Campbell Scientific TGA100 Trace Gas Analyzer Manual - Manual (Page 99)

Figure 9-3. Purge connection for horizontal installation (left) and vertical installation (right)
9.4.3.2 Sample Inlet Filter
Install a filter element (10 µm maximum pore size) in the filter holder at the sample inlet. The PD1000 uses a 47 mm
membrane filter holder, and is shipped with a box of 100 spare filter elements. Be sure to use the filter elements (white)
and not the separator papers (light blue). Additional filter elements can be ordered from a laboratory supply vendor
(Pall Corp. part number 61757) or from Campbell Scientific (part number 9838).
The PD625 uses a Swagelok inline filter holder with a sintered element, and is shipped with one spare element.
Additional filter elements can be ordered from Campbell Scientific (part number 17575) or from your local Swagelok
supplier (part number SS-4F-K4-7). In some cases it may be helpful to have spare filter holders. These can also be
ordered from Campbell Scientific (part number 17574) or your local Swagelok supplier (part number B-4F-7).
9.4.3.3 Tubing Connections
Connect the sample intake assembly to the dryer's sample inlet connection. The PD1000 inlet connection is a ⅜"
Swagelok and the PD625 inlet connection is a ¼" Swagelok.
Connect the sample outlet of the dryer to the sample inlet of the TGA100. If the PD1000 dryer is used, both of these
connections are ⅜" Swagelok fittings, so ⅜" OD tubing may be used directly. Swagelok reducers may be used if
another tubing size is used. The sample outlet of the PD625 has a ¼" Swagelok connection, so a Swagelok reducer
must be used.
Install the purge connector in the 1" ID suction hose between the TGA100 and the sample pump, using the hose clamps
supplied with the dryer. Position the purge connector to minimize the distance to the dryer, thereby minimizing pressure
drop in the purge tubing. Usually this means the connector should be near the TGA100.
Connect the dryer's purge outlet to the purge connector in the 1" ID suction hose. The PD1000 purge connector is a ½"
Swagelok, but a reducer is supplied for ⅜" connection, and the purge connector in the 1" ID suction hose has two sizes,
½" and ⅜". The purge tubing should generally be ½" OD, but ⅜" OD tubing may be used for low flow rates and short
distances. The PD625 is configured for ⅜" OD purge tubing.
Two precautions must be observed when using the dryer:
• Make sure the purge outlet is connected to the sample pump before turning on the sample pump. This will
ensure the sample pressure is always higher than the purge pressure, to avoid tubing collapse.
• Do not allow liquid water to enter or condense inside the dryer. This can cause cooling of the dryer membrane
as the liquid water evaporates into the purge stream. This cooling will cause more water to condense from the
humid air sample, increasing the cooling effect. If a very humid sample is used, the inlet end of the dryer must
be heated to 5 to 10 °C above the dew point of the sample.
9.4.3.4 Split-Sample Configuration
For eddy covariance (EC) applications, it is desirable to have a relatively high flow rate, typically above 10 slpm.
Mount the dryer on the mast, very close to the actual intake, to minimize the length of tubing upstream of the dryer. The
dryer is used in a split sample configuration, in which a portion of the air from the dryer is used for the purge flow. This
method works because the sample air is at high (near ambient) pressure, and the purge flow is at very low pressure.
Dropping the total pressure of the purge flow drops the partial pressure of water vapor in the purge flow. This
difference in the partial pressure of water forces the water vapor from the humid sample air, through the walls of the
Nafion® tubing, and into the dry purge stream.
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