What is the difference between the YSI Pro20 and Pro20i?

The Pro20 features user-replaceable field cables, allowing the user to swap between cable lengths (1, 4, 10, 20, 30, and up to 100 meters) for different deployment depths. The Pro20i features an integral (built-in) cable available in 1-meter (607120-1), 4-meter (607120-4), and 10-meter (607120-10) lengths. Both share the same IP67 instrument body, the same measurement capabilities (dissolved oxygen and temperature), the same One Touch Calibration, and the same 50-point data log. The Pro20 also supports the ProBOD self-stirring BOD bottle probe assembly, which the Pro20i does not. Choose the Pro20i when you always work at the same depth; choose the Pro20 when cable length flexibility is needed.

What is the difference between polarographic and galvanic DO sensors on the YSI Pro20?

Both sensor types measure dissolved oxygen by electrochemical reduction at a cathode through a semi-permeable membrane, but they differ in electrode materials and operational characteristics. The polarographic sensor (black body, model 2003) requires an applied voltage and needs a 5-15 minute warm-up time before use or calibration. It lasts longer with a 1-year warranty. The galvanic sensor (grey body, model 2002) uses dissimilar electrode potentials (zinc anode, silver cathode) that generate current without applied voltage, providing instant-on operation with no warm-up needed. However, the galvanic sensor consumes the zinc anode continuously even when not in active use, so it has a shorter lifespan and a 6-month warranty. For daily field monitoring where instant-on matters, galvanic. For periodic laboratory or compliance monitoring where warm-up is acceptable, polarographic.

How do you calibrate the YSI Pro20 dissolved oxygen sensor?

The primary calibration method is air-saturation calibration using the grey calibration/storage sleeve. Moisten the sponge in the sleeve with water, install it over the sensor guard (not submerging the sensor), and allow 5-15 minutes for the chamber to reach 100% relative humidity. Verify the barometer reading is accurate first. With One Touch Cal enabled (the default), press and hold the Calibrate key for 3 seconds. The Pro20 displays 'Calibrating %DO' and automatically calibrates to the barometer and salinity compensation values. This simultaneously calibrates mg/L and percent readings. A 'Calibration Successful' message confirms completion. The barometer must be accurate before DO calibration because percent calibration uses the barometer value.

Why does the YSI Pro20 require barometer calibration?

The Pro20 has an internal barometer used to calculate dissolved oxygen saturation values. DO percent saturation and mg/L readings are calculated from the measured oxygen partial pressure relative to atmospheric conditions. If the barometer reading is inaccurate, all DO calibrations and measurements will be off proportionally. The factory barometer calibration is accurate for most uses, but altitude changes, barometric pressure changes, and time can require recalibration. Importantly: do not use weather service barometric pressure readings, which are corrected to sea level. Use true, uncorrected barometric pressure from a laboratory barometer. The conversion formula is: True BP = Corrected BP minus (2.5 times Local Altitude in feet divided by 100).

How often should the Pro20 membrane and electrolyte be replaced?

For polarographic sensors (black, model 2003): replace the KCl electrolyte solution and membrane cap at least every 30 days during regular use, or sooner if bubbles are visible under the membrane, dried electrolyte is visible on the membrane, or readings are unstable. For galvanic sensors (grey, model 2002): replace the NaCl electrolyte and membrane cap at least every 60 days during regular use, with the same early replacement triggers. After any membrane change, allow the new membrane to equilibrate overnight before calibrating for best accuracy. The sensor ships with a dry red protective cap — this must be replaced with a membrane and electrolyte before any use.

What is DO Local% on the YSI Pro20 and when should I use it?

DO Local% is an alternative dissolved oxygen percent saturation mode where the calibration value is always set to 100% regardless of altitude or barometric pressure. In standard mode with DO Local% disabled, calibrating at altitude produces a calibration value less than 100% (e.g., at 841 ft elevation with 737 mmHg, the calibration value is 96.97%). In DO Local% mode, the instrument always calibrates to 100% but then applies the barometric pressure correction to each individual measurement rather than during calibration. Both modes produce identical mg/L readings. DO Local% is useful when comparing samples collected at the same altitude, or when the operator wants a 100% calibration point for straightforward QC documentation. DO Local% does not affect mg/L accuracy.

