Jet‐nozzle method for measuring response times of scalar sensors used in liquids and gases

Abstract

Response time is a central characteristic of sensors measuring solute concentrations. Knowing the response time of the sensor under realistic conditions is critically important when measuring rapid changes, e.g., oxygen concentration fluctuations for aquatic eddy covariance flux calculations. Response times therefore should be determined under conditions that are similar to those the sensor is exposed to when deployed for the actual measurements. This study introduces a new method for quantifying the response time of mini‐ or microsensors measuring solute concentrations in liquids and gases. The method is based on the rapid switching of the supply of two fluids with different oxygen concentration to the sensor without removing the sensor from the fluid flow. The device uses a jet nozzle moved by electromagnetic actuators, and the switching of the calibration fluids is monitored by a reference sensor with a response time < 0.001 s. The functionality of the device is demonstrated for a fast oxygen optode system that consisted of a fiber minioptode connected to an oxygen meter that was read by a data logger. With the setup described here, the method permits reproducible response time measurements with a resolution of 0.01 s ± 0.001 s (SD, n = 60). With this technique, the response time characterization of aquatic sensors can be standardized.

Technical note: Measurements and data analysis of sediment–water oxygen flux using a new dual-optode eddy covariance instrument

“…benthic chambers) may be most significant for deployments in continental margins. The magnitudes of biogeochemical benthic processes increase with decreasing water depth, with benthic fluxes reaching the highest rates and dynamics in the shelf environment Middelburg et al, 2005;Jahnke, 2010;Bauer et al, 2013;Reimers et al, 2004). Here light bottom currents and waves may strongly influence benthic fluxes (Gattuso et al, 2006).…”

“…Small deviations in the response time and magnitude of the sensor signal, however, can produce large differences between the measured and true flux (Berg et al, 2015). Some biofouling can be reduced by coating the sensor with antibiotics, but such treatments cannot prevent the adherence of marine snow or detritus particles (Navarro-Villoslada et al, 2001).…”

Technical note: Measurements and data analysis of sediment-water oxygen flux using a new dual-optode eddy covariance instrument

“…The reliability of the flux data hinges on unbiased sensor data that can capture temporal variability of current flow and the oxygen it carries, which may change as rapidly as 1-3 Hz (Kuwae et al, 2006;McGinnis et al, 2008a). The ADV used in the 2OEC can produce calibrated current data non-invasively at a frequency of 64 Hz, while the fiber optode has a slower response time (200-300 ms, (Merikhi et al, 2018)), and its placement near the ADV measuring volume may affect current flow measurements and thereby could bias the flux calculations. The Pyroscience fiber optode used with the 2OEC is one of the smallest and fastest oxygen sensors available, and a comparison with the most common oxygen sensors presently utilized for aquatic eddy covariance (Table A4) favors the selection of this sensor for many field settings.…”

“…due to ambient light or fluorescent substances in the water). The ultra-high speed OXR430-UHS optodes achieve response times (t90) of 150 to 300 ms (Merikhi et al, 2018) and thus can easily capture oxygen fluctuations at the temporal resolution required by the eddy covariance technique (Lorrai et al, 2010;Donis et al, 2015), preventing loss of flux contributions at high turbulence frequencies (McGinnis et al, 2008b). The 430 µm diameter optical fiber of these optodes is relative robust relative to microelectrodes.…”

“…The signal drift over this period was negligible (< 0.03%). The optodes for the in-situ measurements were selected for similar fast response times (< 300 ms) using the jet-nozzle method introduced by Merikhi et al (2018). The acoustic Doppler velocimeter (ADV) used for this eddy instrument was a NORTEK Vector, which is a single-point current meter capable of measuring velocity and current direction in a small measuring volume (14 mm diameter, 14 mm height (user-specified)), at rates up to 64 Hz (Table A3).…”