YSI 550 DO User manual

YSI 550
Handheld
Dissolved
Oxygen and
Temperature
System
Operations
Manual

i
CONTENTS
SECTION 1 GENERAL DESCRIPTION......................................................................................................... 3
1.1 KEYPAD LAYOUT ...................................................................................................................................... 3
SECTION 2 PREPARING THE YSI 550 ......................................................................................................... 4
2.1 UNPACKING ............................................................................................................................................... 4
2.2 WARRANTY CARD..................................................................................................................................... 4
2.3 BATTERIES................................................................................................................................................. 4
2.4 TURNING THE INSTRUMENT ON................................................................................................................ 5
2.5 CHANGING THE TEMPERATURE UNITS...................................................................................................... 5
2.6 CALIBRATION/STORAGE CHAMBER........................................................................................................... 5
2.7 HAND STRAP ............................................................................................................................................. 6
2.8 THE INSTRUMENT CASE ............................................................................................................................ 6
SECTION 3 PREPARING THE PROBE......................................................................................................... 7
3.1 DESCRIPTION ............................................................................................................................................. 7
3.2 CHOOSING THE RIGHT MEMBRANE .......................................................................................................... 7
3.3 PROBE PREPARATION................................................................................................................................7
3.4 MEMBRANE CAP INSTALLATION ............................................................................................................... 7
3.5 PROBE OPERATION AND PRECAUTIONS..................................................................................................... 8
3.6 PROBE MAINTENANCE .............................................................................................................................. 9
3.7 HOW TO REPLACE THE DO PROBE TIP .................................................................................................... 10
SECTION 4 CALIBRATION.......................................................................................................................... 12
4.1 BEFORE YOU CALIBRATE........................................................................................................................ 12
4.2 THE CALIBRATION PROCESS ................................................................................................................... 12
SECTION 5 PRINCIPLES OF OPERATION...............................................................................................15
SECTION 6 TROUBLESHOOTING ............................................................................................................. 17
SECTION 7 WARRANTY AND REPAIR..................................................................................................... 19
APPENDIX A SPECIFICATIONS ................................................................................................................ 24
APPENDIX B REQUIRED NOTICE.............................................................................................................. 25
APPENDIX C ACCESSORIES/REPLACEMENT PARTS.........................................................................26
APPENDIX D SOLUBILITY TABLE........................................................................................................... 27
APPENDIX E CONVERSION CHART........................................................................................................29

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YSI Incorporated YSI 550 DO Handheld Instrument 3
SECTION 1 GENERAL DESCRIPTION
The YSI 550 Handheld Dissolved Oxygen Instrument is a rugged, micro-processor based, digital
instrument with a field-replaceable YSI dissolved oxygen probe. The YSI 550 DO Instrument is
impact-resistant and waterproof.
The YSI 550 DO Instrument is designed for field use and is available with cable lengths of 12 or 25
feet. The body of the probe has been manufactured with stainless steel to add rugged durability and
sinking weight. The large Liquid Crystal Display (LCD) is easy to read and is equipped with a
back-light for use in dark or poorly lighted areas.
The YSI 550 DO Instrument’s micro-processor allows the system to be easily calibrated with the
press of a few keys. Additionally, the micro-processor performs a self-diagnostic routine each time
the instrument is turned on. The self-diagnostic routine provides you with useful information about
the function of the instrument circuitry and the quality of the readings you obtain. For a list of these
diagnostic features, see Section 6, Troubleshooting.
The system displays temperature in either oC or oF by pressing the MODE key and the down arrow
at the same time. The instrument also displays dissolved oxygen in either mg/L (milligrams per
liter) or % air saturation. The system requires only a single calibration regardless of which
dissolved oxygen display you use, however, it will calibrate in mg/L or % saturation. You can
switch back and forth from % air saturation to mg/L with a single push of the MODE key.
A detachable calibration chamber is mounted to the back of the instrument. A small sponge in the
chamber can be moistened to provide a water saturated air environment which is ideal for air
calibration. This chamber is also designed for transporting and storing the probe. When the probe is
stored in the chamber, the moist environment will prolong effective membrane performance and
probe life.
The YSI 550 DO Instrument is powered by 4 C-size alkaline batteries. A new set of alkaline
batteries will provide approximately 2000 hours of continuous operation. When batteries need to be
replaced, the LCD will display a "LO BAT" message. When the LO bat message appears, the
instrument will have approximately 50 hours of life left, depending on how much the back light is
used. If the backlight is used often, the batteries will be depleted quicker.
The YSI 550 DO Instrument case is waterproof with an IP-67 rating. The YSI 550 DO Instrument is
100% corrosion proof and can be operated in a wet environment without damage to the instrument.
1.1 KEYPAD LAYOUT
The YSI 550 DO keypad consists of six keys. See Figure 1.
There are four function keys, and up and down arrow keys. The
top left key that has a green circle and line is the ON/OFF key.
The top right key is the backlight. The bottom left is the Mode
key, and the bottom right is the Enter key.
Figure 1

