4
Configuring The Meter
4.9 PROPORTIONAL BAND (P.BND)
Proportional controls are designed to eliminate the cycling associated with on-off control. A
proportional controller decreases the average power being supplied to the heater, as the
temperature approaches setpoint. This has the effect of slowing down the heater, so that it will
not overshoot the setpoint, but will approach the setpoint and maintain a stable temperature. This
proportioning action can be accomplished by turning the output on and off for short intervals. This
Òtime proportioningÓ varies the ratio of ÒonÓ time to ÒoffÓ time to control the temperature.
The time period between two successive turn-ons is known as the Òcycle timeÓ or Òduty cycleÓ.
The proportioning action occurs with a Òproportional bandÓ around the setpoint temperature.
Outside this band, the controller functions as an on-off unit, with the output either fully on (below
the band) or fully off (above the band). However, within the band, the output is turned on and off
in the ratio of the measurement difference from the setpoint. At the setpoint (the midpoint of the
proportional band), the output on-off ratio is 1:1 that is, the on-
time and off-times vary in proportion to the temperature
difference. If the temperature is below setpoint, the output will be
on longer. If the temperature is too high, the output will be off
longer.
The proportional band is usually expressed as a percent of full
input range scale, or in degrees. It may also be referred to as
gain, which is the reciprocal of the band. In many units, the cycle
time and/or proportional bandwidth are adjustable, so that the
controller may be better matched to a particular process.
Proportional controllers have a manual reset (trim) adjustment,
which may be used to adjust for an offset between the steady
state temperature and the setpoint.
In addition to electromechanical and solid state relay outputs, proportional controllers are also
available with proportional analog signal outputs, such as 4 to 20mA or 0 to 5 Vdc. With these
outputs, the actual output level amplitude is varied, rather than the proportion of on and off times.
To select the proportional band for your proportional controller.
"
P.BND
" only shows if you select analog output as proportional.
1.
Press the
MENU
button until the meter shows "
P.BND
".
2.
Press the
©
/DEV
button. The meter shows the last previously stored 4-digit number
(0000 through 9999) with flashing 4th digit.
3.
Press the
¶
/MAX
button to change the value of the flashing digit.
If you continue to press the
¶
/MAX
button, the flashing digit's value continues to change.
4.
Press the
©
/DEV
button to scroll to the next digit.
5.
Press the
MENU
button to store your selection. The meter momentarily shows "
STRD
",
followed by "
M.RST
" (Manual Reset).
24
Above
Temp.
At Setpoint
Temp.
Below
Temp.
On
Time
Off
Time
Figure 4.1 Proportional Band
Note
☞
