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Total Dissolved Gas Frequently Asked Questions
Q: What is TDG and why would I want to measure it?
A: TDG (Total Dissolved Gas) is a measure of the sum total of all gas partial pressures (including water vapor) in water. TDG can be reported as an absolute overall dissolved gas pressure or relative to atmospheric pressure (“delta P”). Gas bubbles can form in waters that are supersaturated, that is, when total dissolved gas pressure is greater than atmospheric pressure plus water column pressure. More importantly, gas bubbles can also form in the blood and tissues of aquatic organisms when water becomes supersaturated with gas. This results in “Gas Bubble Disease” in the affected organisms. Gas Bubble Disease can, in turn, cause rapid acute mortality as well as increase long-term mortality in populations of those organisms in rivers, natural springs, aquaculture ponds and tanks, aquaria, and other environments. Supersaturation can easily occur if air bubbles are exposed to high water pressure (e.g. - spillways, pumps, filtration systems), the temperature of water rises (e.g. - cooling water discharge, mixing of different water masses), if water pressure is reduced (e.g. - groundwater, springs), or when ice forms.
Q: Can I derive Total Dissolved Gas levels from dissolved oxygen (DO) readings?
A: No. Nitrogen and oxygen are the two most prevalent atmospheric gases. While elevated levels of either of these gases MAY drive waters to overall gas supersaturation, measuring just the concentration of oxygen, nitrogen, any other individual gas, or a combination of individual gases will not yield useful information related to this problem. The total pressure of all dissolved gases combined (including water vapor pressure) is the single factor that determines whether bubbles form or not. Measuring dissolved oxygen individually is certainly an important factor in water quality monitoring, but dissolved oxygen readings cannot be used to derive any information about overall gas saturation levels and whether or not Gas Bubble Disease will occur.
Individual gases do not dissolve into water in the same proportions as they are found in air. Moreover, oxygen (and carbon dioxide) can be produced or consumed by biological activity in water. It is common for oxygen concentration and oxygen partial pressure in water to change without any significant relationship to the overall total dissolved gas pressure of the water.
Q: What is the depth limit of TDG sensors?
A: TDG sensors are capable of accurately making measurements to a depth of approximately 53 meters (about 174 feet). Gases are not able to move across the TDG sensor membranes below this depth. No damage will occur to the membrane if it is submerged below that depth, but damage to the pressure sensing element can occur if the sensor is submerged beyond 60 meters (197 feet).
Q: How often must a TDG sensor be calibrated?
A: The TDG sensor’s pressure element is factory calibrated. No user calibration is possible or necessary. A TDG sensor must be recalibrated at the factory should its pressure response show drift compared to a known reference (barometric pressure, for instance). This is rarely necessary and typically only results from sensor abuse, damage, or age. The following conditions indicate a need for factory recalibration:
Absolute sensors (T506, T507) – the equilibrated TDG reading in air is greater than +/-7 mmHg from true barometric pressure.
Model T900 – the equilibrated delta P reading in air is less than –7 mmHg or greater than +7 mmHg
Model T300E – the difference between the displayed delta P readings with the zero adjust knob turned fully clockwise and fully counter-clockwise is less than 247 or greater than 253.
Q: What are the field maintenance requirements for a TDG sensor?
A: No user calibration of the sensor is necessary. The membrane and membrane cartridge should be cleaned with a soft brush after approximately 2 to 4 weeks of continuous use. Fouling of the membrane and cartridge (biological or otherwise) will not affect the accuracy of the TDG sensor, but will increase the response time of the sensor to changes in dissolved gas pressure.
Q: What is the sensor response time?
A: The sensor works through passive diffusion of gases across the membrane. Typical response time of the sensor to large changes in dissolved gas pressure is 5 minutes or less for a 90% change. This is partly dependent on water temperature. Colder environments require a longer equilibration time.
Q: Can I upgrade my older Alpha Designs model 300C Tensionometer to the newer model 300E?
A: No. The model 300C has some electronics in the probe and some in the display unit. All electronics were moved to the handheld display unit on the model 300E. No upgrade is possible.
Q: Can I purchase one model 300E handheld display and use it with two probes?
A: No. The model 300E handheld displays and probes must be factory calibrated as a matched set. They are not available for sale separately. Probes and displays should not be mixed and matched if a user owns multiple units.
Q: Is a waterproof carrying case available for the model 300E Tensionometer?
A: The model 300E sensor and display set comes with a watertight custom fitted Pelican™ case, which has an environmental rating of IP67. It even floats!
Q: Is the model 300E display waterproof?
A: No. The model 300E display has an environmental rating of IP65. It is resistant to splash, spray, and rain, but should not be submerged.
