Download: TruDO Sensor Troubleshooting Guide
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1. 4 Finesse Application Note TruDO Finesse LLC 3350 Scott Boulevard 1 Santa Clara CA 95054 9351 Irvine Boulevard Irvine CA 92618 www finesse inc com P01 Improved Process Measurement amp Control Truba amp TruDO Sensor Troubleshooting Guide This application note first discusses general limitations of polarographic dissolved oxygen DO sen sors Guidelines for troubleshooting common errors encountered when operating TruDO sensors are then given Such common errors include no output current zero current being too high slow response times and calibration currents in air being too high or too low Limitations of Traditional Dissolved Oxygen DO sensors Recall that TruDO sensors are Clark cell sen sors constructed using Ag AgCl and platinum electrodes and using KCI electrolyte so that the detailed oxygen reduction reaction at each electrode is Cathode Reduction O 2H 0 4e 40H 1 Anode Oxidation 4Ag 4Cl 4e AgCl The overall reaction that occurs is shown in equation 2 Overall 4Ag O 2H 0 4CF 4AgCl 40H 2 From equation 1 it can be seen that every time oxygen is reduced 4 electrons are generated and the cathode is depolarized These electrons pro duce a current that is related to the partial pressure of the dissolved oxygen The current is measured using a transmitter that then reports the oxygen value either in saturation or ppm units A2. zero the transmitter is fine Reconnect the probe and sponse time specifications are valid only for process temperatures around 25 C Media contains a lot of carbon dioxide May require a different DO sensor Membrane is broken or the mechanical ten sion on the membrane has become too low Replace the membrane cap see app note TruDO Sensor Field Service Membrane is covered with dirt or is biofouled The dirty layer may contain living organ isms that consume oxygen so that the sensor response time is artificially slowed First try to wipe off the dirt layer with a wetted soft towel e g Kimwipe If the dirt layer cannot be removed after being wiped several times because of excessive contamination replace the membrane cap with a new one see app note TruDO Sensor Field Service Current in air is too high high probability reasons cable to reproduce the high reading Then disconnect the cable from the probe The transmitter display should again read zero If it does try a different sensor If it does not the cable is faulty and should be replaced Cathode glass body is broken Replace the sensor Membrane is broken or the mechanical ten sion on membrane has become too low Re place the membrane cap see app note Tru DO Sensor Field Service Membrane is damaged in the inside In this case the cathode and the steel mesh are in contact producing a short
3. pins to see if they are covered with liquid and or dirt If they are clean connect the sen sor and transmitter using a different cable to see if the cable is the root cause If it is not try connecting a different sensor to see if the sensor is the root cause Membrane is broken or the mechanical tension on membrane has become to low Unscrew the membrane cap and exchange the elec trolyte solution see app note TruDO Sensor Field Service Clean the glass body of the electrode with hot water dry with a Kimwipe rinse with electrolyte and dry again Replace the membrane cap Check that the resistance between the anode and cathode is low m Anode is contaminated with cyanide sulfide or iodide the polarogram is shifted to more negative values With the sensor exposed to air check to see if the sensor output current is higher than expected for air contacts with a voltmeter set to the diode setting There may be a problem in the cable wiring or the cable connector Membrane body is not filled with sufficient electrolyte 1 5 ml Unscrew the membrane cap and check the electrolyte level If there is less than 1 5 ml of electrolyte refill the elec trolyte level Zero current is too high out of range high probability reasons Liquid inside the sensor connector head In spect the female jack of the cable and the male jack of the sensor connector to see if there are any liquid droplets on either If t
4. circuit Check the anode to cathode resistance using an ohm meter If the resistance is zero replace both the electrolyte solution and the membrane cap see app note TruDO Sensor Field Service 4 Finesse Improved Process Measurement amp Control Truba amp Application Note TruDO Finesse LLC 3350 Scott Boulevard 1 Santa Clara CA 95054 9351 Irvine Boulevard Irvine CA 92618 www finesse inc com P05 Output current in air is too high low probability reasons for bioprocess applications m Anode has become contaminated by cya nide sulfide or iodide sensor response has been shifted to more negative values Check if sensor responds to nitrogen If the sensor does not respond please return the sensor to Finesse for maintenance Output current in air is too low m Membrane broken and or electrolyte contami nated Unscrew the membrane cap and check that electrolyte is not contaminated it should look clear and transparent with no precipita tion If it is exchange it see app note TruDO Sensor Field Service If the electrolyte ap pears fine check the membrane by holding it up to a bright light and examining it from the in side of the cap If the membrane appears non uniform or has holes or micro tears replace it m Membrane is covered with dirt or is biofouled The dirty layer may contain living organisms that consume oxygen so that the sensor read ing is arti
