Control chart: temperature error

Lifted from process control with control strips, whose “four causes, four signatures” section is where the course sets these out, and whose P1 perturbation is this fault performed deliberately. The register keeps the control-chart signatures with Parts XIII and XV even though Part XXVIII owns the bottle defects they correspond to.

What you see

One point off aim, and back on it next run. Kodak’s own definition of an outlier is a point on or beyond the control limit, and a temperature error on a single run is the textbook producer of one.

The shape within the point is what names it. Contrast moves first and furthest, speed follows, and fog moves only for large errors.

Likely causes

1. The bath was at the wrong temperature when development started.
2. The bath drifted during development. A tank cooling towards room temperature spends most of its time somewhere other than where it started.
3. The thermometer was read once, at the start, or in the wrong place — the top of a tall tank is not the bottom of it.
4. The thermometer is wrong. The balance and thermometer check is the course’s procedure for finding out.

What is happening: the chemistry and physics

Development is a rate process, so a warmer bath develops further in the same time. Part III owns the kinetics; what belongs here is the size of it.

ILFORD’s published time-temperature chart makes 1 °C worth about 9 per cent of development time, and Kodak state flatly that variations greater than ±0.3 °C in the developer will affect process control and image quality. So a bath 2 °C warm has added roughly 18 per cent to your development without the timer moving.

Why contrast leads. Contrast is the degree of development, which is what temperature changes. Speed is set mostly by where the toe sits, and the toe moves more slowly with development than the upper scale does. Fog moves last because it takes a substantial change in developer energy to start reducing unexposed grains.

!The confirming test, which Part XIV runs as a deliberate perturbation

Develop one strip at 22.0 °C and another 20 per cent longer at 20.0 °C, and predict before you process that they will land in nearly the same place.

At +2 °C, ILFORD’s chart gives about +18 per cent of development. At +20 per cent of time, you have +20 per cent. They should agree, and the equivalence is the finding.

How big is that on a chart? Kodak adjust development time in 10 per cent increments when moving contrast index towards aim and accept a strip within ±0.02 of aim. So a manufacturer treats 10 per cent as a step worth taking and 0.02 as the width of “close enough”, which puts 18 to 20 per cent comfortably outside any honestly set limit.

And the discriminator against every other cause on the chart is the shape in time: this is an outlier, one point, back on aim next run. A bath that is permanently mis-tempered gives a level shift instead, and a developer ageing in the bottle gives a trend.

Diagnostic questions

• What did the thermometer read at the end? Not at the start. If the two differ, the number to use is neither.
• Where was the thermometer? In the developer, in the water bath, or on the bench are three different measurements.
• Has the thermometer been checked against ice and boiling water? The balance and thermometer check takes ten minutes and is the only way to know.
• Did contrast move more than speed? If speed moved more than contrast, look elsewhere — that is not this fault’s shape.
• Was it one run or four? Kodak’s own first instruction on an outlier is to check the measurement — re-read the strip, then process another — before concluding anything about the process.

Corrective action

Do not adjust the process on one point. This is the fault most likely to provoke over-control, and Kodak’s warning about reacting to random variation is aimed squarely at it.

Annotate the chart. “Unusually warm day”, written on the date axis, converts a mysterious excursion into an explanation. Part XIV argues that the annotations are what make a chart diagnostic rather than merely descriptive.

If the fault repeats, fix the tempering rather than the timer. Compensating a temperature error with time works arithmetically and leaves the process no more repeatable than it was.

Prevention

A water bath, which is the whole argument of Part XIV’s P1 and P2 pair: being 2 °C out is an ordinary morning’s mistake, and being 20 per cent out on a timer is not something you would do by accident.

Read and record the actual temperature, at the end as well as the start, on the lab notebook worksheet rather than in your head.

Keep the chart annotated, so that the next excursion has something to be compared against.