YSI Incorporated YSI 550 DO Handheld Instrument 4
SECTION 2 PREPARING THE YSI 550
2.1 UNPACKING
When you unpack your new YSI 550 DO Handheld Instrument for the first time, check the packing
list to make sure you have properly received everything. If there is anything missing or damaged,
call the dealer from whom you purchased the YSI 550. If you do not know which authorized dealer
sold the system to you, call YSI Customer Service at 800-765-4974 or 937-767-7241.
2.2 WARRANTY CARD
Before you do anything else, please complete the Warranty Card and return it to YSI Incorporated.
Your purchase of this quality instrument will then be recorded in YSI’s customer database. Once
your purchase is recorded, you will receive prompt, efficient service in the event any part of your
YSI 550 DO Instrument should ever need repair.
2.3 BATTERIES
There are a few things you must do to prepare your YSI 550 DO instrument for use. First, locate the
4 C-size alkaline batteries that were included with your instrument. See Figure 2.
NOTE: It is very important that the batteries be installed ONLY as illustrated. The instrument will
not function if the batteries are installed incorrectly.
4 C-cell batteries
Figure 2

YSI Incorporated YSI 550 DO Handheld Instrument 5
2.4 TURNING THE INSTRUMENT ON
Turn the instrument on by pressing and releasing the ON/OFF button on the front of the
instrument. The instrument will activate all segments of the display for a few seconds, which will
be followed by a self test procedure that will last for several more seconds. During this power on
self test sequence, the instrument’s microprocessor is verifying that the instrument is working. If the
instrument were to detect a problem, a continuous error message would be displayed. See Section
6, Troubleshooting for error messages. If the instrument does not operate, consult Section 6,
Troubleshooting.
Because the probe has not yet been prepared, the data displayed on the screen is not accurate.
You may also want to take the instrument into a dark room and, with the instrument turned ON,
press the LIGHT key. The light key is at the top right of the keypad. The instrument back-light
should illuminate the LCD so that the display can be easily read. Pushing the LIGHT key again
will turn the back light off. The light will power off automatically after two minutes of non-use.
2.5 CHANGING THE TEMPERATURE UNITS
The YSI 550 DO Instrument can display the temperature units in either Fahrenheit or Celsius. To
change the units that are displayed, turn the instrument on. Pressing both the down arrow key and
the mode key at the same time will switch the units between C or F.
2.6 CALIBRATION/STORAGE CHAMBER
The YSI 550 DO Instrument has a convenient calibration/storage chamber that can be attached to
the instrument’s back. See Figure 3. The calibration chamber can be used from either side of the
instrument, by moving the rubber stopper to either end. This chamber provides an ideal storage area
for the probe during transport and extended non-use. If you look into the chamber, you should
notice a small round sponge in the bottom. Carefully put 3 to 6 drops of clean water into the sponge.
Turn the instrument over and allow any excess water to drain out of the chamber. The wet sponge
creates a 100% water saturated air environment for the probe that is ideal for dissolved oxygen
calibration.
Hand Strap Figure 3