5. e sensor is disconnected from the transmitter or polarizer the voltage sup ply is cut off and the sensor will depolarize After the sensor and voltage source are re connected the user must wait for the probe to become polarized or more precisely for the current loop to become stabilized For TruDO sensors this waiting period is approximately 30 minutes Any measurement taken before this polarization period has elapsed will be result in incorrect higher in value oxygen readings 5 Membrane mediated effects Typical DO sensor membranes are generally made from Teflon and often suffer from cold flow deformation as a result of the stress caused by being mounted to a rigid stainless steel shaft The change in tension changes the effective thickness of the membrane Because the DO output current is an inverse function of the thickness of the membrane any stretching or thinning of the membrane will produce an effective increase or upward drift in the DO reading Conversely membrane clogging will Troubleshooting create a downward drift in the DO reading Fur thermore if the sensor is sterilized numerous times e g autoclaved the membrane can un dergo significant physical deformation some times even to the point of rupture or failure TruDO sensors have a thicker steel mesh reinforced membrane which is more robust and reliable than other DO sensors Please refer to the application note entitled T
6. ficially low First try to wipe off the dirt layer with a wetted soft towel e g Kimwipe If the dirt layer cannot be removed after be ing wiped several times because of excessive A short circuit has been created between the anode and the stainless steel shaft Unscrew the membrane cap Check that the helix of the anode is sitting firmly around the glass body of the internal electrode assembly Do not apply any mechanical force to the electrode during this pro cess If the anode is attached please return sen sor to Finesse for maintenance If the anode is no longer attached maintenance is not possible contamination exchange the membrane cap for a new one see app note TruDO Sensor Field Service Cathode has lost all activity This problem can arise if the membrane ruptures and the sample media has penetrated through the membrane to the cathode First check the cathode to see if there are any deposits on the tip of the glass body If the glass body is contaminated polish the cathode see app note TruDO Sensor Field Service Polarization potential is incorrect First make sure that a voltage is being applied Then us ing a voltmeter check that the polarity plus and minus sign and magnitude of the applied voltage are correct
7. for the membrane tip such that air bubbles cannot become trapped Recall that the response of the sensor is slower when measuring in an unstirred solution because the rate of oxygen diffusion is slower in liquids Note that the time response specification stated in the TruDO datasheet is only valid for measurements in the gas phase i e calibration For measurements in the gas phase a drop let of water is sitting on the top of the sensor In this case the sensor response to nitrogen is slow Dry the sensor with a soft paper towel e g Kimwipe Process temperature is too low The response of TruDO sensor slows m Liquid inside the sensor connector head In spect the female jack of the cable and the male jack of the sensor connector to see if there are any liquid droplets on either If there is liquid dry the cable and connector Note that it is very difficult to dry restore a VP female connector however one can attempt to clean the inside with a swab and rubbing alcohol Once the connectors are dry reconnect the probe and cable and using a voltmeter check that the an ode to cathode resistance is low Then check that the sensor responds to nitrogen Finally check that the current goes to zero if the sen sor is disconnected from the transmitter Short circuit between the anode and cathode at the cable or transmitter Disconnect the cable from the transmitter and check that the current display shows zero If it is
8. here is liquid dry the cable and connector Note that it is very difficult to dry restore a VP female connec tor however attempt to clean the inside with a swab and rubbing alcohol Once the connec tors are dry reconnect the probe and cable and first check that the anode to cathode resis tance is low using an ohmmeter Then check that the sensor responds to nitrogen Finally check that the current goes to zero if the sen sor is disconnected from the transmitter Polarization time too short Apply the polarizer and wait at least 15 minutes The test medium is not completely oxygen free Use oxygen free nitrogen Zero current is too high out of range lower probability reasons Sample media contains gaseous species that can be reduced or oxidized at the cathode One indicator of this problem is a positive zero current reading l Finesse TruDo amp amp Improved Process Measurement amp Control Application Note Response time is too slow m For measurements in liquid media an air down with decreasing temperature The re TruDO Finesse LLC 3350 Scott Boulevard 1 Santa Clara CA 95054 9351 Irvine Boulevard Irvine CA 92618 www finesse inc com P04 bubble is sitting in front of the membrane In this case the sensor output current will pro duce a constant reading that is close to the value in air Change the position of the sen sor in the process and find a location
