How to Calculate Dilution Ratio in the Darkroom

September 12, 2015  •  1 Comment

How to Calculate Dilution Ratio in the Darkroom by Tim LaytonHow to Calculate Dilution Ratio in the Darkroom by Tim Layton In this article, I discuss how to correctly figure out the amount of chemical for the total volume desired to be used for any dilution factor.  

This can be confusing because people sometimes use different terms to try and explain the same concept.

Anytime there is measurement involved, confusion often follows.  

Watch the video and let me know if my approach is clear.

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How would you prepare 1000ml of a 1:100 dilution for PyroHD?

Step 1: Determine the required ratio by dividing final volume by dilution factor: 1000ml/100 = 10ml (developer)

In this case we know that we want to mix 1000ml of PyroHD and we want it diluted at 1:100.

Step 2: Subtract the ratio volume from the final volume:  1000ml - 10ml = 990ml (water)

Step 3: Measure out 990ml of water, add 10ml PyroHD developer to it, mix thoroughly

It really is that simple.  

Here is another example using a different dilution so you can practice.  

Try and figure out the two parts (developer and water) for the dilution before looking at the answer to make sure you understand how to use the process.


Dilute HC110 1:63 
final vol / dil factor
1- 500ml/63 dilution factor = 8ml (developer)
2 - 500ml - 8ml = 492 (water)
3 - mix two together and enjoy!

Dilute Stop Bath 1:19
final vol / dil factor
1- 500ml/19 dilution factor = 26ml (stop bath chemical)
2 = 500ml - 26ml = 474ml (water)
3 - mix the two together

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Technically Pyrocat HD has parts A and B so would be 1A:1B:100W so you need 10+10+980 to get a Liter. And then there is also decisions about the correct dilution for ones processing method. 2A:2B:100W for Rotary seems to be the norm, while 1A:0.75B:200W may be appropriate for semi-stand method.

I would like to see a deeper dive article on minimum and maximum developer in dilution per square inch for a given processing system and what the impacts are. Especially as it relates to square inches of film for a single sheet of 4x5 vs 8x10 or processing multiple 8x10 sheets. Example the basic HC110 at normal 1:31 can create too much contrast for a single 4x5 sheet in a tank following published times, and too little contrast if processing 3 8x10s in a Jobo tank.
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