J-KEM Scientific Gemini User Manual - page 22
22
3.12
Changing Between PID and ON/OFF Operating Modes.
The controller can heat in either of 2
operating modes, PID (Proportional, Integral, Derivative) or ON/OFF mode. The difference between them
is the way they supply power to the heater.
In ON/OFF mode (the simplest heating mode),
the controller is ON when it’s below the set point
and OFF when above. The disadvantage of this
mode is a large over shoot of the set point (5 -
30
o
) on initial warm up and oscillation of
temperature around the set point thereafter. The
reason for the overshoot is because the heater
turns off only after crossing the set point and
until the heater cools down the temperature
continues to rise. This method works well for
heaters that transfer heat rapidly (such as heat
lamps), it’s acceptable for heaters such as heating mantles (
≈
5
o
overshoot), but it’s terrible for heaters that
transfer heat slowly (vacuum ovens, heating blocks, etc.).
In PID mode the controller monitors the shape of
the heating curve during initial warm up and
decreases power to the heater before the set
point is reached so that the solution reaches the
set point with minimal over shoot. [Notice that
the heater turns off for varying periods of time
before the set point temperature is reached].
The second feature of PID mode is that it adjusts
the percent of time the heater is on so that the set
point is maintained precisely. The advantage of
PID mode is that it delivers stable temperature
control with any heater
from heat lamps to vacuum ovens. The disadvantage is that the controller must be properly tuned to the
heater being used, whereas ON/OFF mode requires no tuning. Since both heating modes have their
advantages (simplicity vs. accuracy), instructions to change the controller to ON/OFF mode are given
below (though PID mode will probably give better results 95% of the time). The controller can be set back
to PID mode by following Procedure 4 in Section 3.10.
1. Press and hold in both the
or
keys on the front of the temperature meter for the
channel you want to change until the word “
tunE
”
appears in the display, then let go
of the buttons.
2. Press
until the word “
CyC.t
” appears in the display.
3. While holding in the * key, press the
key until the word
“
on.of
”
appears in the
display. NOTE: if the display shows the letter
“
A”
when the * key is held in, press
the
key until “
on.of
” is in the display, then let go of all the keys.
4. Press the
key until the word
“
bAnd
”
appears in the display. While holding in the
* key, press the
key until the value “
0.1
” appears in the display, then let go of all
the keys.
5. Press and hold in both the
or
keys on the front of the controller until the
temperature is displayed, then release both keys.
Procedure to change
controller to ON/OFF mode
The controller can be set
back to PID control by
following Procedure 4 in
Section 3.10. To completely
reset the controller to
original factory settings,
follow the procedure in the
Appendix, Section II.