Section 3
Operation
3-9
Equation 3-6 shows that the same 5 kPa over-pressure now causes nearly
a 5% reduction in vapor pressure when measured in the LI-6262 (or other
instrument) at atmospheric pressure.
☞
Errors due to condenser over-pressure can be minimized in two ways.
First, condenser pressure can be measured and an appropriate correction
can be applied using equation 3-6. This should be done when the LI-610
is used as part of a photosynthesis system, because high flow rates are
sometimes required, and the gas exchange system may have extensive
plumbing, both leading to significant condenser over-pressures.
The 610-03 Digital Pressure Meter is available as a useful accessory to
make this measurement. It has an analog output signal that can be
automatically recorded and used in the calculations of the LI-6200
Portable Photosynthesis System, for example. Alternatively, one can use
a water or mercury manometer to measure the condenser over-pressure,
and make the corrections manually.
☞
A second method for minimizing condenser over-pressure is to use low
flow rates and a short flow path between the LI-610 and the water-
measuring instrument. This would be the normal configuration when
using the LI-610 to calibrate another instrument. The device to be
calibrated should be attached to the LI-610 with a short piece of tubing
with relatively large bore diameter [1/8 inch (3.2 mm) or larger], and the
flow rate should be kept low when the measurement is actually made
(0.25 liters per minute or less). A small condenser over-pressure may
still develop, but the condenser fill tube can be used to measure water
column height with sufficient accuracy to estimate the condenser
pressure. One centimeter of water is equivalent to 0.097 kPa, or about
0.1 kPa, at ordinary temperatures. See Section 4 for further information
about calibration protocols.