Product Review & Technical Reference — YSI Instruments

YSI Pro20 / Pro20i Review & Complete Reference (2026): Dissolved Oxygen & Temperature Meter

Source: YSI Pro20/Pro20i User Manual Document #605597 Rev C (September 2016) • IP67 • 3-year instrument warranty

Dissolved oxygen (mg/L, %, ppm) • Temperature • Internal barometer • One Touch Calibration • 50-point data log • 400-hour battery life • Polarographic or galvanic sensor

YSI Pro20 dissolved oxygen meter being used at a wastewater aeration basin — field monitoring in an activated sludge process

Operator holding YSI Pro20 with cable probe submerged in aeration basin at municipal wastewater treatment plant

Verdict: The Field Standard for Dissolved Oxygen Monitoring

The YSI Pro20 has been the reference instrument for dissolved oxygen measurement in environmental monitoring, wastewater treatment, aquaculture, and industrial process control for decades. The design is deliberately straightforward: IP67-rated body, a six-button keypad, and a single-key One Touch Calibration that eliminates the most common source of field measurement error. The internal barometer is what separates it from lesser DO meters — altitude compensation happens automatically rather than requiring operator lookup tables. The polarographic vs galvanic sensor choice is the key decision point, and it is often misunderstood. This reference covers everything from first power-on through electrode resurfacing.

YSI Pro20 Dissolved Oxygen & Temperature Meter

IP67 • One Touch Cal • Internal barometer • 50-point log • 400-hr battery • 3-year warranty. Pro20 (field cable) and Pro20i (integral cable) configurations available.

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DO Range

0–50 mg/L • 0–500% sat

DO Accuracy

±2% of reading or ±0.2 mg/L

Temp Range

−5 to 55°C (±0.3°C)

Barometer

400–999.9 mmHg (±5 mmHg)

Data Log

50 data sets (non-volatile)

Battery Life

400 hours (2×C alkaline, no backlight)

IP Rating

IP67 Waterproof

Sensor Options

Polarographic (#2003) or Galvanic (#2002)

Calibration

One Touch (air sat.) or manual %/mg/L

Cable options

1, 4, 10, 20, 30, 100 m (Pro20)

Instrument warranty

3 years

Polaro sensor warranty

1 year

Pro20 vs Pro20i — Which Configuration to Buy

Feature	Pro20 (6050020)	Pro20i
Cable type	Replaceable field cable — user-swappable	Integral (built-in) cable — fixed length
Available cable lengths	1, 4, 10, 20, 30, and up to 100 m	1 m (607120-1), 4 m (607120-4), 10 m (607120-10)
BOD probe compatibility	Yes — ProBOD self-stirring assembly (605780)	No
Instrument body	Same IP67 body	Same IP67 body
Sensor interchangeability	Yes — polarographic or galvanic	Yes — polarographic or galvanic
Best for	Variable depth deployment; ProBOD BOD testing; multi-site use	Single-depth routine monitoring; simpler setup; lower cost

Polarographic vs Galvanic Sensor — The Most Important Decision

Both sensor types measure dissolved oxygen by electrochemical reduction at a cathode through a semi-permeable membrane. The difference is in electrode chemistry, warm-up requirements, consumable life, and warranty coverage.

POLAROGRAPHIC vs GALVANIC — COMPLETE COMPARISON

Polarographic — Model 2003 (Black)

Part #605203 (includes 5908 membrane kit)
Anode: Silver • Cathode: Gold
Electrolyte: KCl (white label)
Warm-up required: 5–15 minutes
Applied voltage required: Yes
Membrane change interval: Every 30 days
Sensor warranty: 1 year
Sensor lifespan: Longer
Best for: Scheduled compliance monitoring, lab use, infrequent field use

Galvanic — Model 2002 (Grey)

Part #605202 (includes 5913 membrane kit)
Anode: Zinc • Cathode: Silver
Electrolyte: NaCl (blue label)
No warm-up — instant-on
Applied voltage required: No (self-generating)
Membrane change interval: Every 60 days
Sensor warranty: 6 months
Sensor lifespan: Shorter (zinc consumes continuously)
Best for: Daily field monitoring, aeration basins, rapid spot-checks

Critical note: Galvanic sensors consume zinc continuously even when the display is off. Never leave a galvanic sensor installed and inactive for extended periods — the zinc anode depletes even without active measurement.