YSI Incorporated YSI 550 DO Handheld Instrument 6
The YSI 550 DO Instrument’s storage chamber can be conveniently used from either side of the
instrument. To attach the rubber stopper to the calibration chamber, first remove the chamber from
the instrument by unscrewing the two screws. After the chamber is free from the instrument, the
rubber stopper can be removed from the chamber by pulling the “tail” free of the small hole on the
chamber. The stopper can then be re-attached to the storage chamber by threading the “tail” through
the small hole on the chamber. The storage chamber can then be re-attached to the instrument using
the two screws. See Figure 4.
2.7 HAND STRAP
The hand strap is designed to allow comfortable operation of the YSI 550 DO Instrument with
minimum effort. If the hand strap is adjusted correctly, it is unlikely that the instrument will be
easily dropped or bumped from your hand. The YSI 550 DO Instrument’s hand strap can be
conveniently used from either side of the instrument.
To switch the hand strap from one side to the other, simply pull the two velcro strips apart. Pull the
strap free of the upper and lower hooks, and then feed the strap through the hooks on the other side
of the instrument, and adjust the strap length so that your hand is snugly held in place. Press the
two velcro strips back together.
2.8 THE INSTRUMENT CASE
The instrument case is sealed at the factory and is not to be opened, except by authorized service
technicians.
Figure 4
Stopper
Tail
Small hole for the
stopper’s tail
Caution: Do not attempt to separate the two halves of the instrument case as this
may damage the instrument, break the waterproof seal, and will void the
manufacturer's warrant
y
.

YSI Incorporated YSI 550 DO Handheld Instrument 7
SECTION 3 PREPARING THE PROBE
3.1 DESCRIPTION
The YSI 550 dissolved oxygen probe is a replaceable, polarographic sensor designed specifically
for the YSI 550 Handheld Dissolved Oxygen System. Probe cables are available in lengths of 12 or
25 feet. See Figure 5.
3.2 CHOOSING THE RIGHT MEMBRANE
The YSI 5906 Standard Membrane Kit is supplied with the YSI 550 DO Instrument. This kit
contains six cap membranes and a bottle of electrolyte solution. YSI recommends the 5906
membranes for all applications.
3.3 PROBE PREPARATION
The YSI 550 DO probe is shipped dry. Before using the YSI 550, the protective membrane cap
on the probe tip must be removed, the new membrane cap must be filled with electrolyte
solution and installed. Follow the instructions below to replace the membrane cap.
3.4 MEMBRANE CAP INSTALLATION
To install a new membrane cap on your YSI 550 dissolved oxygen probe:
1. Unscrew and remove the probe sensor guard.
Figure 5

YSI Incorporated YSI 550 DO Handheld Instrument 8
2. Unscrew and remove the old membrane cap.
3. Thoroughly rinse the sensor tip with distilled or DI water.
4. Prepare the electrolyte according to the directions on the KCl solution bottle.
5. Hold the membrane cap and fill it at least 1/2 full with the electrolyte solution.
6. Screw the membrane cap onto the probe moderately tight. A small amount of electrolyte should
overflow.
7. Screw the probe sensor guard on moderately tight.
CAUTION: Do not touch the membrane surface.
3.5 PROBE OPERATION AND PRECAUTIONS
NOTE: The YSI 550DO Instrument should not be used in a purpose other than that specified by
YSI Incorporated. See Section 7, Warranty and Repair for details.
1. It is important to recognize that oxygen dissolved in the sample is consumed during probe
operation. It is therefore essential that the sample be continuously stirred at the sensor tip. If
stagnation occurs, your readings will be artificially low. Stirring may be accomplished by
mechanically moving the sample around the probe tip, or by rapidly moving the probe through
the sample. The rate of stirring should be at least 1 foot per second.
2. Membrane life depends on usage. Membranes will last a long time if installed properly and
treated with care. Erratic readings are a result of loose, wrinkled, damaged, or fouled
membranes, or from large (more than 1/8" diameter) bubbles in the electrolyte reservoir. If
erratic readings or evidence of membrane damage occurs, you should replace the membrane
and the electrolyte solution. The average replacement interval is two to four weeks.
3. If the membrane is coated with oxygen consuming (e.g. bacteria) or oxygen producing
organisms (e.g. algae), erroneous readings may occur.
4. Chlorine, sulfur dioxide, nitric oxide, and nitrous oxide can affect readings by behaving like
oxygen at the probe. If you suspect erroneous readings, it may be necessary to determine if
these gases are the cause.
5. Avoid any environment that contains substances that may attack the probe materials. Some of
these substances are concentrated acids, caustics, and strong solvents. The probe materials that
come in contact with the sample include FEP Teflon, acrylic plastic, EPR rubber, stainless
steel, epoxy, polyetherimide and the PVC cable covering.
6. For correct probe operation, the gold cathode must always be bright. If it is tarnished (which
can result from contact with certain gases), or plated with silver (which can result from
extended use with a loose or wrinkled membrane), the gold surface must be restored. To
restore the cathode, you may either return the instrument to the factory or clean it using the
sanding disk in the YSI 5238 Probe Reconditioning Kit. Never use chemicals or abrasives not
supplied with this kit.

