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CHAPTER 7
CALIBRATION
Every instrument must occasionally be calibrated against some known value of a
parameter in order to make sure that its accuracy has not deteriorated.
The instrument software makes sure that the display readout is always a linear
function of the source excitation (i.e. gas concentration or temperature, etc.). You
therefore need only two points on the straight line to calibrate a parameter over its
entire range. Usually, the first point chosen is the zero value (called zeroing the
instrument). The second point has to be set by using some known value of the
parameter being calibrated (for example, using 200 PPM certified carbon
monoxide gas to set the display to read 200). Sometimes the second point is not
needed: if the slope of the parameter is known and is always the same. For
example, for the stack temperature the slope of the curve is well known and you
don't need a span calibration.
A. AUTOZEROING THE INSTRUMENT
Every time you turn the instrument on, wait for two minutes to allow the ENERAC
to warm up. You can then press the ZERO button to start the autozero procedure.
At the end of the autozero period the ENERAC reads the output of all sensors and
sets them all to zero, with the exception of the oxygen that it sets to 20.9%. (The
ambient temperature is read directly). Consequently, it is very important that at the
moment of "zeroing" the probe tip is at room temperature and the environment is
clean from traces of carbon monoxide or other gases.
You can set the countdown time for autozeroing the analyzer by first pressing the
SHIFT button to enter the SETUP MENU, and then choosing the ZERO/SPAN
SETUP submenu. Scroll down until the arrow appears beside the message “ZERO
TIME: XX SEC”. Press ENTER to edit, use the UP/DOWN keys to set the desired
auto zero period, then press the ENTER key.
NOTE: In practice AUTOZEROING is only needed once at the beginning of a day
of measurements. The ENERAC will not have sufficient zero drift during the next
24 hours to require additional autozeroing procedures.
To carry out the autozero procedure, follow these steps:
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