Sensor type must be configured before first use. The System Setup menu Sensor Type setting MUST match the sensor installed. Configuring polarographic when a galvanic sensor is installed (or vice versa) can permanently damage the sensor and voids the warranty. The instrument steps you through this on first power-on, but verify after any sensor change.

Keypad Reference

#	Key	Short Press / Single Press	Hold 3 Seconds
1	Calibrate	Opens Calibrate menu (when One Touch Cal is disabled)	Initiates One Touch Calibration (when One Touch Cal is enabled)
2	Up Arrow	Navigate menus; navigate Run screen bottom boxes; increase numerical inputs	—
3	Power / Backlight	First press: power on. Second press: backlight on. Third press: backlight off.	Power off (from any screen)
4	Menu	Enter System Setup menu from Run screen	Alternate contrast adjustment (hold + Up/Down arrow)
5	Enter	Confirm selections and entries	—
6	Down Arrow	Navigate menus; decrease numerical inputs	—
Backlight auto-off: turns off after 2 minutes of no key press, or when pressed a third time. Battery life without backlight: ~400 hours. With backlight on continuously: significantly shorter.

First Power-On — Initial Configuration

The Pro20 steps through three selections the first time it is powered on. These must be set correctly before any measurement.

Select Language

English, Spanish, German, or French. Note: the bottom-of-screen function boxes always display in English regardless of language selection.

Select Sensor Type — Polaro (black) or Galvanic (grey)

Match to the physical sensor installed. This is the most critical setup step — incorrect selection can damage the sensor. Confirm by the sensor body color and the model number engraved on it: 2003 = polarographic (black), 2002 = galvanic (grey).

Select Membrane Type — 1.25 mil PE (Yellow) or 2.0 mil PE (Blue)

Match to the membrane cap color installed on the sensor. Yellow cap = 1.25 mil PE (faster response, more flow-dependent). Blue cap = 2.0 mil PE (slower response, less flow-dependent).

Membrane Type Selection — Performance Trade-Offs

Membrane	Color	Thickness	Flow Dependence (4 min)	Min Sample Movement	T-95 Response
5908 / 5913 (Yellow)	Yellow cap	1.25 mil PE	25%	6 in/sec (15 cm/sec)	~8 seconds
5909 / 5914 (Blue)	Blue cap	2.0 mil PE	18%	3 in/sec (8 cm/sec)	~17 seconds
5908/5909 = polarographic membrane kits. 5913/5914 = galvanic membrane kits. T-95 response = time to reach 95% of final reading. Flow dependence = accuracy degradation if the probe is not moving through the sample at minimum speed.

Choosing a membrane: Use the 1.25 mil yellow membrane when response time matters and you can maintain adequate sample movement (probes in flowing streams, aeration basins with mixers). Use the 2.0 mil blue membrane when sample movement is limited or unpredictable (stagnant ponds, tank profiles, slow-moving streams) — the reduced flow dependence prevents underreading in still water.

Barometer Calibration — Do This Before DO Calibration

The Pro20’s internal barometer is factory-calibrated and accurate for most conditions. However, altitude, temperature changes, and time may require recalibration. Barometer calibration must be performed before any dissolved oxygen calibration because percent DO calibration uses the barometer value as its reference.

Do not use weather service barometric pressure. Weather service readings are corrected to sea level and will produce systematic DO errors if used directly. Use true (uncorrected) barometric pressure from a laboratory barometer. If only sea-level corrected pressure is available, use this formula:

True BP = Corrected BP − (2.5 × Local Altitude in ft ÷ 100)

Example at 500 ft elevation with 1013 mbar (corrected): True BP = 1013 − (2.5 × 5) = 1013 − 12.5 = 1000.5 mbar (750.4 mmHg)

From Run screen: highlight Barometer box (bottom row) — press Enter

The barometer box shows the current barometric pressure reading.

