Current weighing options for home darkroom use

There is a current trend in formulating one’s own chemistry, either for alternative processes, or just for interest. One of the key requirements is a method of weighing chemicals in a consistent and convenient manner.

Weighing devices vary a lot. Older laboratory balances used a set of weights to balance the pan and a knife edge pivot. Modern electronic balances use a load cell to read pressure on the pan as an electrical signal. An older laboratory balance in good condition is very accurate, but if it has not been looked after it may be worse than useless.

There are a lot of ‘pocket’ electronic balances available very cheaply. They are sold for weighing coins or other objects, but they will work for chemicals. The main thing is to look at the maximum capacity and the display precision in terms of the formulae you expect to be making up. If you habitually need to weigh more than 500g, you need a larger maximum. If you need to weigh fractions of a gram, look for a device with at least one decimal place better than you need. For some chemicals the best method is to make up a concentrated solution and dilute that, provided the formula will allow a liquid substitution.

For general photographic purposes, the key elements are precision/repeatability, linearity at the lower weights, and accuracy. A slightly weak or strong solution is less of a problem than incorrect proportions or variations. You can calibrate for a consistent deviation.

The pocket balances may be too small if you weigh large amounts. Although you can tare off a larger container, you have to be able to see the scale.


This is a measure of how weights of the item are reported - the number of decimal places the device will measure. Digital displays are subject to rounding in the last digit displayed.


This is the amount of deviation from a known standard weight. This may vary over the range, and is normally given as plus or minus a certain weight or sometimes as a percentage.


This is a measure of consistency. If you weigh an item, and then weigh it again later, do you get the same result?


Most balances and scales have a maximum limit on the weight they can handle. Taring usually works within that maximum.

Display units

Many electronic balances can measure in grams, ounces (avoirdupois (general weights) or Troy (specie) ), grains, pennyweights and other specialized units. For photographic purposes the usual choice is grams and ounces (avoirdupois).

The precision of the display may not represent the precision of the device. A display reading to 0.1 is more precise in grams than it is in ounces. Decimal displays are only readable to +/- half of the last digit - there is bound to be some rounding up or down. If the number of divisions are stated, then the capacity of the balance divided by the number of divisions gives the smallest increment. A 500g capacity with 1000 divisions reads to 0.5g. The weighing precision should be at least as good.


Most balances drift over time, usually due to environmental changes.


A measurement of consistency over the range of the device. For example, weighing several items individually and adding them up should be the same as putting them all on the pan at the same time (subject to display units and precision). Linearity is often quoted as a percentage of the load.


The response speed to changes in weight. Highly sensitive balances are subject to environmental influences. Low sensitivity makes for difficult fine adjustments when weighing powders, as small additions or subtractions may not register immediately.

Environmental concerns

Drafts, vibration, temperature changes, uneven or sloping surfaces can all affect the performance of the device. Power fluctuations may be a problem with electronic devices. Mechanical devices that only use illuminated scales are not affected by power fluctuations.


Most of the pocket electronic balances are battery powered, and some offer external power using the usual DC wall plug transformer.

Calibration weights

Laboratory balances usually have an internal calibration weight, or are supplied with a set of weights. The small electronic balances have to use an external calibration weight, or may not permit calibration at all. In that case they may require an external calibration graph created using a set of known weights. Proper calibration sets come with tweezers for handling because the small weights suffer from gains from sweat if touched, and losses if polished. Though the pocket balances reading to at best 0.01gm will not detect such changes. Metric cooking weights in the 5g to 100g range can be useful for check calibrations.

Weighing practice

Most photographic formula are listed in the order of solution (about the only exception is that a pinch of sodium sulphite is added before metol in solutions containing both), so the weighing order should follow. It is up to you whether to weigh then mix, or weigh and mix alternately. Weighing into a container or onto a piece of slick paper is the best practice. Do not weigh onto the balance pan itself.

Choosing a balance

These recommendations assume that the aim is to make up personal stocks of common developers and toners in lots of less than 5 liters. The key factor is the resolution of the balance. If you need to weigh to 0.1gm, then a display reading to 0.01gm is desirable. If you only rarely need to weigh to less than 1gm, then it is often possible to weigh ten times the desired amount and dissolve it, taking one tenth of the solution. The next factor is the maximum capacity. This should be large enough to encompass the common formulae you intend using. You can always split large weights into two or more fractions, but this is tedious to do on a routine basis. The ability to tare off a container or weighing paper is useful.

As an example, I have one small electronic balance that will measure to 500g and report to 0.1g. That is one part in 5,000. I have another that has a maximum of 100g, and will read to 0.01g, or one part in 10,000. If I was regularly weighing in 100’s of grams, the bulk would probably dictate a balance with a larger top plate, and reading to 0.1g.

After that the balance of features - size, calibration, illuminated scale, etc. - is up to you.