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. Recalibration by a direct approach Tools needed A flat screwdriver with a narrow blade mm. An adjustable lightbox or equivalent.
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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. Recalibration by a direct approach Tools needed A flat screwdriver with a narrow blade mm.
An adjustable lightbox or equivalent. Unless your lightbox is calibrated in EV, you will need a known, good, accurate light meter. Center-weighted is best. 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. For high light: use EV14 19 on the dial and EV8 13 instead, and tweak the right two pots. 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. Recalibration using substitute resistances 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. Tools needed A digital multimeter. A known, good, accurate light meter. Normal, center-weighted is best. This file: LDR calibration sheet.
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.
Calibrating Gossen Lunasix 3 (LunaPro)
Vikora This entry was posted in rants etc. You might also have decided to try recalibrating for silver-oxide batteries without using a diode. A 1N should do. It might still need a slight adjustment, but it will be tiny. Yesterday I stumbled upon a very nice, mint-looking original grey Lunasix at a garage sale. Here are all the pages of the operating manual for the Gossen Lunasix 3 photographic light meter.
GOSSEN LUNASIX 3 MANUAL PDF