A minimal cell voltage of 1. Alkaline manganese and silver oxide batteries of identical size and shape with cell voltages of 1. The older GOSSEN exposure meters with cadmium sulfide photoresistor are matched to the above-mentioned characteristics of the mercury batteries. Operation with batteries which have a higher cell voltage may lead to damage or incorrect measured values, beyond which measured values would change during the service life of alkaline manganese batteries.
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It beats my Canon 20D built-in meter in terms of actual low-light performance by 5 stops. Unfortunately it belongs to one of these designs, that relied on mercury battery voltage staying constant throughout their life and mercury batteries were banned few years ago.
This article describes, how to deal with this problem. There is a battery chemistry available that can replace the now obsolete mercury batteries. Silver oxide cells have equally stable voltage output and are based on the same principle, as mercury batteries, the main difference being use of silver in place of mercury, which makes these batteries more expensive to manufacture.
The Gossen Lunasix is a purely passive circuit, i. Light is detected using a CdS Cadmium Sulphide LDR light dependent resistor that directly influences amount of current allowed into the galvanometer. Being a purely passive, resistive, linear circuit it has no principal problem with a slightly higher battery voltage, besides showing wrong exposure values of course. As CdS cells are not made to very tight tolerances CdS based meters have to provide for a relatively wide adjustment range.
Because of this, in most cases it is possible to readjust the meter for 1. This, however, could require moving some of the adjustment pots towards their extremes and limit the adjustment precision, so as long, as you can solder two wires together, putting in a voltage-dropping diode is a good idea.
It has a hole in it, through which the serial number can be seen. The plate is glued in, you have to pry it carefully off taking care not to bend it too much. It helps to warm the glue up, so either use your hair-dryer or put the meter belly-up in the sun in either case remember to remove batteries first!
Get it pleasantly warm to the touch, but not very hot! Use a needle or a sharp screwdriver blade to lift the plate through the serial number opening at the back.
Under the plate you will find six screws see photo to the right, above. Remove four : all that are outermost at the corners in one corner there are two screws, remove only the one closer to the corner. The upper half of the case will lift off. Inspect carefully the front part where the sensor is.
The last element in the list you have to be extremely careful about: it will likely spring out if you try switching the meter on. If you lose this part or damage it, it will be extremely hard to replace or repair.
Inspect the left-top side of the meter. You will notice back of the battery compartment and two wires coming from it. The negative pole goes to the corner of the board, while the positive ends somewhere around the center. Solder the diode into the negative wire, the small bar on the diode housing shall point towards the battery compartment see photo to the right. Insert batteries and activate the battery check no need to reassemble the meter at this point.
The needle should stop now within the red field or very close to it. If your meter was dead-on with mercury batteries and the meter needle on battery check stops now within the red field you are most likely done. It will cost you less work than recalibrating the meter. Resist the urge to recalibrate, as it is not as trivial task as it sounds and getting the meter back to factory-quality calibration requires either proper tools or some patience.
However, all parts age, so if your meter is old it could be necessary to recalibrate anyway. You might also have decided to try recalibrating for silver-oxide batteries without using a diode. There are two approaches to calibration: you can either do it directly, by exposing the meter to different light levels and succesively adjusting the trimm-pots, or indirectly, by recording the LDR characteristics and calibrating with substitute resistors.
The first approach is easier when you either have an adjustable lightbox, or several white walls with different levels of illumination accessible at all times. The second approach is easier for a casual tinkerer, it requires more steps and could take more time, but makes calibration without a good reference set possible. When you switch the low-light range on, this filter is removed from in front of the sensor for the period of measurement.
The corresponding scale points that result in the same effective illumination of the CdS sensor are listed here:. As you can see, for calibrating the point 20 on the high-light scale you either need a lightbox outputting EV15, or you cheat, remove the filter, set the lightbox to EV9 and adjust the meter as if the filter was there, i.
Equally, producing an accurate EV0 for calibrating low-light scale might turn tricky, so you can instead cheat, put the filter in decoupled from its drive spring and measure EV6 instead. The net result is, you need only EV5 to EV10 reference range to adjust this meter.
Towards the bottom of the PCB you will find four trimm-pots available through holes in the pcb see photo. The left pair is responsible for low light range , no ND filter , the right pair — for the high light range , ND6 filter in the light path. The two pots closer to the center are for adjusting offset, while the two pots that on the outside adjust the slope more or less, they are actually cross-linked. Adjustments done for the low-light range do not influence the high-light range, nor the other way around.
Next step is to completely assemble the meter and check the calibration throughout the range. It should be a centered peak. You will likely find the meter to be off around the bottom of the scale. If it reads good above 13 on high, and above 4 on low, just accept it. While you can get these to match perfectly, it will throw the other end of scale off by much more. Remember, that the upper scale end is more accurate and around cross-over between the scales just take the reading from the lower range as more reliable.
