Support Engineering Group : Reliable High Precision pH Sensor This page last changed on Mar 31, 2011 by tm. Links HRPH Sensors for May 2009 Deployment HRpH Sensors for May 2010 Deployment HRpH Sensors for Apr 2011 Deployment HRPH Tech Transfer Package For Heron Island - tech transfer package was prepared but not used, UQ Heron Island decided to have Nido Instruments do the build instead ##### HRPH Project Background Ocean Acidification Instruments / Studies such as FOCE require a precision pH sensor capable of operating up to full ocean depth (4000m). The Seabird SBE 18-I is presently used. The pH sensor problems found on FOCE and BRS after talking with Ed Peltzer, Bob Herlein and Craig Okuda. The housing leaks. The sensor rails possibly due to physical placement near motors and either does not recover or takes 20 to 30 minutes to recover. Seawater diffuses into the KCL electrolyte resulting in an offset over time. The pH sensor also exhibits slow response when the reference KCL has been infiltrated with seawater. Notes from Thom on possible solutions: Improve the housing design. Also look at documenting a procedure to seal and test the existing housing reliably. Redesign the sensor electronics and improve the grounding and isolation. Provide an isolated digitized output. (Possible Innovative idea) Use 'standardized' Seawater as the electrolyte to mitigate affects of prolonged exposure to seawater. Preliminary reading about the chemistry of the sensor and a brief discussion with Ed Peltzer last wee (7/31) seemed to indicate this approach might work. Notes on FOCE system issues found during pH testing Mink-10 connector on FOCE can is not straight thru wired to DJ's Mars simulator can Motor back EMF problem due to 24v motor being operated at 24v. EMF problem is there at 2000 rpm and very pronounced at 3000 rpm. The result is current slewing 4A to 8A at 3000 rpm. Manufacturer recommends operating at 48v however the EZ servo upper limit is 42v. There is a blown channel (#3) on the A/D card in the PC104 stack. The reading is always 4.999v. Loose connectors to the A/D card in the PC104 stack. Wiggling this connector causes the pH readings to rail. Hardisk cable rubbing on a sharp edge caused a hard disk failure Ethernet connection is sometimes flaky Loose power connection to PC104 relay board - screw terminals need tightening FOCE pressure housing is tied to Ground through the metal chassis 24v Fuse is 8A, should be at least 10 to 12 slo blow. FOCE vibrates noticably when operated just below the surface - this vibration may have been interacting with the A/D connector. Modifications to address FOCE system issues No modification for Mink-10 connector as the ODI connector defines the MARS interface. Team educated to always use the test cable. Note: as a lessons learned, perhaps the use case of bench testing should be a requirement and the Mink-10 connector pinouts should be the same to enable use of a straight thru cable. 36v power supply added to drive the motor and give headroom for operation at 2000 rpm (max spec'd speed) Replaced A/D card fixed the blown channel #3 problem Replaced the pins in the connector for a more positive contact Replaced harddisk mounting bracket and removed sharp edges. Also re-routed harddisk cable away from the Ethernet. Replaced ethernet connector with a factory connector from Lippert. Tightened screw terminals. Removed chassis from pressure housing lid - used mylar to insulate chassis from pressure housing lid. FUSE replaced with 10A. #### Design of sensor electronics with improved grounding and isolation Requirements Meet or exceed Seabird18 for accuracy and performance without being susceptible to railing or errors when operated with the full range of the FOCE motors. Match electrical spec of Seabird 18 as used by FOCE Power: 6-24v dc, 7mA. (Measured at 14mA) Analog signal output: 0 to +5v Isolation - isolate analog front end from connection to FOCE (break the ground loop) Option A: is to use an isolated power supply and an analog isolator Option B: isolated power supply, A/D converter, microprocessor for signal processing / correction, digitized ph sent RS232 or RS485 Option C: same as option B, but convert pH back to analog. This option will be used for the engineering test if we don't have the resource to write a SIAM serial driver. Best in class analog front end - pH probes are typically 40M to 400M ohms impedance and a proper front end should present 10x or more of that impedance to properly read the sensor. Key factors in opamp selection are: super high input impedance super low bias current offset voltage drift / temperature stability availability power consumption Seabird18 uses a single AD549 op amp, BRS team uses LM6001. INA116 is an instrumentation amp with 3 fA bias and 0.1 Peta ohm input impedance (10 to the 14 - yikes) AD549 AD648 Improve analog front end grounding Several articles on high resolution pH show grounding the analog front end to the solution being measured #### Hardware Design ##### Analog