead blames the unit for overall experience - "this meter sucks" rather than "this battery sucks".

Bill Burk said:

Luna pro is funny because there are three switches, low high and battery check.

I put a micro switch in the path of the needle release mechanism and routed to the regulator.

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Low/High switch (more exactly, 3 positions - Low, None, High) is a single same SPDT momentary type switch (Single Pole Double Throw), so two switches in the device, not three. If you look at the schematic of the device, when in released position it disconnects the battery completely. "Batt check" electrically is also just a momentary ON button, it's off (battery disconnected) when released. So adding any micro-switches in series that also interrupt already interrupted battery current path is redundant and would serve no purpose.

336v said:

Regarding Luna Pro meters, when the rocker switch ...

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For additional clarity that's true of the CdS Luna Pro. There may be some confusion in what meter some folks are talking about. The Luna Pro SBC (and Luna Lux SBC) does not have a rocker switch. It has a power button on the left side that is pushed to activate the measurement function. Then the meter circuit turns itself off in the read-and-hold mode. The power button can also be configured to an always on mode. It seeems potentially plausible that one possible failure mode would involve that circuit malfuctioning and not automatically turning off the meter. Or, as mentioned earlier, if the measurement button stays pushed through mechanical means other than the intent of the user, the meter would remain metering until the battery expires.

The two modes are explained on pages 10 and 11:

The only downside to the older LunaPro with the rocker switch is to remember which way to press, given the light conditions. If you are out in the brilliant July sun and press the "low light' position, the CdS cell will be blinded for a time. CdS cells ave a memory effect. About like somebody popping off a #5 flashbulb in your face. It's 10 or 15 minutes before that your eyes recover from the crazy bright spot in your vision. Same way with the CdS cell. If in doubt, press the high direction first. If the needle barely comes up, re-meter in low. Another thing to keep in mind that silicon cells can lose linearity and become un-calibratible as badly as CdS or selenium. A lot of Luna-Pros, like camera meters from that era are half dead and must be replaced. 'Ive worked on enough exposure meters of all kinds to know there's no rhyme or reason why one meter died and another of the same model is good as the day it was made.

"Blinded" CdS cell recovers within 5-6 seconds. Under normal (not very rush photography) circumstances this makes no practical difference. It'll take longer to compose a good shot...

336v said:

"Blinded" CdS cell recovers within 5-6 seconds. Under normal (not very rush photography) circumstances this makes no practical difference. It'll take longer to compose a good shot...

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I put a CdS cell under a light which has constant output. The resistance raises somewhat after some time. It's very repeatable.