Use Up/Down arrows to adjust to true local barometric pressure — press Enter to save

Hold Up or Down arrow to change value rapidly. The instrument will not allow large adjustments far from its measured value — this is a built-in safeguard.

One Touch Calibration — Primary DO Calibration Method

One Touch Calibration (the default, enabled in System Setup) is the fastest and most reliable DO calibration method. It calibrates to air saturation using the moist calibration sleeve — no buffer solutions or sample titration required.

Moisten the sponge in the grey calibration/storage sleeve — install over sensor guard

The sponge must be moist but NOT have standing water that could contact the membrane. If water droplets get on the membrane, remove the sleeve, dry the membrane, and restart. The goal is 100% relative humidity air surrounding the membrane, not liquid water contact.

Wait 5–15 min (polarographic) or 5–10 min (galvanic) for chamber to saturate and sensor to stabilize

Set Auto Shutoff to Off or at least 20 minutes before starting. A polarographic sensor that has just been powered on requires the full 5–15 minutes for the applied voltage to equilibrate. Do not try to shorten this — premature calibration is the leading cause of calibration failures and DO reading errors.

Verify barometer reading is accurate

Recalibrate barometer if needed before proceeding.

Press and hold Calibrate key for 3 seconds

Display shows “Calibrating %DO.” The Pro20 automatically calibrates to the barometer and salinity compensation values. Takes up to 2 minutes. “Calibration Successful” confirms completion.

One calibration covers all DO units. Calibrating in % simultaneously calibrates mg/L and ppm. There is no need to calibrate separately in each unit. YSI recommends calibrating in % for both ease and accuracy.

DO Local% — Altitude and Barometric Pressure Mode

DO Local% changes how the percent saturation value is calculated and displayed. With DO Local% off (default), the calibration value decreases with altitude — at 841 ft elevation (737 mmHg), you calibrate to 96.97%, not 100%. With DO Local% on, the calibration value is always 100% but the barometric pressure correction is applied to each individual measurement instead.

Both modes produce identical mg/L readings. The difference is only in how %DO is expressed. Enable DO Local% when your application needs a 100% calibration setpoint for documentation or QC, or when comparing samples collected at the same altitude.

Taking Measurements — Field Procedure

Calibrate before measuring — allow 5–15 min warm-up for polarographic sensors

A galvanic sensor is instant-on; a polarographic sensor requires the warm-up period from first power-on.

Install sensor guard — place probe in sample — give probe a quick shake to release air bubbles

Bubbles on the membrane produce falsely low readings. A quick shake at initial immersion clears them.

Maintain minimum sample movement: 6 in/sec (yellow membrane) or 3 in/sec (blue membrane)

Both membrane types are flow-dependent. In still water, the sensor depletes oxygen at its tip faster than natural diffusion can replenish it, causing readings to slowly drift down. Move the probe continuously through the sample, or allow the sample to stir around it. In flowing streams, orient the sensor perpendicular to the current — not facing into the flow.

Allow temperature to stabilize — then stir until DO value plateaus — record or save

Temperature stabilization is required for accurate ATC. Once temperature is stable and DO has plateaued, use Down/Up arrow to highlight SAVE on the Run screen and press Enter to store the reading.

Aeration basin measurements: Bubbles bursting on the membrane cause unstable, erratic DO readings in aeration tanks. To prevent this, orient the sensor upward (facing skyward) and curve the cable so the probe hangs with the membrane facing up. Bubbles travel up and away from the membrane surface rather than bursting on it.

Salinity Compensation

The Pro20 applies a user-entered salinity value (0.0–70.0 ppt) to compensate dissolved oxygen mg/L values for salt content. Higher salinity reduces DO solubility — saltwater holds less dissolved oxygen than fresh water at the same temperature. The factory default is 0.0 ppt (fresh water).

Adjust the salinity compensation value on the Run screen (highlight Salinity box, press Enter, adjust with Up/Down, press Enter to save) to match the salinity of your sample. The salinity compensation can be adjusted at any time without recalibration.

