Box 1 The Total Dissolved Gas Pressure (TDGP) Probe
The TDGP of groundwater can be measured with a probe as illustrated in Figure Box 1-1. A TDGP probe consists of small-diameter tubing that is permeable to gases (including water vapor) but is not permeable to liquid water. One end of the tube is connected to a pressure sensor and the other end is plugged. When placed in water, gas inside the tube will either migrate into the water (if the TDGP is less than the initial gas pressure inside the tube) or dissolved gases from the water will migrate into the tube (if the TDGP is greater than the initial gas pressure). For example, if the probe is inserted into a bottle of carbonated water, the probe will read the pressure of [latex]{CO}_{2}[/latex] that was used to create the carbonated water but will not respond to the fluid pressure. If the probe is inserted into degassed water, it will read zero pressure independent of the fluid pressure.
Figure Box 1‑1 ‑ Dissolved gas pressure probe.