Pre-Deployment Laboratory Calibration and Performance Checks

Prior to each site visit (preferably the day before, but no more than 5 working days in advance), calibration checks are performed for the repTDG1 and repTDG2 monitors and for a field monitor that will be used to detect changes in TDG during cleaning procedures in the field. Calibration notes are documented on the laboratory calibration form (app. 1). Monitors can be stored with or without a membrane installed. If a monitor was stored without a membrane installed, a clean, dry membrane is installed, and gas pressure inside the membrane (in air) is allowed to equilibrate for at least 1 hour. If the monitor was stored with a membrane installed, the membrane is visually inspected to ensure that it is clean and intact and is replaced if its integrity is in question. The TDG reading (in air) is compared to an air pressure reading from a certified reference barometer. If the in-air TDG reading differs from the certified reference barometer by more than 2 mm Hg, the sensor is calibrated to the air pressure reading. The 2mm Hg criterion was adapted from Tanner and Johnston (2001), who used 2.5 mm Hg as a standard. Experience within the USGS PAWSC has indicated that the lower value of 2 mm Hg is consistently achievable.

After the calibration procedures are complete, two qualitative tests are performed to verify the integrity of the TDG membrane. These tests are critical because the in-air calibration check will not detect a torn or otherwise compromised membrane, and a sensor with a compromised membrane acts like a pressure transducer in water and reacts to changes in water depth. The first check is performed in a “zero-oxygen” solution (a solution of approximately 2 grams of sodium sulfite per liter of deionized water; U.S. Geological Survey [2020]), which tests the sensor response in undersaturated dissolved gas environments. TDG readings in zero-oxygen solution should drop by approximately 30 mm Hg (compared to the in-air reading and known barometric pressure) within roughly 5 minutes. If the sensor passes the zero-oxygen solution check, a second test is done in supersaturated (carbonated) water. In this test, TDG readings should increase to 1,000 mm Hg or greater within 2 minutes (Tanner and Johnston, 2001). If either check test fails, the membrane is replaced, and these steps (beginning with the calibration check versus air pressure as described above) are repeated.

An optional third test can be performed in a pressure chamber using a Druck DPI 104 pressure gage connected to a vacuum pump and a modified calibration cup through ¼-inch silicone tubing. This calibration cup is mounted on the monitor with the membrane placed on the TDG sensor. The absolute pressure (BP not subtracted) in the chamber is increased to exceed the expected maximum TDG value during deployment, typically 800 mm Hg (fig. 4). The TDG readings of the monitor are then compared to the pressure gage readings. Values are recorded (typically every 15 minutes) until the readings have stabilized. Full stabilization may not be achieved because getting a perfect air-tight connection is difficult. TDG monitor and pressure gage readings should agree within plus or minus (±) 5 mm Hg after about 30 minutes. An instantaneous response during this test indicates a damaged membrane. All pre-deployment TDG notes, calibration checks, and performance checks are recorded on a paper TDG pre-deployment form (app. 1) and digitally archived on local servers and within the USGS Aquarius Time-Series (AQTS) database. Table 1 summarizes each calibration and performance check that is performed on a TDG membrane.

Field Procedures

The field procedures for collecting TDG data were adapted from procedures described by Wagner and others (2006). Field notes for TDG and BP are recorded on the paper TDG field form (app. 2) and archived on local servers and within AQTS. As noted above in the sections titled Site Operation and Pre-Deployment Calibration and Performance Checks, the repTDG1 and repTDG2 monitors, along with the field monitor, will have undergone calibration and operational checks in advance of the field visit.

Upon arrival at the site, each monitor is placed in a clean, dry location that is not in direct sunlight. Readings from the repTDG1 and repTDG2 monitors, in air, are checked against a reference barometer and are recorded at approximately 10-minute intervals until readings have stabilized (stable over a 10-minute period or changing at the same rate as barometric pressure). Changing temperatures and field factors may slow stabilization. If the TDG monitors pass all laboratory performance checks, then an in-air TDG reading within ±3 mm Hg of the field barometer is accepted. This is slightly less stringent than the ±2 mm Hg criteria from the pre-deployment checks performed in the laboratory, in recognition of the less controlled conditions in the field. If the difference is greater than ±3 mm Hg, the sensor is recalibrated in the field. The purpose of these readings is to ensure the sensor was not damaged during transport from the laboratory to the field site.

