7
Application Notes
If the distance from the solution to the sensor is further than the recommended
length of 25 feet, the maximum lagtime must be calculated from the desired control
band to determine a pump flow rate based on a given distance of standard, uniform
tubing. The maximum lagtime is the maximum allowable time for the solution to
continuously get to the sensor in order to achieve the desired control band.
To calculate maximum lagtime:
Max. Lagtime =
Desired Control Band*
4
x
Depletion
Rate
where Control band = Maximum deviation of concentration
Depletion rate = Rate at which the bath will deplete per unit
of time
* The deadband should be adjusted so that it is 1/4 the desired control
band.
For Example:
The set point is 4.00 g/L.
If the desired control band is 0.20 g/L (± 0.10 g/L or 2.5%) and the bath is
depleting at a rate of 1.25 g/L every 15 minutes (0.08333 g/L every minute),
then Max. Lagtime =
0.20 g/L
4
x
(0.08333
g/L
/min)
=
0.60 minutes
So, 0.60 minutes is the maximum time it should take for the solution to reach the
sensor.
To calculate pump flow rate:
Minimum Pump Flow Rate = Volume of System*
Maximum
Lagtime
where Volume of system =
( Tubing I.D.)2 x Length of tubing
2
Maximum lagtime = Previously calculated time to get solution to
sensor.
* Volume is based on length from solution to sensor, not the return.
For Example: If the system parameters are: Tubing is 3/8" O.D. x 1/4" I.D.
Length
is
30
feet
(360
inches)
then the volume of the system =
( 0.25 in )2 x (360 in)
=
17.7 in3
2