Data Logging — 50-Point Memory

Function	Procedure
Save a reading	From Run screen, highlight SAVE box (bottom row), press Enter. Data set number confirms storage. “Memory Full” displays if all 50 sets are saved.
View saved data	Highlight DATA box, press Enter to enter Data mode. Use Up/Down arrows to scroll through data sets 01–50. Press Enter to access erase functions.
Erase one data set	In Data mode, press Enter → highlight Erase → press Enter → highlight Erase Data Set → press Enter. Remaining data sets renumber sequentially.
Erase all data sets	In Data mode, press Enter → highlight Erase → Erase All Sets → Enter. Clears all 50 records.
Exit Data mode	Press Enter → highlight ESC-Exit → press Enter to return to Run screen.
Data is stored in non-volatile memory and retained through power-off cycles and battery changes. A Factory Reset does NOT erase saved data.

System Setup Menu — Complete Parameter Reference

Parameter	Options	Default	Notes
Sensor Type	Polaro (black) / Galvanic (grey)	Last confirmed setting	Must match sensor installed. Incorrect setting can damage sensor.
Membrane Type	1.25 mil (Yellow) / 2.0 mil (Blue)	Last confirmed setting	Must match membrane installed. Incorrect setting causes inaccurate readings.
One Touch Cal	Enabled / Disabled	Enabled	When enabled: hold Cal key 3 sec to calibrate. When disabled: manual % or mg/L calibration from Cal menu.
DO Local%	Enabled / Disabled	Disabled	Enabled: always calibrates to 100% at any altitude. Disabled: calibration value decreases with altitude/lower barometric pressure.
DO Units	mg/L, ppm, %	mg/L and %	Multiple units can display simultaneously. mg/L and ppm are equivalent.
Temperature Units	°C / °F	°C	Only one unit at a time.
Pressure Units	mmHg, inHg, mbar, psi, kPa	mmHg	Only one unit at a time. Used for barometer display.
LDS (Last Digit Suppression)	Enabled / Disabled	Disabled	When enabled: rounds DO to nearest tenth (e.g., 8.25 → 8.3 mg/L). Auto-disabled during calibration.
Auto Stable	0.0–1.9% DO change over 3–19 sec	Off (0.0% change, 10 sec)	When enabled: AS symbol blinks during stabilization, displays steadily + 2 beeps when stable. Disable by setting DO% Change to 0.0.
Audio	Enabled / Disabled	Enabled	When enabled: beeps on key press and double-beeps on Auto Stable confirmation.
Contrast	Adjustable	Mid range	Up arrow = darker. Down arrow = lighter. Alternate: hold Menu key + Up/Down arrows from any screen.
Auto Shutoff	0–60 minutes	30 minutes	Set to 0 to disable. Disable or set to ≥20 min before calibration to prevent instrument shutoff during warm-up.
Language	English, Spanish, German, French	English	Bottom-of-screen function box labels always display in English regardless of language setting.
Reset to Factory Default	Yes / No	No	Resets all settings to defaults. Does NOT erase saved data. Requires barometer and DO recalibration after reset.

Electrode Maintenance — Membrane & Electrolyte

Membrane Replacement Procedure (Both Sensor Types)

Remove sensor guard — remove old membrane cap by unscrewing counterclockwise

Hold the sensor body with one hand while unscrewing the membrane cap with the other. Discard the used cap and electrolyte.

Rinse sensor tip with distilled or deionized water

Remove any electrolyte residue from the electrode surface.

Prepare electrolyte solution per bottle instructions — allow to sit 1 hour before use

Allowing the mixed solution to rest for 1 hour releases dissolved air that would otherwise form bubbles under the new membrane, which would require another membrane change. KCl = polarographic. NaCl = galvanic. Do NOT interchange electrolytes between sensor types.

Fill new membrane cap 3/4 full with electrolyte — tap to release bubbles — do NOT touch the membrane

Even fingerprints on the membrane surface can affect its permeability. Handle the cap by the sides only.