Next, the replacement monitors and the field monitor are placed in the tailrace pool, near the operating site monitors that are interfaced to the Satlink 3 and XLink 500 DCPs. Readings from the site monitors, field monitor, and replacement monitors are recorded at approximately 10-minute intervals until readings have stabilized or are changing at approximately the same rate. The last values from TDG1 (or TDG2 if the primary has malfunctioned) and the field monitor are used as “before-cleaning” readings as described in Wagner and others (2006). This process also serves as a functional test for the replacement monitors while allowing them to stabilize to environmental conditions. This will shorten stabilization time when the monitors are eventually swapped.

After TDG readings have stabilized and before-cleaning readings have been recorded, the primary and backup site barometers are checked against a reference barometer to ensure accuracy within ±2 mm Hg. The primary and redundant site TDG monitors are then removed from their deployment tubes for cleaning. The monitor body is wiped down with a microfiber cloth and (or) foam eraser and cleaned as necessary (fig. 5A). The probe guard is removed and cleaned thoroughly using a soft-bristled nylon brush and deionized water. The sensors are rinsed thoroughly with deionized water. Dirt and debris that have accumulated in the TDG membrane (fig. 5B) are carefully removed using foam or cotton swabs (fig. 5C). If fouling is severe, a small amount of diluted soapy water can be used. If soap is used, three tap-water rinses are performed to remove the soap solution, followed by three rinses in deionized water. If soap is not used, then only the three deionized water rinses are needed. Visible debris on the outside of the deployment tubes is removed, and a nylon brush mounted to a chimney cleaning rod is used to remove debris from the inside of each deployment tube.

The cleaned site monitors are then placed back in their deployment tubes. Readings are again recorded at approximately 10-minute intervals until they have stabilized or are changing at approximately the same rate. The last values from TDG1 (or TDG2 if the primary has malfunctioned) and the field monitor are now recorded as “after-cleaning” readings, as adapted from Wagner and others (2006).

Finally, the TDG1 and TDG2 monitors are removed from their deployment tubes and the repTDG1 and repTDG2 monitors are installed and interfaced to the Satlink3 and XLink 500 DCPs, respectively. Readings from the repTDG1 and repTDG2 monitors and the field monitor are again recorded at approximately 10-minute intervals until readings have stabilized. These readings provide confidence that the replacement monitors are functioning properly in the deployment tubes.

Post-Deployment Calibration Checks

Within 5 business days of returning from the site visit, calibration checks must be performed on the TDG1 and (or) TDG2 monitor that were removed from the site. The same steps that were followed for pre-deployment (table 1) are performed again. The purpose of this step is to verify that the monitor(s) were operational and to determine if a correction for calibration drift will be needed. All post-deployment TDG notes, calibration checks, and performance checks are recorded on a paper TDG post-deployment laboratory calibration form (app. 1), which is digitally archived on local servers and within AQTS. Finally, the monitor and TDG membrane are soaked in soapy water for approximately 10 minutes, rinsed thoroughly with tap water, and allowed to air dry. If necessary, a new TDG membrane could be installed at this time. Monitors are stored with the sensor guards attached, and the membranes are stored dry and open to air.

Collection of Quality Assurance Data

Quality assurance checks are performed at least once per data collection season (June 1–November 30) to verify that data collected at the site monitoring location are representative of water quality in the dam tailrace. Procedures for completing water quality cross-section surveys found in Wagner and others (2006) were modified for this purpose. Data are collected using independent monitors equipped with TDG sensors either from a crewed boat anchored at a minimum of three locations in the tailrace pool, or from a minimum of three locations along the bank. Collecting data from the bank eliminates hazards associated with operating a crewed boat in the turbulent water of the tailrace. TDG readings are not instantaneous because ample time is needed for gases to diffuse through the membrane; thus, two monitors are used to collect data. One monitor is placed on the stream bed, and the other is held just below the water surface (fig. 6). Readings are allowed to stabilize for at least 15 minutes before data are recorded. Collecting data at depth and just below the water surface allows for evaluation of the effect of hydrostatic pressure and depth compensation on the readings. Pool depth near the monitors is typically 3 to 5 feet. Data are recorded in a standard field notebook, and notes are scanned and appended to AQTS and local servers for archiving.

