Question: Do sensors need to be removed/sheltered/maintained in the winter?
Answer:
The pressure transducer in the CTD sensor can be damaged if ice freezes around it. If your sensors are in shallow water they will be at risk for this type of freezing during cold winter weather. Please see the below section from the manual for guidance:
3.2.6 Winter time freezing risk – when to remove sensors from the stream
The issue of highest concern during winter is damage to the pressure transducer (measures depth) in the Hydros 21 CTD sensor (see Section 2.4). The pressure transducer, as the name implies, is sensitive to pressure changes, so when water expands during freezing the pressure of the ice directly against the pressure transducer can damage it.
If freezing does occur around the CTD and turbidity sensors all data will be inaccurate during this period time and should not be used. Upon thawing, turbidity data should return to normal. Depth data will be suspect and should be closely checked after thawing to determine if the sensor was damaged. Temperature and conductivity data should return to normal but should also be checked to ensure accuracy after thawing, as these can be affected if the pressure transducer is damaged.
Generally, if sensors are >6 inches below the water surface, freezing of the sensors is unlikely – the shallower the water in which sensors are positioned the higher the risk of freezing. In most cases sensors are installed at depths that are below where ice layers normally form. Monitoring air temperatures during the winter can be important, especially in cases where sensors are located within the part of the water column where freezing may occur. Freezing risk is generally highest in small streams where there are not deep locations available for sensor placement, but freezing may also be a risk for deeper sensors during severe and extended low air temperatures when ice layers get thicker than normal.
Ice layers above sensors are not a high risk to sensor integrity, however, once an ice layer forms it can be difficult to access sensors. If the ice layer has to be broken with a hammer or chisel there is risk of damage via direct contact of the hammer/chisel with the sensors and/or sensor wires. There is also a risk of damage to sensors through shifting pieces of ice as they are broken and through possible damage from the shock of the hammer/chisel.
If freezing of the water surface is an issue and obtaining accurate data during this time period is important it is recommended that ice layers be removed on a frequent enough basis so as to not require a hammer/chisel for breaking the ice (i.e., remove ice when it can be broken by hand). Clearing ice at this frequency will ensure that the sensors are accessible for cleaning and ensuring accurate data.
David Bressler
