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VAPOR-LOGIC
®
VERSION 6 INSTALLATION AND OPERATION MANUAL
THE DERIVATIVE TERM
The derivative term is the measured change in error over time multiplied by the
derivative gain (differentiating error with respect to time) .
If the actual measured RH is below set point and is rising, the derivative term
subtracts from the demand in anticipation of the approaching set point .
If the actual measured RH is below set point and is falling, the derivative term
adds to the demand in anticipation of the need to get the demand up faster
and start climbing toward set point .
If the actual measured RH is above set point and falling, the derivative term
adds to the overall demand in anticipation of the approaching set point .
It generally is used to increase damping and, in some cases, improves the
stability of the system .
However, in the majority of control situations, the derivative term is not needed
and is simply set to zero . The proportional term and integral term provide tight,
accurate control without the addition of the derivative term .
PID BAND
The last user-controlled term in the PID equation is the PID band . The PID band
defines the range of measured RH values (°F/°C for dew point control) where
the PID loop is in operation . The PID loop is in operation when the measured
RH is in the range of (RH set point – PID band) to (RH set point + PID band) . If
the measured RH is below the PID band, the PID calculations are suspended
and the demand is set to 100% . Conversely, if the measured RH is above the
PID band, the demand is set to 0% .
For example, if starting with an RH set point of 35% and a PID band of 10%,
the PID loop operates when the actual humidity is in the range of 25% to 45% .
If the actual humidity is lower than 25%, the humidifier is full on . If the actual
humidity is above 45%, the humidifier demand is 0% . The PID band aids in
speeding the response time of the system . It allows the RH to get somewhat
close to the set point and then lets the PID loop precisely control the RH when it
is within the PID band . The default value for the PID band is 10% .
Large spaces where the humidification system influences the RH very
slowly typically benefit from a smaller PID band . Small spaces where the
humidification system can quickly influence the RH typically benefit from a
larger PID band . Rarely should it be set to less than 10% .
PID tuning
OPERATION
