Gossen Lunasix (CdS) using a single battery

elerion · Mar 30, 2017

I bought an old Gossen Lunasix lightmeter. It's readings are 1 stop off in some light level readings, and dead on on some others.
I opened the battery holder, and I found an alkaline battery (625A, 1.5V).
I was expecting this. I know how to callibrate it for using silver oxide batteries, so that is not a problem.

What I found to be strange, is that my model seems to use a single battery. All manuals and references to these meters speak about two batteries (2 PX13 or PX625). This one seems to use a single 1.35V battery, not two, and not a PX14 one (which is 2,7V).
Next to the battery lid there's a text "Mallory MR625".

Anyone knows about this?
Could I be wrong?

silveror0 · Mar 30, 2017

My Lunasix (from the 1960s) takes 2 batteries, PX625A, which I installed before sending it off the Quality Light Metric in Hollywood for calibration check. I queried them before doing so to make sure those batteries were correct; they confirmed. Never heard of a version that uses only one battery.

flavio81 · Mar 30, 2017

Mine uses two PX625 MERCURY batteries.

elerion said:
I know how to callibrate it for using silver oxide batteries, so that is not a problem.

Internally it has a LOT of variable resistors (potentiometers) for adjustment. Do you have the procedure for recalibration?? If you have please send it to me or point me to where this document is.

Far easier is to fit the #675 zinc-air batteries, which will give the correct, stable voltage.

silveror0 · Mar 30, 2017

flavio81 said:
...Far easier is to fit the #675 zinc-air batteries, which will give the correct, stable voltage.

Zinc air batteries have next to nothing for life after removing the cover from the air holes. I use p675 in my hearing aid and I have to replace them every 2.5 days on average, although that may be dependent on the power they must provide to the device they're used in.

elerion · Mar 31, 2017

Mine seems to use a single battery.
I just replaced it with a single SR44, 1.5 silver-oxide battery, and it seems to works fine (still not precise over the full range, because I didn't callibrated it yet). Strange (and you guys confirmed it), never heard of anyone whose lunasix used one battery.

flavio81 said:
Do you have the procedure for recalibration??

what about this?
http://www.biasedlogic.com/index.php/calibrating-gossen-lunasix-3-lunapro/

BMbikerider · Mar 31, 2017

There were two versions of the Lunasix meters. Bear in mind this meter first saw the light of day way back in the 1970's. The main complaint was the memory displayed by the CDS cell from one reading to another, which brought about a tweak to try and correct this, This 'tweak' may be the point where some meters had two batteries and some only one. (Don't ask me which was which!) I don't think there is any easy way to check to see when the change over was unless you contact Gossen themselves. (When they were first sold they came with a very high price they were asking - several hundreds of pound/dollars in todays money).

On the subject of batteries as originally used in the Lunasix, I.E. the mercury type, it has never been satisfactorily explained to me why they were withdrawn from sale. Yes mercury is poisonous but fluorescent lighting tubes have an internal coating of a mercury oxide is equally so. These were promoted as better for the environment, no mention being that they contained a poison. I am more than sure that the quantity of mercury used over time in lighting tubes is far higher that that used in batteries.

elerion · Mar 31, 2017

What do you mean by "memory displayed from one reading to another"?

BMbikerider · Mar 31, 2017

elerion said:
What do you mean by "memory displayed from one reading to another"?

There is written and anecdotal evidence that if you took a reading from one scene, unless you let the meter 'rest' for a second or two the next reading was influenced by the one previously taken. This was one of the reasons behind the change from CDS meters in cameras, to later types using the 1.5v batteries.

To see more of this Google CDS exposure meter memory

flavio81 · Mar 31, 2017

silveror0 said:
Zinc air batteries have next to nothing for life after removing the cover from the air holes. I use p675 in my hearing aid and I have to replace them every 2.5 days on average, although that may be dependent on the power they must provide to the device they're used in.

Hearing aids consume FAR more power than a meter.

I use zinc-air batteries in many of my cameras and they last a few months.

flavio81 · Mar 31, 2017

elerion said:
Mine seems to use a single battery.
I just replaced it with a single SR44, 1.5 silver-oxide battery, and it seems to works fine (still not precise over the full range, because I didn't callibrated it yet). Strange (and you guys confirmed it), never heard of anyone whose lunasix used one battery.

what about this?
http://www.biasedlogic.com/index.php/calibrating-gossen-lunasix-3-lunapro/

Thanks!!!

silveror0 · Mar 31, 2017

Just remembered something worth mentioning. When I attended the first of several workshops with Ansel Adams in 1966, on the very first day he had all attendees check their light meters and shutter speeds. He cautioned those with CdS meters that it'd be necessary to "pre-condition" them before taking readings in the shadow areas of a scene, telling us that we should note how the needle will drift SLOWLY down to the correct reading and that the meter needs to be aimed at a very bright area (e.g., a white cloud) for about 20 seconds before taking the shadow reading to remove the drift behavior. I confirmed my Lunasix did exhibit the drift (the nature of CdS) and have always heeded his advice. Just so you know.

DWThomas · Mar 31, 2017

BMbikerider said:
There were two versions of the Lunasix meters. Bear in mind this meter first saw the light of day way back in the 1970's.

