So simple...but....

[dave_f2]

When mixing powered developer...or any chemical and the instruction say 1:5...is

that is by volume right?

[steve_levine]

Since water isn't measured by weight, you are correct. Be advised that you should never mix a partial package either. The powder is rarely "homogenized", and different batches won't match exactly.

[ben_hutcherson]

Well, since 1 ml of water has a mass of 1 gram, it doesn't really matter.

 

I'm not sure on what the previous poster's statement that "water isn't measured by weight" is based, but in the lab, it's actually quite common practice to measure water by weight(or mass, more correctly).

 

The reasons for this are

 

1. Balance which read to 1/10 milligram (.0001 grams) are quite common. By contrast, most common volumetric glassware can only accurately measure to 1/100 milliliter. Thus, it's far more precise to find the mass of a small quantity of water than to find the volume of it.

 

2. Volumetric glassware is calibrated to a specific temperature, typically 20 degrees celcius. If the temperature of the glassware rises, the volume increases slightly. Also, the density of water increases slightly at higher temperatures. Thus, if a specific number of moles of water are needed for a reaction, mass is truthfully a more reliable means of finding a specific quantity of water than volume.

[jtk]

Avoid chemists.

 

Photographic dilutions usually involve weights of dry components and volumes of wet. Two systems. The GOAL is to arrive at a certain volume, not weight, of photographic chemicals.

 

It'd be silly to weigh both liquid and dry components as the liquids are typically mixed to relatively large volumes (gallons, liters etc)and photolab scales are not typically good at measuring such weights.

[dave_f2]

hmmm i know one cc of water at room temperature is one gram...but that wouldn't work if i measured the powder by volume that wouldn't really equate weight if the powder is measured in weight.

 

so is consensus that powder is measured in weight?

[david_lyga]

Different dry chemicals take up different volumetric displacements. For years I have used volume to make my formulas. I have coined an acronym "CML" which is compacted millilitre. I simply place the dry chemical into a dry, accurate measuring cylinder and tap it down a few times (hence, 'compacted') to measure what I want. If you want to know which chemicals weigh per CML send me an email or see the attachment which is my latest 'photo data sheet'. My e-mail is david33x@yahoo.com if you have questions (but all messages will be returned 'collect' because I do not want phone bills for my free efforts. If you want a very accurate scale without spending anything the Post Office has one with 1/10th ounce accuracy. Be certain to subract the weight of the empty container first, then take the avoirdupois figure and convert to metric.