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.