Testing the Honeywell Durafet® for seawater pH applications

We report on the first seawater tests at 1 atm of the Honeywell Durafet (R) pH sensor, a commercially available ion sensitive field effect transistor (ISFET). Performance of this sensor was evaluated in a number of different situations including a temperature-controlled calibration vessel, the MBARI test tank, shipboard underway mapping, and a surface mooring. Many of these tests included a secondary reference electrode in addition to the internal reference supplied with the stock Durafet sensor. We present a theoretical overview of sensor response using both types of reference electrode. The Durafet sensor operates with a short term precision of +/- 0.0005 pH over periods of several hours and exhibits stability of better than 0.005 pH over periods of weeks to months. Our tests indicate that the Durafet pH sensor operates at a level of performance satisfactory for many types of biogeochemical studies at low pressure.