Thread cap onto sensor — finger tight — do NOT use tools

Some electrolyte overflow is normal. Replace sensor guard. For best results, allow the new membrane to equilibrate overnight before calibrating.

Electrode Resurfacing — When and How

Resurfacing is not routine maintenance. Do not sand the electrodes every membrane change. Only resurface if the sensor fails to stabilize or calibrate after a fresh membrane installation. Visual tarnish alone does not require sanding — the sensor may still perform accurately.

Gold cathode (polarographic sensors only): Wet the 400-grit sanding disc provided and place face-up in palm. Hold sensor vertically, tip down, and place on the disc. Twist 3–4 times in a circular motion — goal is a matte finish indicating the build-up has been removed. Rinse thoroughly before installing a new membrane.

Silver anode (both sensor types): Two options:

Chemical cleaning (polarographic only — NEVER galvanic): Soak in 14% ammonium hydroxide for 2–3 minutes or 3% ammonia solution 8–12 hours. Rinse heavily in tap water then distilled water. Smell the tip to confirm all ammonia is removed — trapped ammonia under the new membrane gives false readings.
Mechanical cleaning (both sensor types): Wet the sanding disc and gently wrap around the anode shaft. Twist 3–4 times to lightly sand. Rinse, wipe with lens tissue, rinse with distilled water, install new membrane.

Sensor Storage

Storage Duration	Method	Notes
Short-term (<30 days)	Grey calibration/storage sleeve with moist sponge over sensor guard	Keep sponge moist with tap water. Creates humid air environment — sensor should NOT be submerged in water.
Long-term (>30 days)	Remove membrane cap, rinse sensor, air-dry completely, install a DRY protective cap	DO NOT leave the electrolyte-filled membrane on during long-term storage. Rinse, dry, cap with a clean dry cap. Long-term storage temperature: −5 to 70°C.
Return to service	Install new membrane with fresh electrolyte, allow overnight equilibration, recalibrate	Conditioning a sensor that was stored dry is essential before accurate measurement.

Troubleshooting Reference

Symptom	Likely Cause	Resolution
Calibration fails — "Calibration Over," "Calibration Under," or "Unstable Reading"	Multiple possible causes	1. Verify barometer reading. 2. Verify correct Sensor Type and Membrane Type in System Setup. 3. Ensure calibration sleeve sponge is moist (not dry, not dripping). 4. Ensure adequate sample movement for mg/L or ppm calibration. 5. Allow full stabilization time; wait ≥3 seconds before confirming. 6. Replace membrane and electrolyte. 7. Clean electrodes. 8. Return for service.
Instrument will not turn on / battery symbol / "Critical Shutdown"	Low or incorrectly installed batteries	1. Replace both C-cell alkaline batteries. 2. Check polarity alignment. 3. Return for service if batteries are correct.
Barometer reads Over/Under — DO/Temp display Over/Undr — Cal key gives barometric pressure error	Barometer hardware failure	Return system for service — barometer cannot be recalibrated past this point.
Inaccurate readings	Multiple possible causes	1. Verify Sensor Type and Membrane Type settings. 2. Recalibrate; check barometer and salinity. 3. Verify temperature readings are accurate. 4. Sample temperature above 45°C exceeds ATC range. 5. Calibration sleeve was not 100% humidity during calibration — remoisten and recalibrate. 6. Replace membrane and electrolyte. 7. Clean electrodes. 8. Return for service.
DO displays "Over" or "Undr"	Out-of-range reading or configuration error	1. Verify Sensor Type and Membrane Type settings. 2. For polarographic: allow 5–15 min warm-up. 3. Sample DO is above 50 mg/L (500%) or below −0.02 mg/L. 4. Verify barometer and salinity; recalibrate. 5. Replace membrane; recalibrate. 6. Clean electrodes. 7. Return for service.
Temperature displays "Over" or "Undr"	Sample temperature out of range	Sample temperature below −5°C or above +55°C. Bring sample within operating range. If temperature is within range, return for service.
DO reading drifts downward in still water	Insufficient sample movement — sensor depleting O₂ at membrane	Increase probe movement to minimum 6 in/sec (yellow membrane) or 3 in/sec (blue membrane). Consider switching to the 2.0 mil blue membrane for lower flow dependence.
Unstable, erratic readings in aeration basin	Air bubbles bursting on membrane	Orient sensor upward (facing skyward); curve cable so probe hangs with membrane facing up. Bubbles travel up and away from the membrane rather than bursting on it.