9. on in the sensor must be changed TruDO sensors allow the user to easily add electrolyte solution see application note TruDO Sensor Field Service which makes reactivation very simple and fast Finesse also offers fast turn around sensor servic es including probe rebuilding calibration and certification for TruDO sensors see www finesse inc com services Depletion of Chloride The net reaction also consumes chloride ions CF Over time the chloride ions will be fully consumed and this results in the deterioration of the electrolyte Eventually the chloride ion depletion becomes so severe that the electro lyte solution in the sensor must be changed TruDO sensors allow the user to easily add electrolyte solution see application note TruDO Sensor Field Service which makes reactivation both simple and fast Finesse also offers fast turn around sensor servic es including probe rebuilding calibration and certification for TruDO sensors see www finesse inc com services 4 Finesse Application Note TruDO Finesse LLC 3350 Scott Boulevard 1 Santa Clara CA 95054 9351 Irvine Boulevard Irvine CA 92618 www finesse inc com P02 Improved Process Measurement amp Control Truba amp 4 Polarization Time Polarographic DO sensors require approxi mately 675 mV to be applied to the sen sor in order to initiate the electrochemi cal reaction and produce a measurement When th
10. operating input range Finesse can provide a list of transmitters that are compatible with our TruDO sensors In the user manual for most transmitters a wir ing table with color codes is provided for spe cific DO sensors If your TruDO sensor differs from the DO sensors described in the manual be aware that the color code may no longer be valid Please contact your local Finesse repre sentative for technical assistance l Finesse Improved Process Measurement amp Control Truba amp Application Note No current output at all E Wiring of the sensor to the transmitter is incor Faulty cable Check the cable for any open TruDO Finesse LLC 3350 Scott Boulevard 1 Santa Clara CA 95054 9351 Irvine Boulevard Irvine CA 92618 www finesse inc com P03 rect First check the anode and cathode con nections Some transmitters can also handle three electrode oxygen sensors in this case a jumper must be inserted between the refer ence electrode and anode connectors Faulty sensor connector Check to see if sen sor connector pins are covered with liquid and or dirt if they are replace the sensor If the connector appears to be clean first try a new cable If the new cable doesn t help try a different sensor altogether m Cable problem Disconnect the sensor from the cable to make sure that the transmitter reading goes to zero Connector head Visually inspect the connec tor
11. ruDO Sensor Reliability for more details There fore TruDO sensors show greater immunity to drift and have longer operating lifetimes However as no sensor lasts forever TruDO sensors are designed so that the membrane is easily field replaceable by the user see application note TruDO Sensor Field Ser vice Finesse offers fast turn around sen sor services including probe rebuilding calibration and certification for TruDO sen sors see www finesse inc com services Environmental effects Changes in the flow rate of liquid across the DO sensor membrane such as those pro duced by agitation and or stirring in a biopro cess vessel can cause variations in the out put reading Additional effects on the sensor reading can be caused by vibrations Setting up the transmitter other than a TruTransmitter m Program the transmitter so that the absolute current output of the TruDO sensor is dis played Messages such as calibration not possible are difficult to interpret if the actual sensor output current is unknown Most transmitters assume that the calibra tion was performed in air or in air saturated with water Therefore be sure to perform the calibration in air that is correctly satu rated with water Most transmitters accept only a limited range for the TruDO output currents in air For non Finesse transmitters please verify that the TruDO sensor output is compatible with the transmitter s
12. s with any other type of electrochemical sen sor polarographic dissolved oxygen sensors are not completely trouble free As the sensor ages it may begin to drift or produce more noise in its reading Such degradation in sensor perfor mance can be caused by a wide range of factors the common of which are listed below 1 Anode isolation The net result of the electro chemical reaction in a DO sensor is silver chloride AgCl This AgCI product slowly coats the anode Once the anode is completely coated with AgCl the reaction stops because there is no more sil ver available to carry out the reaction and the DO sensor can no longer respond to oxygen The sensor can be reactivated by cleaning the anode to remove the AgCl deposit This process requires some care and can be a time consuming procedure TruDO sen sors allow the user to completely replace the electrode assembly see application note TruDO Sensor Field Service which makes reactivation both simple and fast Finesse also offers fast turn around sensor services including probe rebuilding cali bration and certification for TruDO sensors see www finesse inc com services Zero Shift The electrochemical reaction also produc es OH ions which change the pH value of the electrolyte and move it into the alkaline range This change in pH causes a shift in the sensor zero point over time Eventually the shift becomes so significant that the electro lyte soluti
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