Q: What is the digital data output format of the model 300E?
A: The model 300E is a handheld device designed for spot-checking of TDG levels. The model 300E display has a three digit LCD that shows delta P as whole number values in millimeters of mercury (mmHg). The model 300E display unit does not log values nor are there connections for external logging devices.
Q: What is the format of the available RS232 data output of TDG sensors (excluding the model 300E)?
A: Model T506, T507, and T900 TDG sensors output data as RS232 ASCII strings. The format of these strings for each model is as follows:
T506: xx.x ° C x.xxx Atm
T507: xx.x ° C xxxx.x mmHg
T900: -xxx.x mmHg (for negative delta P) / xxx.x mmHg (for positive delta P)
Communications settings for all TDG sensors listed above are:
Baud rate: 9600
Data bits: 8
Parity: None
Stop bits: 1
Flow control: None
Q: What is the warranty on TDG sensors?
A: The In-Situ® total dissolved gas sensor and In-Situ® RuggedCable® are warranted for one year from date of purchase by the end user against defects in materials and workmanship. The In-Situ® Inc. TDG membranes are warranted for 30 days from date of purchase by the end user against defects in material and workmanship. Within the warranty period, In-Situ® will repair or replace, at its sole discretion, free of charge, any product that In-Situ® Inc. determines to be covered by this warranty.
Q: What equipment and accessories are required to monitor TDG?
A: The model 300E is a stand-alone spot-checking instrument that includes everything necessary to monitor TDG. The model T900 TDG sensor includes a probe, membrane cartridge, and fixed length cable (4, 10, or 20 meters) for connection to an external RS232 or analog (0 to 5 volt) input on third party devices. Model T506 and T507 sensors include the probe and membrane cartridge only. Connection to the probe is via a Sea ConTM marine connector. Optional cables are available for RS232 digital (T506 & T507) or 0.5 to 4.5 volt analog (T507) connections to third party devices.
Q: Should I buy an extra membrane cartridge?
A: Yes! Membrane cartridges may become damaged, fouled, or rendered temporarily inoperable by water vapor condensation inside the membrane tubing. Having an extra membrane cartridge available in the field is inexpensive and easy insurance against interruption of your data collection.
Q: How difficult is membrane cartridge replacement?
A: Membrane cartridge replacement is easy and takes about 30 seconds. Simply unscrew the protective guard, unscrew the old membrane cartridge, screw on a new one, and replace the guard.
Q: Can I buy a used refurbished membrane cartridge?
A: No. In-Situ® will refurbish current models of Alpha Designs TDG sensor membrane cartridges sent to us by customers for such service. We do not, however, offer used or refurbished cartridges for sale.
Q: Is there a “core exchange” on membrane cartridge refurbishing?
A: No. In-Situ® refurbishes and returns the same Alpha Designs membrane cartridge sent to us for service. If the cartridge spool is damaged and unsuitable for refurbishing, we will require that a new one be purchased at an additional charge.
Q: What maintenance services are available for TDG sensors?
A: An evaluation fee applies to each complete TDG sensor or system sent for service. This evaluation is required for every repair, and a quotation will be sent to the customer after the evaluation but prior to the execution of repairs. This quotation will detail the problems found and the estimated services and costs required to correct them. Repairs will be billed at an hourly rate for labor plus the cost of parts. These charges are in addition to the evaluation fee. Recalibration of the TDG pressure sensor is not included in the above services, and is billable as a separate charge and is only done per customer request/authorization.
Q: What services are included with membrane cartridge refurbishing?
A: Membrane refurbishing includes examination of all membrane cartridge components for condition and suitability for reuse, cleaning of all components, and replacement and rewinding of the membrane tubing. The cartridge refurbishing fee does not include any other services or evaluation of any other equipment. No separate evaluation fee is charged for membrane cartridges sent by themselves for refurbishing.
Q: Are TDG sensor readings compensated for temperature?
A: Yes. The sensor electronics are compensated for temperature changes. No further temperature adjustment of the TDG reading is necessary.
Q: Are TDG readings compensated for salinity?
A: No “correction” for salinity is required when measuring TDG. The TDG sensors do not measure dissolved gas concentration, but rather, measure total dissolved gas pressure (or “tension”). While the concentration of gases in water is affected by changes in salinity, the tension of those gases in water is not.
Q: How is the availability of TDG sensors for Hydrolab(TM) and YSI(TM) instruments affected by the acquisition of Alpha Designs by In-Situ® Inc.?
A: The availability is not affected. In-Situ® continues to service and supply Alpha Designs TDG sensors and membrane cartridges designed for use on Hydrolab(TM) and YSI(TM) instruments to Hydrolab and YSI companies and customers.
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