YSI Incorporated YSI 550 DO Handheld Instrument 9
7. It is also possible for the silver anode, which is the entire silver body of the probe, to become
contaminated, which will prevent successful calibration. To clean the anode, follow the Anode
cleaning instructions in the following section. Next, rinse the sensor tip and Electrolyte
reservoir with deionized water, add new Electrolyte solution, and install a new membrane and
O-ring. Turn the instrument on and allow the system to stabilize for at least 30 minutes. If,
after several hours, you are still unable to calibrate, return the YSI 550 DO Instrument to a YSI
authorized service center for service, See Section 7, Warranty and Repair for more
information.
8. To keep the electrolyte from drying out, store the probe in the calibration/storage chamber with
the wet sponge.
3.6 PROBE MAINTENANCE
ELECTRODE CLEANING
It is always a good practice to keep the probe in good operating condition.
SILVER ANODE
After extended use, a thick layer of AgCl builds up on the silver anode reducing the sensitivity of
the sensor. The anode must be cleaned to remove this layer and restore proper performance. The
cleaning can be chemical or mechanical:
•Chemical cleaning: Remove the membrane cap and soak the entire anode section in a
14% ammonium hydroxide solution for 2 to 3 minutes, followed by a thorough rinsing
with distilled or deionized water. The anode should then be thoroughly wiped with a wet
paper towel to remove the residual layer from the anode.
•Mechanical cleaning: Sand off the dark layer from the silver anode with 400 grit wet/dry
sandpaper. Wrap the wet sandpaper around the anode and twist the probe. Rinse the anode
with clean water after sanding, followed by wiping thoroughly with a wet paper towel.
GOLD CATHODE
For correct probe operation, the gold cathode must be textured properly. It can become tarnished or
plated with silver after extended use. The gold cathode can be cleaned by using the adhesive backed
sanding disc and tool provided in the YSI 5238 Probe Reconditioning Kit.
Using the sanding paper provided in the YSI 5238 Probe Reconditioning Kit, wet sand the gold
with a twisting motion about 3 times or until all silver deposits are removed and the gold appears to
have a matte finish. If the cathode remains tarnished, return the probe for service. Wipe the gold
cathode thoroughly with a wet paper towel before putting on a new membrane cap.

YSI Incorporated YSI 550 DO Handheld Instrument 10
3.7 HOW TO REPLACE THE DO PROBE TIP
The YSI 559 Replaceable DO Module Kit includes:
Quantity Item
1 Instruction Sheet
1 5906 Membrane Kit
1 Sensor, DO Module
1 Set Screw 4-40x 1/2 long
1 Hex key Wrench, .050”
DO SENSOR REPLACEMENT
Along with the YSI 559 dissolved oxygen module, a YSI 5906 membrane kit, the set screw that
holds the sensor in place, YSI supplies a hex key wrench to help install the probe, and an instruction
sheet. See Figure 6.
1. Remove probe guard.
2. IMPORTANT: Thoroughly dry the sensor so that no water enters the probe port when the
sensor is removed.
3. Insert the long end of the hex key wrench into the small hole in the side of the DO sensor
module (see figure 1). Turn the wrench counter clockwise and remove the screw. (You do not
have to remove the screw all the way to release the sensor.)
4. Pull the DO sensor out of the module. The DO sensor is keyed, or has a flat side, so that it can
not be removed improperly.
5. Insert the new probe. Make sure that the inside of the module, and the o-ring of the sensor are
clean, with no contaminants, such as grease, dirt, or hair. The DO sensor is keyed, or has a flat
side, so that it can not be installed improperly.
6a. IF you did not remove the screw all the way in Step 3: Use the hex key wrench to tighten the
screw in properly, making sure that the screw does not stick out on either side of the DO sensor
module.
6b. IF you removed the screw all the way in Step 3: Insert the set screw into the small hole in the
side of the DO sensor module, and turn clockwise to rethread.