As you must have already noticed, calibration is a repetitive process that converges on the proper value in several steps. While many meters can be easily calibrated in one-shot by computing necessary resistor values and just setting them, Gossen meters make this approach extremely difficult by using a non-linear scale.
Working the scale nonlinearities back into their original formulas would likely lead to too many assumptions and thus inaccuracies, so I consider it a void attempt in this case. However, repetitive process like this requires you to have a necessary set of reference luminances at hand, e.
For many a hobby tinkerer this is not the case. An observant reader might have noticed, that the CdS cell does not change during calibration. All we really do with the adjustable lightbox is making the cell assume its characteristic resistances for a given set of illumination levels in sequence, repeatedly. You can actually measure it with a multimeter. If you collect enough data, you can pick a few fixed resistors and put them in place of the LDR cell during calibration.
First, disassemble the meter and unsolder the negative battery wire as before. Have a closer look under the galvanometer dial. At the same pad, as the battery wire, arrives another one, in a yellow sleeve or at least it was yellow in my meter. These are the LDR leads. Once one leg is free, connect two thin, flexible wires to the LDR — one to each leg — and route them out of the meter.
Close the meter as far as it goes, making sure that the one leg of the LDR does not make contact anywhere. Now connect your multimeter to these two wires and start collecting data, use the sheet referenced before to evaluate it.
Remember, that each evenly lit, preferably white or gray surface you target, can provide you with two readings: one with and one without the ND filter. The ND filter is very close to 6 stops, but just in case: mark your measured values taken with and without ND filter to track them back later, shall discrepancies show. Take a route around your house, in and out. A wall in a windowless toilet can be as dark as EV-3, a whitewashed wall outside in the sun will hit EV17, try your best to fill at least the gray marked range in the spreadsheet.
As soon as you input few points you should get some line plotted in the plot area. If you did your homework right, it will be almost parallel to the sample data, and it will be not too far from it either.
Your points, marked as red crosses, should not stray too far from the green line, if they do, either you did something wrong, or the CdS cell is cooked, or I screwed the XLS.
Now put your reference resistors between the wires, activate the meter and set the trimmpots as described before, i. It might still need a slight adjustment, but it will be tiny.
Very fresh silver oxides will be 1. Remember the exact needle position to tell any voltage drift in the future. I have corrected the text. The idea is: use low, but not lowest point and high, but not highest.
In the upper scale my Lunasix is spot-on, but in the low light scale I have more trouble due to a faulty potentiometer. My next project will be restoring the old selenium meter of a Yashica-mat LM. May I suggest you to write an article about those old selenium meters and the way to restore them with the electronic components available today? Thank you in advance, Juanjo. While you can revive these meters using silicone solar cells, the spectral response will differ and sensitivity curve will be shot.
Pretty much every meter will be a case on its own how to tackle these problems…. Hi, I hope you can help me with this. I recently received a lunasix 3, which I am now struggling to calibrate. No matter how I adjust the pots, when I take readings with and without the nd filter, there is only a third of a stop difference. Thanks in advance, Augie. It should be silverish and quite opaque, you might see a light source like the sun or a bright bulb through it, but not your desk.
Does the meter provide ballpark readings on the high scale without nd? The filter appears to be fine. I even stuck it in front of another Lunapro, which I am using as a reference, and it lowered the reading by about six stops. The battery check was a bit high. The problem is present in both scales. If I understand correctly, the two sets of pots are on independent circuits, so this would seem to rule out a single bad pot.
Thanks for responding so fast, Augie. Okay, so it seems that the gauge and the ND filter are okay.
Two Gossen Lunasix 3 Lightmeters with different values - which one is correct?
About this item Details About this item. Item description Details Seller's description. Sell One Like This. Gossen Lunasix 3 Light Meter
Calibrating Gossen Lunasix 3 (LunaPro)
I bought a second hand Gossen Lunasix 3 lightmeter as a replacement for my other Lunasix 3, which ISO setting dial got broken and can only be shifted in one direction not a big issue, but anyway I wanted a backup lightmeter. However, when I tried the new second hand Lunasix lightmeter I discovered that values are constantly approximately one light value higher than the old Lunasix one. And this seems to be constant. I made several tests and I even used the same batteries, as I wondered that the problem might be related to different battery state. But even with the same batteries the light value differs between these lightmeter. And it is strange, that the difference had been in all tests constantly the same. The Gossen Lunasix 3 is an old style light meter with a CdS light cell mechanism.
Lunasix 3 manual
It beats my Canon 20D built-in meter in terms of actual low-light performance by 5 stops. Unfortunately it belongs to one of these designs, that relied on mercury battery voltage staying constant throughout their life and mercury batteries were banned few years ago. This article describes, how to deal with this problem. There is a battery chemistry available that can replace the now obsolete mercury batteries. Silver oxide cells have equally stable voltage output and are based on the same principle, as mercury batteries, the main difference being use of silver in place of mercury, which makes these batteries more expensive to manufacture.