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.
No comments:
Post a Comment