YSI Incorporated YSI 550 DO Handheld Instrument 11
7. The YSI 559 DO module is shipped dry. Before using the YSI 550 with a new YSI 559
sensor, the protective membrane on the probe tip must be removed, the membrane cap
must be filled with electrolyte solution and a new membrane must be installed. Follow the
instructions below to install the electrolyte solution and membrane.
CAUTION: Make sure that you do not cross-thread the screw. Use the hex key wrench to
tighten the screw in properly, making sure that the screw does not stick out on either side of
the cable end. The probe guard will not thread on properly and damage may result if the
screw is allowed to stick out on either side of the cable end.
Probe Guard
DO Sensor
Cable End
Set Screw
Hex Key
Wrench
Figure 6

YSI Incorporated YSI 550 DO Handheld Instrument 12
SECTION 4 CALIBRATION
Dissolved oxygen calibration must be done in an environment with a known oxygen content.
Since the amount of oxygen in the atmosphere is known, it makes an excellent environment for
calibration (at 100% relative humidity). The calibration/storage chamber contains a sponge to
create a 100% water saturated air environment.
4.1 BEFORE YOU CALIBRATE
Before you calibrate the YSI 550 DO Instrument complete the procedures discussed in Section 2,
Preparing the YSI 550 and Section 3, Preparing the Probe. You can calibrate the YSI 550 DO
Instrument in either mg/L or % saturation. In the section below, you will find instructions on how to
calibrate and change between these modes.
Note: While testing to EN-61000-4-6, Conducted RF Immunity, per Table A.1 of EN61326,
Electrical Equipment for Measurement, Control and Laboratory Use, the YSI 550 exhibited an
ERROR 8 message from 8.6 MHz 22.8 MHz at induced RF voltages of 3-Volts to 1-Volt RMS on
the 25-foot probe cable. If you observe this interference please relocate the probe-cable away from
heavy industrial equipment power and control cables or communications equipment cables which
may be causing the interference.
To accurately calibrate the YSI 550, you will need to know the following information:
!The approximate salinity of the water you will be analyzing. Fresh water has a salinity of
approximately zero. Seawater has a salinity of approximately 35 parts per thousand (ppt). If you
are uncertain what the salinity of the sample water, use a YSI 30 Salinity-Conductivity-
Temperature instrument to determine salinity.
!IF you are going to calibrate in % saturation mode, you need to know the approximate altitude
of the region where you are located.
4.2 THE CALIBRATION PROCESS
1. Ensure that the sponge inside the instrument's calibration chamber is moist. Insert the probe
into the calibration chamber.
2. Turn the instrument on by pressing the ON/OFF button on the front of the instrument. Wait
for the dissolved oxygen and temperature readings to stabilize (usually 15 minutes is required
after turning the instrument on).
3. To enter the calibration menu, use two fingers to press and release both the UP ARROW and
DOWN ARROW keys at the same time.

YSI Incorporated YSI 550 DO Handheld Instrument 13
TO CALIBRATE IN MG/L:
To switch between mg/L and percent saturation to be displayed as the oxygen readings units, press
the Mode key. Either a “mg/L” or “%” will be displayed on the right side of the screen.
4. Make sure that the DO reading (large display) is stable, then press the ENTER button. The
YSI 550 DO instrument should now display CAL in the lower left of the display, the
calibration value should be displayed in the lower right of the display and the current DO
reading (before calibration) should be on the main display.
5. Make sure that the DO reading (large display) is stable, then press the ENTER button. The
LCD will prompt you to enter the approximate salinity of the water you are about to analyze.
You can enter any number from 0 to 40 parts per thousand (PPT) of salinity. Use the arrow
keys to increase or decrease the salinity setting. When the correct salinity appears on the LCD
(zero for fresh water), press the ENTER key. The instrument will return to normal operation.
Once the calibration process is complete, the only keys that will remain operational are the MODE
key, the LIGHT key and the ON/OFF key. You can move back and forth from reading dissolved
oxygen in the mg/L mode or the % air saturation mode by pressing the MODE key. If you are
working in a dark area and have difficulty reading the LCD, press the LIGHT key to activate the
back-light of the YSI 550. The ON/OFF key turns the instrument on or off.
For best results:
!Each time the YSI 550 DO Instrument is turned off, re-calibrate before taking measurements.
!Calibrate at a temperature within ±10°C of the sample temperature.
TO CALIBRATE IN % SATURATION:
To switch between mg/L and percent saturation to be displayed as the oxygen readings units, press
the Mode key. Either a “mg/L” or “%” will be displayed on the right side of the screen.
4. IF you are calibrating in % saturation mode, the LCD will prompt you to enter the local
altitude in hundreds of feet. Use the arrow keys to increase or decrease the altitude.
EXAMPLE: Entering the number 12 here indicates 1200 feet.
6. When the proper altitude appears on the LCD, press the ENTER key. The YSI 550 DO
Instrument should now display CAL in the lower left of the display, the calibration value
should be displayed in the lower right of the display and the current DO reading (before
calibration) should be on the main display.
7. Make sure that the DO reading (large display) is stable, then press the ENTER button. The
LCD will prompt you to enter the approximate salinity of the water you are about to analyze.
You can enter any number from 0 to 40 parts per thousand (PPT) of salinity. Use the arrow