Procedures for collecting independent quality-assurance data were introduced in June 2023. Discrete water samples were collected at the site monitor location and near the power plant and analyzed by the USGS Groundwater Dating Laboratory in Reston, Virginia. Concentrations of dissolved nitrogen, argon, carbon dioxide, methane, and oxygen were determined, and their individual partial pressures were summed and compared to readings from the continuous monitor. Beginning in 2024, these data were collected and analyzed using methods adapted from Busenberg and others (1998) and made available in the USGS NWIS database (U.S. Geological Survey, 2025) to complement data collected using independent monitors.

Review and Deletion of Erroneous Data

TDG data that are clearly erroneous are deleted. Deletion of erroneous data often happens automatically within the USGS NWIS database through screening thresholds or occurs during routine review (generally daily) of real-time data. If the sensor or membrane on the TDG1 sensor is presumed to have failed, data are backfilled with data from the TDG2 sensor within the AQTS software.

Fouling Corrections

Debris and organic matter can deposit on the sensor during deployment, which may result in inaccurate readings. These data can often be corrected. Fouling corrections for TDG are typically applied if fouling exceeds ±1 percent. The BP sensors are not subject to fouling because they are not deployed in the water. Calculation of fouling error of the TDG sensor (in percent) was adapted from methods described by Wagner and others (2006) and is computed as follows:

Corrections are usually applied in a prorated fashion, beginning at 0 percent following an initial site visit and prorating to full effect at the next visit, when a fouling error was determined using equation 3. For example, a clean, calibrated monitor was deployed on June 1, and a routine site visit was completed on June 15. On June 15, a fouling error of 2 percent was computed from before- and after-cleaning and field readings using equation 3. The correction would be prorated from 0 percent on June 1 to 2 percent on June 15, at the time that the before-cleaning reading was recorded. The correction would terminate on June 15 (resetting to a 0 percent correction) when the cleaned monitor was redeployed in the water. Applying a correction in this manner assumes that fouling occurred gradually, beginning at the end of the previous visit. If there is evidence that fouling began later, the start time can be adjusted. In the example provided above, the monitor was deployed on June 1, and a 2 percent fouling correction was computed from the June 15 field data. If discharge to the tailrace was dramatically increased on June 7 at 12:00 p.m., an analyst may determine that the fouling was caused by the increased flow and prorate a correction from 0 percent on June 7 at 12:00 p.m.to 2 percent on June 15 at the time that the before-cleaning reading was recorded.

Drift Corrections

After completing a site visit, the monitors that had been deployed (but were removed and replaced) are brought back to the laboratory to undergo calibration drift checks. As with the preparation of monitors that are to be deployed, in-air TDG readings are compared to readings from a reference barometer. These readings are used to determine drift error as follows:

Drift corrections are applied in the same prorated fashion described for fouling corrections. Corrections are applied when drift exceeds ±1 percent. Calibrations for the TDG sensors have been shown to be robust; the need for drift corrections is rare.

Other Corrections

Other corrections may be applied at the analyst’s discretion. An example of when such a correction might be applied is an instance where the TDG membrane was damaged during the cleaning process, preventing the computation of fouling and drift error. In this case, the analyst might apply a correction to force data from the site monitor to match that from the field monitor at the time that the “before-cleaning” readings were obtained. The rationale for applying such a correction must be explained in detail in a station analysis report that is stored within the USGS internal RMS database.

Professional Judgment

The general correction criteria described above may be superseded by the analyst’s professional judgment. TDG readings are complicated by the slow diffusion rate of gases through the membrane, which is exacerbated by saturation of membrane tubing with water and the fragility of the membranes. For example, changes in the power plant or dam operations that occur while a site visit is taking place can change the TDG concentration in the tailrace. This can complicate efforts to determine when readings have stabilized because the membranes on the TDG1 and TDG2 are likely water-saturated (after a multiweek deployment), which may result in much slower response times than those of the field, repTDG1, and repTDG2 monitors. If readings are recorded before stabilization, equation 3 may indicate a correction that is not valid. All decisions are documented in an internal analysis report and stored in the USGS internal RMS database.