What are the differences between the YSI 550A and Pro20i?

What are the Differences Between My YSI Model 550A and the Pro20i?

Although many were sad to say goodbye to the 550A in 2016, we have two ideal replacements – the YSI Pro20i and the YSI Pro20. 


These instruments measure DO with an electrochemical sensor (membrane-covered), just like the Model 550A. The Pro20i features an integral (i.e. built-in) cable like the Model 550A, which keeps the cost low and makes the Pro20i the direct replacement for the 550A. The only difference between the Pro20i and Pro20 is the cable, as the Pro20 cable is detachable from the instrument via a rugged, locking mil-spec connector.

Model 550A, Pro20i, and Pro20 Comparison Video

The Pro20i and Pro20 have many advantages compared to the Model 550A. The top 5 reasons to upgrade your Model 550A to the Pro20i or Pro20 are:

1) Easier to Use

The Pro20i and Pro20 are much more convenient field instruments. Unlike the 550A, the Pro20i and Pro20 can easily be operated with one hand. They are also lighter, which is important when taking measurements all day.

The graphic display and backlight on the Pro20i and Pro20 have been improved to ensure it is even easier to see in any lighting condition, even with polarized sunglasses.

There is no need to remember key sequences like you did with the 550A, as labeled keys and a simple menu structure make the Pro20i and Pro20 very easy to use. When you need to calibrate, you simply press the CAL key. When you need to see the menu, press the MENU key. It couldn’t be easier to use!

International users will also appreciate the language options on the Pro20i and Pro20 – Spanish, French, and German.

2) Quick Calibration

The most convenient feature of the Pro20i and Pro20 is the Quick One Touch Calibration feature. To calibrate, all you need to do is press and hold the CAL key for 3 seconds. You don’t even have to press ENTER! Check out a demonstration of this feature in the comparison video above.

The Pro20 and Pro20i also feature a built-in barometer for automatic barometric pressure compensation. This makes calibration easier and ensures more accurate measurements!

3) Higher Accuracy

Accuracy is important for most customers, so we have made the Pro20i and Pro20 more accurate (±0.2 mg/L from 0 to 20 mg/L) than the Model 550A (± 0.3 mg/L from 0 to 20 mg/L). Like the Pro20i and Pro20, the Model 550A features a measurement range that is wider than the YSI Model 55.  

4) More Sensor and Cable Options

There are two sensor options for the Pro20i and Pro20. Galvanic sensors are instant-on, while polarographic sensors have a warm-up time but tend to have a longer usable life. The only sensor option for the 550A was a polarographic DO sensor.

>>> Learn more about the difference between polarographic and galvanic DO sensors in the YSI DO Handbook!

When it comes time to change the membrane on the Pro20i and Pro20, they use the same convenient cap membranes as the 550A.

Unlike the Model 550A and Pro20i, the Pro20 features user-replaceable cables. Cable lengths are available up to 100 meters for the Pro20, while maximum cable lengths for the Pro20i (10 meters) and the Model 550A (approximately 30 meters) are much shorter. Additionally, a BOD probe and cable assembly is available for the Pro20, allowing the instrument to be used in the lab.

5) More Rugged Design – Longer Warranty!

The Pro20i and Pro20 are part of a very rugged and reliable instrument series, the Professional Series. These instruments are notoriously rugged and reliable. (learn more, Rugged, Field-Proven Water Quality Meters | 3 Real Life Examples).

These instruments feature an IP-67 (waterproof) rating and a 3-year warranty on the handheld, just like you are used to with the Model 550A. However, we have expanded the warranty on the strong, robust cable to 2 years. We protect your investment!

When you are looking to replace your 550A, we highly encourage you to consider the Pro20i and Pro20. You will not be disappointed!

We recently reviewed the Top 6 Reasons to Upgrade Your Model 55 DO Meter to a Pro20i. The 55 and 550A were both discontinued in 2016.

Do you have other legacy YSI handheld instruments that you’d like to replace? Check out the YSI Pro Series Guide. If you are specifically interested in dissolved oxygen instruments, please see the Dissolved Oxygen Instrument Selection Guide.


Additional Blog Posts of Interest

Dissolved Oxygen Meters Q&A | The Ultimate List

The Dissolved Oxygen Meter You Need at the Price You Want | YSI Pro20i

Which Water Quality Handheld Instrument Do I Need? | We Have the Answer​

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