Wednesday, 23 April 2014

Writing Your Own Schedules, Part 2

Time Versus Rate

Schedules can be expressed as a rate per hour, or a time to get to the target temperature. What you feel most comfortable with relates largely to your background and teaching. Most ceramics based people use the time to get from one temperature to another. Most kiln formers without a background in ceramics tend to use rates per hour when writing schedules.

The rate of 100/hour to 100 degrees is the same as 1 hour to 100. 2.5 hours to 200 is the same as 80/hour to 200. So the conversion to a time to get to a target temperature is a simple one of dividing the temperature by the rate per hour to give the number of hours to achieve the target temperature. Some controllers will allow hours and minutes to be programmed; others allow only minutes – in which case multiply by 60 to give 150 minutes.

This is the same thing you do to find out how long a firing will take. If you see a schedule expressed as time e.g.,
3 hours to 677 for 0.5 hour,
1.25 hour to 800,
asap to 482 for 1 hour,
2.5 hours to 370
you already know approximately how long this firing will take – a bit more than 8.25 hours (3+0.5+1.25+1+2.5) plus cool down.

It can also be expressed as
225/hr to 677 for 30 mins,
102/hr (800-677=123/1.25) to 800,
afap to 482 for 30 mins,
45/hr (482-370=112/2.5) to 370.

The time to target temperature method of writing a schedule comes into its own when dealing with thick castings that require very slow cool downs. For example, a 60mm thick casting calls for an initial annealing cool of 2.4 degrees per hour over the range 482 to 428. I don't know of a programmer than can deal with decimals. So the alternative is to programme in time to target. In this case it would be a time of 22.5 hours.

The reason for avoiding the choice of 2 or 3 degrees per hour is accuracy. If you had put in 2 degrees per hour you would have spent 27 hours, possibly excessively long. If you had put in 3/hour it would have taken 18 hours, possibly not enough time for the glass to adequately anneal. So, for very slow rates of advance, time to target is much the most accurate method of writing the schedule.