Actually they go back further than that. A sticker on the back of mine indicates I had it repaired in 1968 and that was after several years of use. It took 2 PX13 cells. I don't remember it having nearly the memory problems that I've seen with the Super Pilot which is such a slug I ditched it and bought a Digisix when I resurrected the need for a manual meter a few years back. The meter movement itself appears to be dead in my original Lunasix (I still have it as a souvenir of my distant past ). (Ah, but it was fun to take readings in moonlight ...)

silveror0 · Mar 31, 2017

DWThomas said:
...Ah, but it was fun to take readings in moonlight ...

But really a pity it couldn't handle the reciprocity correction (any meter that can do that for various films would be a godsend).

elerion · Apr 3, 2017

silveror0 said:
the meter needs to be aimed at a very bright area (e.g., a white cloud) for about 20 seconds before taking the shadow reading to remove the drift behavior.

Just aimed at? or do I need to press the button down to actually make the meter work during those 20 secs?

silveror0 · Apr 3, 2017

elerion said:
Just aimed at? or do I need to press the button down to actually make the meter work during those 20 secs?

Yes, turn the meter on as if to get a reading. Sorry for being vague about this.

elerion · Apr 7, 2017

I just compared it against a Lunasix F, which uses silicon photodiode, and the first difference between both meters is speed. The F model response is much quicker.
Talking about precision, the older Lunasix using a single 1.55 V battery is usually just 1/2 stop off. In low light sometimes up to 1 stop. In very bright conditions, 1/3 stop, or even less.
It seems that it will require just a little callibration.
Maybe installing a germanium diode will even fix these offsets.
Any experiences?

jlbruyelle · Apr 11, 2017

elerion said:
Maybe installing a germanium diode will even fix these offsets.
Any experiences?

Yes, there: http://www.butkus.org/chinon/batt-adapt-us.pdf. See the graphs on the right column of page 4. Germanium diodes don't really fit the bill, Schottky diodes are better but the main problem comes from the battery losing voltage as it discharges. Silver batteries are much better than alkalines under this respect, although they still don't match the regularity of mercury or zinc-air batteries. You need to remember that constant battery voltage in an important parameter in the design those old CdS meters, without it the reading will vary as the battery ages

Alex V Flinsch · Jun 5, 2017

elerion said:
What I found to be strange, is that my model seems to use a single battery. All manuals and references t these meters speak about two batteries (2 PX13 or PX625). This one seems to use a single 1.35V battery, not two, and not a PX14 one (which is 2,7V).
Next to the battery lid there's a text "Mallory MR625".

Anyone knows about this?
Could I be wrong?

You aren't wrong, I have a LunaPro, and someone recently gave me a LunaSix and indeed there is only a single battery in it. Two just won't fit, and it works well with one. The label on the back indicates using a single RM625, while the label on the back of the LunaPro states 2 IEC-MR9 (which seems to be the same as the RM625/PX13). All of the manuals I can find for the LunaSix definitely show 2 batteries.

Is it possible that there were two versions of this meter? One with a single battery, and an earlier/later version with two?

E. von Hoegh · Jun 28, 2017

flavio81 said:
Hearing aids consume FAR more power than a meter.

I use zinc-air batteries in many of my cameras and they last a few months.

If you block all but one of the airholes in the z-a cell, it will last far longer, I had one in a Nikkormat for 27 months.

jlbruyelle · Jun 30, 2017

Alex V Flinsch said:
You aren't wrong, I have a LunaPro, and someone recently gave me a LunaSix and indeed there is only a single battery in it. Two just won't fit, and it works well with one. The label on the back indicates using a single RM625, while the label on the back of the LunaPro states 2 IEC-MR9 (which seems to be the same as the RM625/PX13). All of the manuals I can find for the LunaSix definitely show 2 batteries.

Is it possible that there were two versions of this meter? One with a single battery, and an earlier/later version with two?

Yes you are correct, MR9, PX13 and PX625 are different names of the same battery. Gossen made a mess with their model names. In the Lunasix (European name) / Luna Pro (US name) series, 4 different models share almost exactly the same name :

- Lunasix = Luna Pro = CdS sensor, 2 PX625 batteries, does not take attachments.
- Lunasix 3 = Luna Pro S = CdS sensor, 2 PX625 batteries, does not take the "Profi" attachments (no electrical connections).
- Lunasix F = Luna Pro F = Silicon sensor, 1 6F22 battery, built-in flash meter function, takes the "Profi" attachments (except the Profiflash which is useless on this model).
- Profisix = Luna Pro SBC = Silicon sensor, 1 6F22 battery, takes the "Profi" attachments (the Profiflash provides the flash meter function).

However I have never seen or heard of a Lunasix that would take only one PX625 battery. As far as I know the closest Gossen that does is the Sixtar (not an earlier model, just a different one).

MattKing · Jun 30, 2017

jlbruyelle said:
Gossen made a mess with their model names.

Actually, it wasn't Gossen. It was the US distributor, who probably chose to re-name the product line in order to deal with "grey" market imports.
Us Canadians got meters with the regular, world-wide, non-USA names.

Gossen Lunasix (CdS) using a single battery

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