I just bought and started using this DMM from Amazon costing $15.00 here in the US.
This meeter and accessories are amazing for this price. It has many, many accessories, including four sets of test leads with various clips and clamps, thermocouple, small screwdrivers and even a roll of electrical tape. This meter is much much better than the one I've been using. It is autoranging with manual control of measurement type, and has a backlit display, something really needed for darkroom work. My old meter didn't have that, making it harder to use. These meter measures impedance without drifting, and is also likely more accurate than my old. Open circuit on Ohm scale is 2.5 volts..
The secret of making a CdS cell dollar densitometer work directly in the simplest way is to use a specific type of cell. The cell needs to have a "gamma" as close to one as possible. There are many CdS cells on the market, but the smallest ones are about 5mm in diameter. The sensitive area is abouut 2x3 millimeters on these two I recommend which is a good size for scanning large formet negatives directly, or for 35mm and 120 on the baseboard of an enlarger:
GM5549 or GL5549 Have gamma of 0.9 which is the highest you can get, I believe. These are available from ebay dealers in US and through other sources. Usually in bags of 20-30 for $5-6. Good to have choices, since these will read differently. Cells with the highest gamma will also have the highest dark readings, around 10M Ohms. I used a bag of 30 GM5539 from Amazon:
These are 5M Ohm with gamma 0.8, but sorting through them I found one with 10MOhm dark that had the higher gamma as well. I lucked out a bit, but the 5549 I could not find a few years ago.
My dollar densitometer works on my enlarger baseboard, something most cannot do. This is ideal for measuring negatives or positives smaller than 4x5, and also makes it a good enlarger exposure meter. Measuring negative density range helps select the contrast grade for variable contrast enlarging paper as well as developing time for enlarging negatives photographically. It also measures the light intensity in the highlight areas to help determine exposure times. This saves wasting time and materials in the darkroom.
I mounted my sensor in a 3 layer sandwich of 1/8 in mdf, medium density fiberboard. I used pieces about 4x4 inches square. I drilled a 1/4 inch hole in the top and middle piece, and cut slots about that same width in the middle piece and running to one flat edge. The CdS leads and the wire soldered to them run through that slot. The three pices are bolted together near the corners for short machine screws. I darkened the hole in the top piece with magic marker, then cut a narrow strip of black electrical tape and wrapped that around the outside of the sensor to make a snug fit into the 1/4 inch hole. I used a #20 twinplex red/black wire connected to bananna jacks of matching colors. A white card can be taped to the top side with matching hole to see more clearly the area being measured. Or, the top surface can be painted white for that matter, but the paint could come off on your negatives if not careful.
To measure a density, you need a calculator that can divide and calculate LOG. The denser region is measured first in Ohms, then divided by the clearer region measured in Ohms, then push the LOG button and you get the density units. This applies in reading two densities on the negative or reading a density relative to no density which is when the reading is taken with no negative in place. For the highest and lowest negative densities, it's known as the density range. It's just that siimple. Density=LOG(hiohms/lowohms).
To verify density ranges of several you have measured with this system, you can use you smartphone with a lux app and a high intensity desk lamp to read a bit over a 3.00 density.
1.With the lamp on an no density on the phone sensor (you need to find where it's located) measure the LUX and enter into calculator. You will need 10,000 lux to read a 3.000 density, 1000 lux for a 2.00 density.
2. Place density on phone sensor and measure the LUX. This should read 10 lux or more.Divide the higher LUX by this lower LUX.
3. Push LOG button on calculator to find density.
Why not just use the phone app? You need to measure a small area that you can control, very hard to do with the phone, and you need way too much light to measure using the phone. The CdS cell is much more sensitive.
This meeter and accessories are amazing for this price. It has many, many accessories, including four sets of test leads with various clips and clamps, thermocouple, small screwdrivers and even a roll of electrical tape. This meter is much much better than the one I've been using. It is autoranging with manual control of measurement type, and has a backlit display, something really needed for darkroom work. My old meter didn't have that, making it harder to use. These meter measures impedance without drifting, and is also likely more accurate than my old. Open circuit on Ohm scale is 2.5 volts..
The secret of making a CdS cell dollar densitometer work directly in the simplest way is to use a specific type of cell. The cell needs to have a "gamma" as close to one as possible. There are many CdS cells on the market, but the smallest ones are about 5mm in diameter. The sensitive area is abouut 2x3 millimeters on these two I recommend which is a good size for scanning large formet negatives directly, or for 35mm and 120 on the baseboard of an enlarger:
GM5549 or GL5549 Have gamma of 0.9 which is the highest you can get, I believe. These are available from ebay dealers in US and through other sources. Usually in bags of 20-30 for $5-6. Good to have choices, since these will read differently. Cells with the highest gamma will also have the highest dark readings, around 10M Ohms. I used a bag of 30 GM5539 from Amazon:
These are 5M Ohm with gamma 0.8, but sorting through them I found one with 10MOhm dark that had the higher gamma as well. I lucked out a bit, but the 5549 I could not find a few years ago.
My dollar densitometer works on my enlarger baseboard, something most cannot do. This is ideal for measuring negatives or positives smaller than 4x5, and also makes it a good enlarger exposure meter. Measuring negative density range helps select the contrast grade for variable contrast enlarging paper as well as developing time for enlarging negatives photographically. It also measures the light intensity in the highlight areas to help determine exposure times. This saves wasting time and materials in the darkroom.
I mounted my sensor in a 3 layer sandwich of 1/8 in mdf, medium density fiberboard. I used pieces about 4x4 inches square. I drilled a 1/4 inch hole in the top and middle piece, and cut slots about that same width in the middle piece and running to one flat edge. The CdS leads and the wire soldered to them run through that slot. The three pices are bolted together near the corners for short machine screws. I darkened the hole in the top piece with magic marker, then cut a narrow strip of black electrical tape and wrapped that around the outside of the sensor to make a snug fit into the 1/4 inch hole. I used a #20 twinplex red/black wire connected to bananna jacks of matching colors. A white card can be taped to the top side with matching hole to see more clearly the area being measured. Or, the top surface can be painted white for that matter, but the paint could come off on your negatives if not careful.
To measure a density, you need a calculator that can divide and calculate LOG. The denser region is measured first in Ohms, then divided by the clearer region measured in Ohms, then push the LOG button and you get the density units. This applies in reading two densities on the negative or reading a density relative to no density which is when the reading is taken with no negative in place. For the highest and lowest negative densities, it's known as the density range. It's just that siimple. Density=LOG(hiohms/lowohms).
To verify density ranges of several you have measured with this system, you can use you smartphone with a lux app and a high intensity desk lamp to read a bit over a 3.00 density.
1.With the lamp on an no density on the phone sensor (you need to find where it's located) measure the LUX and enter into calculator. You will need 10,000 lux to read a 3.000 density, 1000 lux for a 2.00 density.
2. Place density on phone sensor and measure the LUX. This should read 10 lux or more.Divide the higher LUX by this lower LUX.
3. Push LOG button on calculator to find density.
Why not just use the phone app? You need to measure a small area that you can control, very hard to do with the phone, and you need way too much light to measure using the phone. The CdS cell is much more sensitive.
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