YSI Incorporated YSI 550 DO Handheld Instrument 14
keys to increase or decrease the salinity setting. When the correct salinity appears on the LCD
(zero for fresh water), press the ENTER key. The instrument will return to normal operation.
Once the calibration process is complete, the only keys that will remain operational are the MODE
key, the LIGHT key and the ON/OFF key. You can move back and forth from reading dissolved
oxygen in the mg/L mode or the % air saturation mode by pressing the MODE key. If you are
working in a dark area and have difficulty reading the LCD, press and hold the LIGHT key to
activate the back-light of the YSI 550. The ON/OFF key turns the instrument on or off.
For best results:
Each time the YSI 550 DO Instrument is turned off, re-calibrate before taking measurements.
!Calibrate at a temperature within ±10°C of the sample temperature.

YSI Incorporated YSI 550 DO Handheld Instrument 15
SECTION 5 PRINCIPLES OF OPERATION
The sensor consists of a silver body as the anode and a circular gold cathode embedded in the end.
In operation, this end of the sensor is filled with a solution of electrolyte containing a small amount
of surfactant to improve wetting action.
A thin semi-permeable membrane, stretched over the sensor, isolates the electrodes from the
environment, while allowing gases to enter. When a polarizing voltage is applied to the sensor
electrodes oxygen that has passed through the membrane reacts at the cathode causing a current to
flow.
The membrane passes oxygen at a rate proportional to the pressure difference across it. Since
oxygen is rapidly consumed at the cathode, it can be assumed that the oxygen pressure inside the
membrane is zero. Hence, the force causing the oxygen to diffuse through the membrane is
proportional to the partial pressure of oxygen outside the membrane. As the oxygen partial pressure
varies, so does the oxygen diffusion through the membrane. This causes the probe current to change
proportionally.
It is important to recognize that oxygen dissolved in the sample is consumed during the test. It is
therefore essential that the sample be continuously stirred at the sensor tip. If stagnation occurs,
your readings will be artificially low. Stirring may be accomplished by mechanically moving the
sample around the probe tip, or by rapidly moving the probe through the sample. The rate of stirring
should be at least 1 foot per second.
DISCUSSION OF MEASUREMENT ERRORS
There are three basic types of dissolved oxygen errors. Type 1 errors are related to limitations of
instrument design and tolerances of instrument components. These are primarily the instrument
linearity and the resistor tolerances. Type 2 errors are due to basic probe accuracy tolerances,
mainly background signal, probe linearity, and variations in membrane temperature coefficient.
Type 3 errors are related to the operator's ability to determine the conditions at the time of
calibration. If calibration is performed against more accurately known conditions, type 3 errors are
appropriately reduced.
TYPE 1 ERRORS
•Instrument linearity error: ±1% of full scale reading, or ±0.15 mg/L
•Component and circuitry error: ±0.05 mg/L
TYPE 2 ERRORS
•DO errors caused by temperature compensation for measurements at ±10°C from calibration
temperature: ±1% (0.08 mg/L at 25°C)
•DO errors caused by temperature measurement errors: A maximum ±0.2°C temperature error is
equal to ±0.5% (0.04mg/L at 25°C).

