YOKOGAWA EXA TB TB400G, User Manual

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<5. FUNCTIONS>
5-10
IM 12E04A02-02E 9th Edition : Mar. 16, 2018-00
How It Functions
To suppress a rapid change due to air bubbles or dust, the TB400G:
• Checks a turbidity signal before averaging;
• Calculates the difference between the current signal and the previous signal;
• Compares the difference with the Detection Level;
• If it is less than the Detection Level, proceeds to averaging and gives the turbidity reading;
• If it is greater than the Detection Level, holds the output for a specified Hold Time period;
• During that period, suspends the Detection Level check;
• After the Hold Time has elapsed, averages signals without the Detection Level check for a
specified Sampling Time period and gives turbidity readings; and,
• After the Sampling Time h as elapsed, performs the Detection Level check again.
Instructions for Use
The spike detection function is effective in processes where turbidity changes are relatively small
under normal conditions, but it may not be suitable for processes where rapid changes occur
often. Even if rapid changes do not usually occur, this function may be activated and may cause
a delay in the response of the reading or output if stepwise turbidity changes occur occasionally.
Therefore, before using this function process conditions should be carefully considered, and in
order to use it properly, determine optimum setpoints for the operating conditions while changing
them little by little.
When using the function, pay attention to the following.
• Adjust the Detection Level according to the degree of reading change due to air bubbles or dust.
• If an excessively long Hold Time is set, the turbidimeter may not be able to detect a change
in reading that may have occurred and should have been recognized as a real change.
• If the function is responsible for the delay in the response of the turbidity reading, try setting
the Sampling Time longer without changing the Hold Time.
• When an abnormal turbidity reading is suspected, turn off the function once and wait for
a while and see. The function may have prevented the turbidimeter from giving readings
properly.
5.6.2 Settings of Spike Detection Function
To enable the function, set necessary parameters in <PROGRAM1> mode following the
procedure below.
• Turn ON the function (FUNCTION “F.”)
• Set the Detection Level (FUNCTION “D”)
• Set the Hold Time (FUNCTION “E”)
• Set the Sampling Time (FUNCTION “F”)
a. Detection Level (%)
The Detection Level is a value used to determine whether a reading is a spike due to air bubbles
or dust or not. It is defined as a percentage of the upper limit of the analog output range currently
selected. The percentage setpoint is converted to the corresponding turbidity signal in the
computer. (The ratio of the sensitivity correction factor is also calculated automatically).
For the Detection Level check, a pre-averaging turbidity value is used. To determine the
Detection Level, it may be needed to know the degree of fluctuation in pre-averaging turbidity
signals. Set the averaging factor to 1 in FUNCTION “4” in <PROGRAM1> mode to allow the
turbidimeter to give turbidity readings directly without averaging.
Check readings to know the degree of fluctuation. Generally, the Detection Level is set to a value
greater than the fluctuation.