Choose
OK
to continue. The calibration procedure then displays a
Pressure Calibration
dialog box.
The
Measured Pressure
value shows the current instrument pressure reading.
Enter the current atmospheric pressure, in mbars, in the
Calibration Pressure
entry box. Choose
Calibrate
to direct the instrument to read and display the
Measured Pressure
using this calibration
value. Choose
Quit
when you are satisfied with the pressure calibration to return to normal
operation.
7.2 Sensor calibration
The TC sensor can be calibrated in pure H
2
or N
2
gas at atmospheric or elevated pressure. The
following table gives information regarding the different methods.
Table 3 Sensor calibration information
Calibration Method
Time
Benefits
Drawbacks
Reference
Gas at atmospheric
pressure
5 min
Quick and easy
Less accurate than
other methods
Gas with known
purity, certified
barometer
Gas at elevated
pressure (2-3 bar)
10 min Higher accuracy than
atmospheric method.
Validation of the sensor
linearity
Requires a certified
pressure gauge
Certified pressure
gauge
Before calibrating check that the sensor membrane surface is tight, smooth and wrinkle free. Select
the gas measurement phase for calibration. Refer to
select the calibration medium using the Hach 3654 program. Refer to
on page 20.
Ensure that normal measurement mode is selected in the
Measurement Mode
menu of the Hach
3654 program. Refer to
on page 20.
Calibration must only be carried out once the TC sensor is giving a stable measurement. Expose the
membrane to the calibration sample until this stable reading is obtained. This usually takes about
5 minutes.
For calibration it is important that the sensor's membrane is at the same temperature as the
calibration sample. Therefore, allow some sample to pass through the flow chamber for 3 minutes
before calibrating.
7.2.1 Calibration setup
Ensure that the purge gas is flowing at the recommended rate through the sensor by placing the
purge gas exit tubing into water. This rate should be more than four bubbles during the purge which
lasts for 4 seconds. This purge cycle is repeated every 20 seconds.
The procedure requires a source of pure gas (regulated by a pressure valve) connected to the
central entry port of the flow chamber, an accurate pressure gauge (better than ± 2%) at the exit of
the flow chamber, a needle valve to control the flow on exit, and a beaker of water to monitor the gas
flow. Refer to
.
English
21