YSI Incorporated YSI 550 DO Handheld Instrument 16
TYPE 3 ERRORS
•Altitude
•Operator Error: A 1000 feet error in altitude (when calibrating) is equal to an error of
approximately 3.6% at the 10 mg/L level.
•Instrument Error: The maximum DO error caused by calibrating to altitude in increments
of 100 feet: ±0.18% (< 0.015 mg/L at 25°C)
•Humidity: Errors occur if calibration is performed at less than 100% humidity.
The worst possible case would be calibration at 0% humidity. The error varies with the calibration
temperature as follows:
Temperature Calibration Error at 0% humidity
0oC 0.09 mg/L
10oC 0.14 mg/L
20oC 0.21 mg/L
30oC 0.33 mg/L
40oC 0.50 mg/L
APPROXIMATING THE ERROR
It is unlikely that the actual error in any measurement will be the maximum possible error. A better
error approximation is obtained using a root mean squared (r.m.s.) calculation:
r.m.s. error = ±[1a2+ 1b2+ 2a2+ 2b2+ 3a2+ 3b2]½mg/L
NOTE: This sample calculation is for a near extreme set of conditions.

YSI Incorporated YSI 550 DO Handheld Instrument 17
SECTION 6 TROUBLESHOOTING
NOTE: An error displayed briefly during the first few seconds after turning the instrument
on does NOT indicate a problem.
SYMPTOM POSSIBLE CAUSE ACTION (Section)
1. Instrument will not turn on A. Low battery voltage
B. Batteries installed incorrectly
C. Instrument requires service
A. Replace batteries (2.3)
B. Check battery polarity. (2.3)
C. Return system for service (7)
2. Instrument will not calibrate A. Membrane is fouled or damaged
B. Probe anode is fouled or dark
C. Probe cathode is tarnished
D. System requires service
A. Replace membrane and Electrolyte (3)
B. Clean anode (3)
C. Clean cathode (3)
D. Return system for service (7)
3. Instrument "locks up" A. Instrument has received a shock
B. Batteries are low or damaged
C. System requires service
A. Remove batteries, wait 15 seconds for
reset, replace batteries. (2.3)
B. Replace batteries (2.3)
C. Return system for service (7)
4. Instrument readings are inaccurate A. Cal altitude is incorrect
B. Salinity setting is incorrect
C. Probe not in 100% water saturated
air during Cal procedure
D. Membrane fouled or damaged
E. Probe anode is fouled or dark
F. Probe cathode is tarnished
G. System requires service
A. Recalibrate w/correct value (4)
B. Moisten sponge and place in Cal
chamber w/ probe and Recal (2&4)
C. Replace membrane (3.4)
D. Clean anode (3)
E. Clean cathode (3)
F. Return system for service (7)
5. LCD displays "LO BAT"
or
Main display flashes “OFF”
A. Batteries are low or damaged A. Replace batteries (2.3)
6. Main display reads "Er 0" A. Calibration current out of range
B. Instrument's self-test detects
improper probe voltage during
calibration
A. Replace membrane and Electrolyte (3.4)
B. Clean anode (3)
C. Clean cathode (3)
D. Return system for service (7)
7. Main display reads "Er 1" A. Instrument's self-test detects a
variance in RAM
B. System requires service
A. Remove batteries, wait 15 seconds for
reset, replace batteries. (2.3)
B. Return system for service (7)
8. Main display reads "Er 4" A. Sample O2concentration is more
than 20 mg/L or below 0 mg/L
B. System requires service
A. Recalibrate using correct altitude and
salinity compensation (4).
B. Replace membrane and Electrolyte (3.4)
C. Clean anode (3)
D. Clean cathode (3)
E. Return system for service (7)

YSI Incorporated YSI 550 DO Handheld Instrument 18
9. Main display reads "Er 6" A. Sample O2concentration is over or
under range (% mode)
B. System requires service
A. Recalibrate using correct altitude and
salinity compensation (4).
B. Replace membrane and Electrolyte (3.4)
C. Clean anode (3)
D. Clean cathode (3)
E. Return system for service (7)
10. Secondary display reads "Er 8"
or
Main display reads "Er 7"
A. Sample temperature is more than
+50oC (122oF)
B. System requires service
A. Reduce the sample temperature
B. Recalibrate using correct altitude and
salinity compensation (4).
C. Return system for service (7)
11. Secondary display reads "Er 9"
or
Main display reads "Er 7"
A. Sample temperature is more than
+50oC (122oF)
B. System requires service
A. Reduce the sample temperature
B. Recalibrate using correct altitude and
salinity compensation (4).
C. Return system for service (7)