Factory Default Reset Values

Parameter	After Reset
Temperature Units	°C
Dissolved Oxygen Units	mg/L and %
Pressure Units	mmHg
Sensor Type	Last setting confirmed — does NOT reset. Verify after reset.
Membrane Type	Last setting confirmed — does NOT reset. Verify after reset.
Salinity Compensation	0.0 ppt
DO Local%	Off
One Touch Cal	On
Display Contrast	Mid range
Auto Shutoff	30 minutes
Auto Stable	Off (0.0% change, 10 sec)
LDS	Off
Audio	On
Language	English
DO Calibration	Reset to factory default — recalibrate immediately after reset
Barometer Calibration	Reset to factory default — recalibrate barometer immediately after reset
Saved data	NOT affected by factory reset

Specifications

Parameter	Range	Resolution	Accuracy
Temperature	−5 to 55°C / 23 to 113°F	0.1°C / 0.1°F	±0.3°C / ±0.6°F
DO (% saturation, 0–200%)	0–200% air sat.	1% or 0.1% (user selectable)	±2% of reading or ±2% air saturation, whichever greater
DO (% saturation, 200–500%)	200–500% air sat.	1% or 0.1% (user selectable)	±6% of reading
DO (mg/L, 0–20 mg/L)	0–20 mg/L	0.1 or 0.01 mg/L (user selectable)	±2% of reading or ±0.2 mg/L, whichever greater
DO (mg/L, 20–50 mg/L)	20–50 mg/L	0.1 or 0.01 mg/L (user selectable)	±6% of reading
Barometer	400–999.9 mmHg	0.1 mmHg	±5 mmHg within 5°C of calibration temp*
*At operating temperatures below 10°C or above 40°C, the barometer must be recalibrated to maintain accuracy specification. ATC range: −5 to 45°C.

Part Numbers & Accessories Reference

Part Number	Description
6050020	Pro20 instrument (field cable sold separately)
607120-1 / -4 / -10	Pro20i with 1m / 4m / 10m integral cable (DO sensor sold separately)
607125	Pro20i with 4m integral cable + polarographic DO sensor included
60520-1, -4, -10, -20, -30	Field cables for Pro20 (1m through 30m)
605203	Polarographic DO sensor (black, model 2003) — includes 5908 membrane kit
605202	Galvanic DO sensor (grey, model 2002) — includes 5913 membrane kit
605780	ProBOD self-stirring BOD sensor assembly (Pro20 only)
5908 / 605306	1.25 mil PE membrane kit, 6-pack, polarographic (yellow cap)
5909 / 605307	2.0 mil PE membrane kit, 6-pack, polarographic (blue cap)
5913 / 605913	1.25 mil PE membrane kit, 6-pack, galvanic (yellow cap)
5914 / 605914	2.0 mil PE membrane kit, 6-pack, galvanic (blue cap)
603074 / 603075	Carrying case: hard-sided (603074) or soft-sided (603075)
601205	O-ring grease
603070	Shoulder strap

Warranty Summary

Component	Warranty Period	Exception
Pro20/Pro20i instrument	3 years from end user purchase date	Batteries and battery damage excluded
Pro20/Pro20i cables	2 years from end user purchase date	—
Polarographic sensors (2003)	1 year from end user purchase date	—
Galvanic sensors (2002)	6 months from end user purchase date	—
When purchased by rental agencies	90 days (instrument, cables, and probes)	—
Warranty service: YSI Customer Service • 1725 Brannum Lane, Yellow Springs, OH 45387 • 1-800-897-4151 • ysi.com

YSI Pro20 / Pro20i — Dissolved Oxygen & Temperature Meter

IP67 • One Touch Cal • Internal barometer • 50-point log • 400-hr battery • 3-year warranty • Polarographic or galvanic sensor.