How small can I go

[Paul Howell]

I have an old digital lab scale that measures in grams and grains, so thinking about mixing up enough GAF 17 to develop one roll at a time. I think my scale will measure down 10 grains, but at such a small amount might not be as accurate as I need. Question is, my version for ANSCO/GAF 17 makes 128oz, I gallon I need 10 oz for a Patterson Tank for a roll of 35mm, by doing the math will the formula suffer my mixing up 12.8% at a time? Unlike mixing up a kit such as D76 in which the chemistry may not be mixed well enough I will be mixing from the ingredients. Or is there a threshold for any the ingredients to work, a amount less than the threshold and the formula does not work.

 

[albada]

A formula will work fine in single-roll quantities. In fact, you could mix only a few ml to develop one frame of 35mm. I've done so when experimenting with developers.
I suggest buying an electronic scale for about $20 that measures hundredths of a gram. Amazon sells several models.

 

[AgX]

It is not that simple. A digital scale based on pressure sensitive sensors has a minimum load necessary to achieve the error stated (if at all...) by the manufacturer. Beneath this load it gets tricky.

To me a classic beam-scale with a knife bearing seems to be much less dependant on such minimum load. Though here too, even at repeated attempts with same result, one may argue that the bearing has a certain bias or end-angle.


If one gets into a range where one no longer trusts the scale, and if longevity is not a concern, one may make make a concentrate of the lowest mass measurable of the smallest ingredient. And then take just the volume necessary off that concentrate. Thus shifting the measuring problem to another medium where it can be handled better.

 

[Donald Qualls]

To me a classic beam-scale with a knife bearing seems to be much less dependant on such minimum load.

I use a reloading scale that I bought almost forty years ago to (help) turn empty, fired ammunition cases back into new ammunition. It measures to 0.1 grain (15.4 grains to the gram, so minimum scale setting about 7.1 mg), and I can interpolate smaller increments than that. The blade bearing rests in synthetic agate cradles -- it's as close to frictionless as anything mechanical can get. To prevent endless oscillation, there's a non-contact magnetic damper; the beam will stop after only two or three cycles after adding a small amount of sample. When I used it for reloading, I could measure a single grain weight accurately to the precision of the scale. I could see the pointer move when I'd trickle a single powder granule into the pan.

Yes, there's a minimum weight for this kind of scale, but it's in the milligram range. Better yet, reloading scales are preset; you can trickle the sample into the pan until the pointer needle centers and get far better than 7 mg accuracy -- I'm pretty sure I could get within 0.7 mg, maybe closer to correct weight, with mine (and did I mention it's forty years old and has been stored suboptimally for years at a time?).

These things cost more than an electronic "drug dealer scale" -- but only by about a factor of two. They're still sold for around $40, the design unchanged since the 1940s, perhaps longer.

 

[Paul Howell]

Mine is a lab scale certified to .1 grain, got it a surplus sale at ASU, maybe 10 years ago, in storage I have Kodak balance scale, but it does weigh the small weights I would need, smallest is 3.8 grains. The lab scale will also measure in grams if needed.

 

[albada]

It is not that simple. A digital scale based on pressure sensitive sensors has a minimum load necessary to achieve the error stated (if at all...) by the manufacturer. Beneath this load it gets tricky.

I put a plastic cup on my electronic scale, and then zero it by pressing the Tare button. The cup weighs much more than the minimum load, so inaccuracy due to minimum load is not a problem. Also, the quantities of chemicals needed to develop one roll of film will be considerably higher than the 0.01g resolution of cheap electronic scales, so I would expect them be accurate enough even without the added cup.

Note that Phenidone/Dimezone are needed in such tiny amounts that folks often dissolve them in alcohol or propylene glycol to make measuring small amounts of them easier. I use a more expensive milligram scale for those.

 

[koraks]

I put a plastic cup on my electronic scale, and then zero it by pressing the Tare button.

This works well in my experience as well. I looked at the GAF17 formula and I don't see a reason why it wouldn't work when you mix a one roll quantity. In my case that would be 150ml (jobo rotary) which means the minimum quantity to be weighed is something like 200mg or thereabouts. In my experience this is feasible with sufficient accuracy for film development.

 

[Paul Howell]

Good advice, I'll order the chemistry in a day or so, first project is to make a liter of Edwal 20, the chemistry is arriving in a day or so, then move to GAF 17, which could be primary developer, Edwal 20 comes to about $44 a liter.

 

[relistan]

On my digital scale I use a a few shot glasses for measuring (dedicated to this purpose). This very much overcomes the minimum and is infinitely re-usable.