Metex 8760CLP Total Free Chlorine & pH Analyzer - Manual (Page 55)

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55
Adjusting Electronic Calibration
1. Remove any offset calculated by a previous software calibration of the temperature input. Select
[CONF] [in] [°C] [OFFS] from the menu and edit the offset to read 0.0.
2. Set up a precision multimeter, Fluke 8051A or equivalent, to read VDC.
3. Use TB200, terminal 3, as common. See wiring diagram. Place a 1000 Ω 1% resistor across T+ and
T-. Adjust blue trimpot VR202, located at the top-left side of TB201, for a reading of 0.225 V at
TP202. Refer to wiring diagram and drawing D5030269 for component locations.
4. Place a 1,385 Ω 1% resistor across T+ and T-. Adjust blue trimpot VR203, located at top-right side
of U201, for a reading of 4.80 V at TP202. Refer to drawing D5030269 for component locations.
5. Close case and press SAMPLE key followed by the Down arrow key to display the temperature
reading.
6. Re-insert the 1000 Ω 1% resistor and adjust VR202 until the display reads 0.0 ± 0.1 °C.
7. Re-insert the 1,385 Ω 1% resistor and adjust VR203 until the display reads 100.0 ±0.1 °C.
Calibration of pH Input
Input for measurement circuit zero, 0.00 V, at high impedance BNC connector, normally found on
preamp; refer to drawing D5030269, 876 main board component locations. Measured voltage at TP201
(Pin 1 of U201) should be 2.50 VDC. Adjust voltage using blue trimpot VR204.
NOTE: There is no span adjustment because the chemical calibration always varies it to suit the
electrode.
Calibration of 4 mA to 20 mA Outputs
Use one of the following two approaches to get the analyzer to output the desired current level, and then
make electronic adjustments to calibrate the output.
Approach 1: Simulated 4 mA to 20 mA Output (Self Calibration)
1. Select [cur] from the output 1 menu to display the present output current in mA. The display will be
updated as the output current.
2. To simulate a different 4 mA to 20 mA output signal, press ENTER to enter edit mode. Use the
arrow keys to display the desired output signal. Press ENTER to select the displayed value. The
output signal will be adjusted to put out the desired current. This process can be repeated as often as
necessary to output different signal levels.
3. The output signal is held at the displayed level until the program leaves this menu selection. Make
calibration adjustments while the analyzer shows the output at 20.00 mA.
4. Repeat the above steps for output 2.
Approach 2: Use Voltage Source to Adjust Input
This faster calibration approach requires a voltage source for the input.
1. To calibrate output 1, set [in] = [°C], input a low enough signal to cause analyzer to indicate [- Err];
the analyzer will output 4.00 mA. Reverse the polarity or input a high enough signal to cause the
analyzer to indicate [+ Err]; analyzer will output 20.00 mA.
2. Repeat step 1 for output 2.
Tip: Both outputs can be simultaneously calibrated if you set [in] = [°C] for both inputs.