It has been over 30 years since i worked on one of these instruments!
From the manual:
2.4.5 Recorder Output Span Settings
The recorder output of the turbidimeter can be set to cover
any portion of the 0 - 100 NTU range. This feature allows
the operator to bracket the normal turbidity level with the
recorder output for the desired level of resolution on the
recorder. If the turbidity in the sample normally is 0.5 to
0.7 NTU, the recorder output span can be set for 0 to 1
NTU.
Initially, an output span the operator knows will cover the
range of turbidity in the sample should be selected. With
experience, the operator will be able to determine the best
span for the particular application by monitoring turbidity
levels over time.
Once the proper output span range is determined, use the
numeric keypad and press the REC MIN (recorder
minimum) key to enter the low end of the range into the
display. To enter the top end of the range into the display
use the numeric keypad and press the REC MAX (recorder
maximum) key. For example to enter an output span of O- ,
1 NTU, press 0 REC MIN and 1 REC MAX. Press the
REC MIN and REC MAX keys again to verify the correct
settings.
2.4.6 Recorder Output Calibration
The 1720C Turbidimeter is capable of self-calibrating the
voltage or current limits to ensure a 0-100 mV output is
tr uly0 - 100 mV or a 4-20 mA output is exactly 4-20 mA.
This programmed capability eliminates the need for a volt
meter or milliammeter for output calibration. Calibrating
the recorder output involves a two part procedure. First, a
coarse adjustment to set the voltage (follow step 1 below)
or current limit (follow step 2 below, is completed.
Second, a fine tuning (step 3 below) is performed.
1. If a voltage output has been selected , voltage limits are
calibrated automatically by entering 4 DIAG.
Microprocessor programming routines will measure and
calibrate the output limits. This procedure will set the
output to approximately +/ - 1%. Fine tune the recorder
output by following the procedure in step 3.
2. I f the 4-20 mA output is used, it is self-calibrating by
selecting the 4-20 mA CAL output with the recorder output
selector switch (see Figure 8).
a. Set the 4-20 mA CAL switch to on (closed). Because
only one switch should be on at any one time, all other
switches should be off (open)while the4-20 mA CAL
switch is on during self-calibration. There will be no
recorder output while the 4-20 mA CAL switch is selected.
b. Using the keyboard, enter 3 DIAG.
c. Using the recorder output selector switches, turn the 4-
20 mA CAL switch off and then turn the 4-20 mA on. This
procedure will set the output to approximately + / - 1%. Fine
tu ne the recorder output by following the procedure in
step 3.
3. To fine tune the calibration, press 0 DIAG and observe
The recorder reading. The recorder should indicate the low
end of the chart exactly. If the recorder zero reading is not
correct, proceed as follows:
a. If the recorder zero reads too high, press 8 DIAG. The
recorder reading will move downscale. When the recorder
indicates zero, press any key.
b. If the recorder zero reads too low, press 7 DIAG. The
recorder reading will move up-scale. When the recorder
indicates zero press any key.
Press 1 DIAG and observe the recorder reading. The
recorder should indicate the high end of the chart exactly.
If the recorder reading is not correct. proceed as follows:
a. If the recorder full scale reads too high, press 6 DIAG.
The recorder reading will move downscale. When the
recorder reads full scale, press any key.
b. If the recorder full scale reads too low. press 5 DIAG.
The recorder reading will move upscale. When the
recorder reads full scale, press any key.
