18
average concentration for the most recently
completed 15 minutes of operation.
Note: For the first 15 minutes after the PhD
Ultra is initially turned on the STEL reading is a
projected value. The PhD Ultra will begin
projecting a STEL value after the first 30
seconds of operation. For the first 30 seconds
the STEL screen will show an "X" where the
reading should be.
The STEL reading is constantly updated. Audible
and visual gas alarms will be activated whenever
the most recent 15-minute average exceeds the
STEL alarm set point.
Appendix A discusses Permissible Exposure
Limit alarm calculations in greater detail.
2.2.3.3 TWA readings
Press the mode button again to display the TWA
(Time Weighted Average) exposure levels. TWA
values are calculated by projecting exposures over
an eight-hour period.
It is not possible to compute a toxic gas TWA until
the PhD Ultra has been monitoring for at least 15
minutes. Until the minimum monitoring time has
elapsed, the TWA screen will show an "X" where the
reading should be.
Note: After 30 minutes the screen will also
begin to indicate how long the instrument has
been on. This “run time” indication will be
given in hours and completed 15-minute
intervals.
2.2.3.4 Runtime screen
Press the mode button again to display the runtime
screen, which shows battery voltage, running time
for the instrument since last turned on (in hours and
minutes), and temperature (in both Fahrenheit and
Centigrade).
Press the mode button again to return to the current
gas readings screen.
2.2.4 Changing operating modes
To change operating modes, simply slide the belt
clip located on the back of the PhD Ultra case
downward to expose the four push buttons on the
instrument keypad. Push the "+" and "-" buttons at
the same time to change operating modes. Each
time the operating mode is changed the display
screen will briefly indicate the operating mode that
has been selected. The instrument will not incur
any loss of data due to a change in operating mode.
Note: Shifting modes or otherwise
reprogramming the instrument should only be
done by employees who are authorized to do so.
2.3 Batteries
PhD Ultra batteries are housed in removable battery
packs. Two types of battery packs (rechargeable
NiCad and alkaline) are available for use. Battery
packs are fully interchangeable and can be replaced
while the instrument is in the field. It is not
necessary to open the instrument case to replace or
change the battery pack.
Battery packs are clearly labeled as containing
either NiCad or disposable alkaline batteries. The
gasketed battery packs are held firmly in place by a
spring-loaded catch on the instrument. The PhD
Ultra should be turned off before changing or
replacing battery packs.
Note: The PhD Ultra is designed to turn itself on
whenever a battery pack is removed and then
replaced. This is to ensure that in the event of a
power interruption the instrument is not
accidentally turned off. Any time the battery
pack is momentarily removed or replaced with
another it will be necessary to manually turn the
PhD Ultra off if the instrument is not going to be
put to immediate use.
Figure 2.3. PhD Ultra with Battery Pack Removed
To remove the battery pack pull back on the spring-
loaded catch, and pull the battery pack upward and
backwards (towards the rear of the instrument). To
replace the battery pack, seat the notch at the front
of the battery pack housing to the matching lip in the
PhD Ultra instrument case. When the battery pack
is properly positioned, press down gently on the rear
of the battery pack to engage the spring loaded
catch.
