6.6MAXIMUM % SOLIDS
To complete the calibration and to enable the system to automatically select the optimum frequency of operation., it is necessary to set the maximum % solids that the system is required to measure.
In the AUTOCAL Menu, select ‘Max % Solids’ (P 160) and enter the value required.
If the maximum % solids are low then the system will choose the higher operating frequency (3.3 MHz). This will give the best possible resolution. If the maximum % solids are higher than can be measured at 3.3 MHz then the system will automatically select the lower frequency (1 MHz). The figure for the maximum % solids that can actually be measured can be seen in MONITOR - DIAGNOSTICS - SENSOR – Max Measurable (D861)
The control unit is now calibrated and ready for operation.
6.7CALIBRATION- Alternative calibration methods
Calibration is normally done via AUTOCAL. However, in special cases, if required, calibration can be done manually.
6.8ZERO SETTING PROCEDURE
Firstly enable access using the “TOGGLE RUN” command in the Main menu. See section 5.2. Next, ensure that the frequency of operation corresponds to the frequency of the sensor by checking parameter D860 located in Monitor\Diagnostics\Sensor\Frequency. If necessary it can be changed. The relevant parameter is “Frequency” (P630) located in Set up\ Engineer\. When the sensor is in “clear” liquid note the value of “Attenuation (D852) located in Monotor\Diagnostics\ Sensor.
To complete the zero setting, enter this value in the appropriate “Zero ref” parameter, located in Calibration\ Manual Entry\Zero ref.. “Zero ref A” (P120) is used for 1MHz sensors, “Zero ref B” (P121) is used for 3.3MHz sensors.
6.9SPAN CALIBRATION/GRADIENT METHODS
There are three alternative ways of setting the gradient relationship between the measured attenuation and the % solids displayed (See Graph shown in Figure2). It is recommended that if AUTOCAL is not used then the Initial Setup should use Method 1: when later, figures are entered according to Methods 2 or 3, these automatically take priority over an original Method I calibration.
The First alternative method uses previous Mobrey experience of slurries/sludges, and the slurry type is chosen by name from a list. The MSM400 then uses the appropriate calibration line.
The Second alternative method uses actual site samples, and as such it is usually the most accurate calibration method. When the MSM400 reading is stable, a sample of slurry is taken for Lab analysis, and the attenuation measured at that time is recorded/entered in the MSM400 memory. Later the Lab result is also entered into a different location in the MSM400 memory, and the microprocessor computes the relationship.
The Third alternative method uses a known mathematical value of attenuation versus suspended solids for the slurry to be monitored from site experience on other tanks or other installations with the same sensor arrangement and slurry.
6.10CALIBRATION METHOD
Enter the CALIBRATION option on the MAIN MENU screen. Then ENTER ‘MANUAL ENTRY’. There are four selections possible here. Select SENSOR\ Sensor Gap and enter the space between sensor faces, in mm. This tells the MSM400 how big the sensor is, to relate it to memory figures of attenuation. Select SLUDGE TYPE (access through CALIBRATION, MANUAL ENTRY, SENSOR menu) and for Method I calibration select one of the listed types to suit the application. The unit will now work with this typical sludge calibration.
6.11
This Method of calibration offers the highest accuracy (and is used by AUTOCAL), since the MSM400 is set up based on actual site sample analysis. It does therefore require quite a lot of site work in taking samples, and analysing the solids %, to enter this later into the MSM400 microprocessor memory.
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