Front End Board pH probe interface with ultra low bias Instrumentation amplifier used (INA116) no filter capacitors in the high impedance signal chain to improve recovery times. Next rev add linear regulators for power filtering to the front end board. ##### pH Meter Board MSP430F2618 / 169 chosen for software reuse from Valve Pack Controller project ADS1251 24bit converter (19 ENOB) chosen for it's precision and software simplicity plus the 4th order 50/60 Hz notch filter and low power consumption ADM1411 digital isolator chosen for the microprocessor to A/D converter interface. Also provides 3.3v to 5.0v level shift 12v input, isolated dual output (+/- 15v) DC/DC converter - VInfinity chosen for it's lower noise figure. LM337 / LM317 chose for adjustable linear regulators for the sensitive analog section MAX3222 for RS232 interface MAX3483 for RS485 MSP430F169 12-bit D/A converter chosen for analog output (temporary) option. With over-sampling and proper filtering it is believed we can get 15 to 16 bit accuracy. 2nd order Sallen-Key filter designed for approx. 50 Hz cutoff frequency. http://sim.okawa-denshi.jp/en/OPstool.php&nbsp;&nbsp;filter calculator was used and standard resistor (39K) and cap parts (0.1uf) selected. Cutoff Freq: 57.7 Hz R1=39K, R2=39K, C1=0.1uF, C2=0.05uF Q = 0.707 29M2f08 Micron NAND flash AD8609 Quad OpAmp for bipolor to unipolar translation, Reference buffering, Analog output and DAC filtering, and 2 channel vibration sensor input AD680 2.5000v Reference ##### Assembly Notes Controller Board Conformal coating JTAG connector Front End board connector Front End Board pH electrode attachement Pressure Housing Assembly Board mounting 3 wire connection White = Ground Green = +12v Black = pH Analog out or TX data Digital out Testing 55 O-Ring grease Aqua Lube Testing Note: the DGH can (see Dale Graves for info) converts the pH sensor from analog to digital and sends the digital by serial bit stream (RS232 or RS485). Oct. 3rd Test Pin 1 - white - GND Pin 2 - black - pH Pin 3 - green - +12v With 12.0v applied, the Seabird pH instrument pulls 12mA (as indicated by the Tenma power supply) HRPH For FOCE 2 Development boards, 6 deployment boards Serial protocol definition / implementation / test Request / Response protocol Sampling configurability study DGH protocol, decide between DGH and proprietary implement test harness (C program) implement and test the protocol document the protocol Finish Flash data logging driver circular upload erase (clear) 6 conductor, 4 Wire connector modification for the digital interface RS232 (RX,TX), GND, +12v Build Test and Qualify 6 new probes Housing selection from Peter Walz and Ed Peltzer New o-rings Pressure test (Nalgene bottle or attaced to the ROV during a dive) Calibration Modify the Calibration software if needed. Run Ed's calibratinion routine FOCE - Deep Ocean Wireless Power and Data Transfer (Discussed with Bill Kirkwood / Mike Risi) Inductive Power Transmitter and Receiver (2W for HRpH, 10W to cover other instruments like CTD) 38400 Baud serial data comms (RF or Optical) 6 conductor, 5 wire connector pinout Software PUCK enabled Packaging and Mechanical #### Protocol Definition The DGH D5000 series products use a simple command and response protocol. A module must be interrogated by a host computer to obtain data. A module will never initiate a command sequence in order to prevent communications collisions. A command is initiated with a command prompt, may be a dollar sign ($) or pound sign (#). Following the prompt a single address character must be transmitted. Each module on a communications bus must be setup with a unique address. The address is followed by a two character command. Every command is terminated with a carriage return. The module response begins with a response prompt, which is an asterisk followed by the necessary data. Every response is terminated with a carriage return. Linefeeds after the carriage return are user selectable. See the table below for typical commands and their respective response messages.
DGH D5000 Series Command Set
Command Definition | Typical Command | Typical Response |
RB - Read Block of Data | $1RB | *+00072.00 *+00200.00 *+00125.76 *+00347.95
RD - Read Data | $1RD | *+00072.00 | IMPLEMENTED
REA - Read Extended Address | $1REA | *3031
RID - Read Identification | $1RID | * BOILER
RMN - Read Display Minimum | $1RMN | *-00100.00
RMX - Read Display Maximum | $1RMX | *+00025.00
RS - Read Setup | $1RS | *31020142
RZ - Read Zero Register | $1RZ | *+00000.00
WE - Write Enable | $1WE | *
Write Protected Commands |  |
CZ - Clear Zero Register | $1CZ | *
ID - Write Identification | $1ID BOILER | *
RR - Remote Reset | $1RR | *
SU - Setup Message | $1SU31070142 | *
TS - Trim Span | $1TS+00745.00 | *
TZ - Trim Zero | $1TZ+00000.00 | *
WEA - Write Extended Address | $1WEA3031 | *
WMN - Set Minimum Display Value | $1WMN-00200.00 | *
WMX - Set Maximum Display Value | $1WMX+00760.00 | * |

Document generated by Confluence on Feb 03, 2026 16:22