Biosystems PhD Ultra Reference Manual - page 28
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Since fresh air contains 20.9 percent oxygen, the
fresh air “zero” calibration is the only calibration
needed for the oxygen sensor in the PhD Ultra.
Toxic and combustible gas sensors must also
undergo span calibration to ensure accuracy.
If the PhD Ultra cannot be taken to an area where
the air is fresh, or if it is not certain whether or not
the air is uncontaminated, special procedures are
required. These special procedures are discussed
at greater length in Appendix C.
3.3
Functional (bump) test
The accuracy of the PhD Ultra may be verified at
any time by performing a simple functional (bump)
test.
To perform a functional (bump) test, do the
following:
(1) Turn the PhD Ultra on and wait at least three
minutes to allow the readings to fully stabilize.
(2) Make sure the instrument is located in fresh air.
(3) Verify that the current gas readings match the
concentrations present in fresh air. If the PhD
Ultra is in Basic or Technician operating mode
the fresh air readings should equal 20.9 % O
2
, 0
% LEL or 0.0 % CH
4
(by volume), and 0 PPM
for any toxic sensors installed. If the instrument
is operated in the Text Only mode all readings
should indicate that conditions are “OK”. If
necessary, fresh air calibrate the instrument
using the procedures discussed in section 3.4.1.
below.
(4) Apply the calibration gas as shown above in
figure 3.4.2.
(5) Wait for the readings to stabilize. (Forty-five
seconds to one minute is usually sufficient.
Reactive gas sensors may take longer.)
(6) Note the readings. LEL and toxic readings are
considered accurate when they are between
90%* and 120% of the expected concentration
as given on the calibration gas cylinder.
Oxygen readings should fall within ±0.5%/(vol.)
of the expected concentration as given on the
calibration gas cylinder.
Note: If LEL or toxic gas concentration readings
are not between 90%* and 120% of the expected
values during a functional (bump) test, the
instrument must be adjusted using the "span"
calibration procedures discussed in section
3.4.2 before further use.
* The Canadian Standards Association (CSA)
requires the instrument to undergo calibration
when the displayed value during a bump test
fails to fall between 100% and 120% of the
expected value for the gas.
Always check the expiration
date on calibration gas cylinder(s) prior to use.
Expired calibration gas can lead to inaccurate
and potentially dangerous readings.
Note: It is necessary to be in either the Basic or
Technician operating mode to make calibration
adjustments. When the instrument is operated
in the Text Only “OK” mode a functional (bump)
test is the procedure used to verify accuracy. If
the readings are accurate, it is safe to use the
instrument without further adjustment.
3.4 Auto-calibration
Biosystems one-button “Auto-Calibration” mode may
be used to verify accuracy any time during normal
operation while the instrument is being used in
either the Basic or Technician operating mode.
Press the mode button three times in rapid
succession to place the instrument in the “Auto-
Calibration” mode. Adjustments are made
automatically simply by pressing the on / off mode
button.
Auto-calibration is a two step procedure. In the first
step the PhD Ultra is taken to an area where the
atmosphere is fresh and a "zero" adjustment is
made automatically by pressing the on / off mode
button. The second step is the sensor response or
"span" calibration adjustment. In this step the
accuracy of the PhD Ultra sensors is established by
exposing them to known concentration calibration
gas. Once again, the sensitivity or “span” is
automatically adjusted.
3.4.1 Fresh air "zero" auto-calibration
sequence
The fresh air zero procedure may only be done
while the instrument is being operated in either the
Technician or Basic operating mode.
(1) Turn the instrument on and make sure gas
readings are given in numbers.
If readings are given in the form of “OK” text
messages the instrument is currently being
operated in the “Text Only” mode. It will be
necessary to change to either the Basic or
Technician operating mode. Switch modes (if
necessary) by simultaneously holding down the
"+" and"-" key. Each time the operating mode is
changed, the LCD screen will briefly indicate the
new operating mode.
(2) Wait at least three minutes after turning the
instrument on to allow sensor readings to
stabilize fully before initiating auto-calibration
procedures.
(3) Make sure the instrument is located in an area
where the air is known to be fresh.