11 Top Blowdown
Mk8 MM Manual Page | 214
11.6 Ways of Controlling TDS Level
11.6.1 Continuous TDS Control
1. The first stage of the TDS control cycle is the sample time where the valve is fully opened to take a fresh
sample for a time period set in expansion option 48.
2. After a fresh sample has been taken and the sample time elapsed, the second stage is the settle time. In
the settle time, the valve fully closes to let the sample settle, for a time period set in expansion option 49.
3. Once the settle time is over, the valve will remain closed for another time period called measurement
time, set in expansion option 50. The TDS probe will measure this sample and this is the reading in the
TDS control.
4. If the measured value is less than the target value set in expansion option 42 minus 100ppm, the valve
will not blowdown, and remain fully closed for the blowdown. For example, if the target TDS value was
set as 2200pppm, the measured value was 2099ppm or less the valve would not blowdown. At the end
of the blowdown time, the cycle will repeat and the TDS control will progress to the sample time.
5. If after the settle time the measured value is above the TDS target value minus 100ppm, the valve will
drive open to a position determined by the PI loop to blowdown the valve and try to maintain the TDS
target value. For example, if the target value was set to 2200ppm, the measured value would need to be
2100ppm or more for the valve to blowdown. At the end of the blowdown time set in expansion option
51, the valve will go to fully open for the sample time, to repeat the TDS control loop.
11.6.2 Solenoid and Servomotor 2-State TDS Control
1. The first stage of the TDS control cycle is the sample time where the valve is fully opened to take a fresh
sample for a time period set in expansion option 48.
2. After a fresh sample has been taken and the sample time elapsed, the second stage is the settle time. In
the settle time, the valve fully closes to let the sample settle, for a time period set in expansion option 49.
3. Once the settle time is over, the valve will remain closed for another time period called measurement
time, set in expansion option 50. The TDS probe will measure this sample and this is the reading in the
TDS control.
4. If the measured value is less than the target value set in expansion option 42 minus 100ppm, the valve
will not blowdown, and remain fully closed for the blowdown. For example, if the target TDS value was
set as 2200pppm, the measured value was 2099ppm or less the valve would not blowdown. At the end
of the blowdown time, the cycle will repeat and the TDS control will progress to the sample time.
5. If after the settle time the measured value is above the TDS target value minus 100ppm, the valve will
remain fully open for part of the blowdown time; this fully open interval is determined by the P element,
and then the valve will go to fully closed for the remainder of the blowdown time. For example, if the
target value was set to 2200ppm, the measured value would need to be 2100ppm or more for the valve
to blowdown. At the end of the blowdown time, the valve will go to fully open for the sample time, to
repeat the TDS control loop.
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