Extech Instruments 96VTR Instruction Manual page 15

Appendix C: Proportional Band and Deadband Details
The two diagrams shown below illustrate how the Proportional Band (Pb), Cooling
Proportional Band (CPb), and deadband (db) interact when controlling a process.
The region identified by the letter "A" reflects the action of the Proportional Band (PB). The
proportional band is the region in which the controller's primary output is permitted to throttle
between 0% and 100%. Outside this region the controller's primary output is either 0%
(above the band) or 100% (below the band).
The region indicated by the letter "B" reflects the action of the Cooling Proportional Band
(CPb) which is the region in which the controller's secondary (cooling) output is permitted to
throttle between 0% and 100%. Outside this region the controller's secondary output is either
0% (above the band) or 100% (below the band).
The region indicated by the letter "C" reflects the action of the deadband (db) which
permits the user to decide whether or not the primary and the secondary outputs should
ever be ON at the same time. By setting the "db" to a positive value (Fig. 6a) the two
control outputs will never be ON at the same time (notice the deadband region in Fig. 6a).
Fig. 6b illustrates a negative deadband value which permits the primary and secondary
outputs' proportional bands to overlap causing the two outputs to potentially be ON at the
same time.
100
OUTPUT %
Heat
0
Figure 6b - Negative "db"
100
O U TP U T %
Heat
0
Figure 6a - Positive "db"
Figure 6
A C
SP
B
A
Cool
C
FIGURE 6
15
B
Cool
PV
P V
S P
96VTR Ver. 2.0 11/99