Performance Verification Statement for the Eureka Manta2 pH Sensor

Ref. No. [UMCES] CBL 2015-009

ACT VS15-02

followed by depth profiling (50%), then hand-held portable use (48%), then flow-through

systems (26%). Respondents used a variety of calibration procedures including commercial

buffers (68%), CO

chemistry (35%), seawater CRMs (23%), pH indicator dyes (18%), and

supplied by manufacturer (13%). The four areas where respondents expressed the greatest

concern over the use of in situ pH sensors were ruggedness (49%), calibration life (46%), level

of measurement uncertainty (43%), and reliability (41%). The complete needs and use

assessment reports can be found at:

INSTRUMENT TECHNOLOGY TESTED

ACT released an additional request-for-technologies for companies to participate in the

pH verification after the initial Laboratory component had taken place. The Eureka Manta2

instrument was submitted during this call and therefore this statement only includes results for

the Moored Field Deployment component of this pH sensor Verification.

The Eureka pH sensing system uses a robust glass sensor optimized for stability and

reliability in the pH 2 - pH 12 range. The pH sensor is short, minimizing the amount of buffer

solution needed during calibration. The system is equipped with a separate, refillable reference

electrode with a replaceable junction cap, designed to reduce operational costs and increase

accuracy. Most combination pH electrodes fail due to reference electrode problems, requiring

the entire probe to be replaced.

The Eureka calibration software allows the user to calibrate with either 2 or 3 reference

buffers, in any order. While calibration is often done with pH 4, 7, & 10 buffers, the user has the

option to use any pH buffer, in an attempt to increase the accuracy of the system in a particular

sample water. The calibration software also alerts the user if the values of the sensor are outside

of the expected range, alerting the user to potential buffer contamination or data entry

errors. The software creates and maintains a Calibration Log that records all calibration results,

including the temperature, millivolt reading, and pre- and post- readings in each buffer. The

logging software will also display and record probe millivolt readings, as well as pH values.

Eureka uses a high impedance, high resolution circuit to measure the output of the pH

probe. The circuit board is sealed with a conformal coating and designed to maintain isolation of

the probe connections to avoid faulty readings from electrical leakage or moisture

condensation. The probe is also electrically disconnected from the instrument during sleep

intervals, to avoid any possible leakage currents that might compromise the probe's lifetime or

performance.

In addition, the unique design of the reference electrode improves the performance of the

system in several ways:



The electrolyte chamber is easily refillable, and designed to purge the probe of air

bubbles and clean the Teflon junction during refilling, to optimize accuracy.



The reference electrode uses a porous Teflon junction to reduce fouling; its large surface

area ensures constant electrolyte diffusion, minimizing the junction potential errors

often encountered when ceramic junctions get dirty.



The large volume of reference electrolyte extends the field deployment time up to 6

weeks, and allows the addition of KCl pellets to even further extend deployment time.