What is the Difference Between a YSI Model 55 and Pro20i

What are the Differences Between the YSI Model 55 and the Pro20i?

(Updated: August, 2021)

Have you been looking for the Model 55? The 55 was discontinued in 2016 after 23 years of service. There is no doubt the Model 55 was a true workhorse, but replacement options for the 55 are packed with improvements and new features!


YSI offers many instruments that measure dissolved oxygen (Learn more, YSI Dissolved Oxygen Instrument Selection Guide), but there are two dissolved oxygen instruments that are ideal replacements for the Model 55 – the Pro20i and the Pro20. These instruments measure DO with an electrochemical sensor, just like the Model 55. The only difference between the Pro20i and Pro20 is the cable, as the Pro20i features an integral (i.e. built-in) cable like the Model 55. This integral cable keeps the cost low and makes the Pro20i the direct replacement for the 55.

The Pro20i and Pro20 have many advantages compared to the Model 55. The top 6 reasons to upgrade your Model 55 to the Pro20i or Pro20 are:

1) Cap Membranes are MUCH Easier to Install

The Pro20 and Pro20i, as well as all YSI Pro Series instruments (Learn more, YSI Classic Handhelds vs Pro Series Guide) that utilize electrochemical DO sensors, feature a screw-on membrane cap that is much easier to change. The cap has a membrane that is pre-stretched at the factory, so there is no need to worry about stretching the membrane too much or not enough when installing it on a sensor.

Additionally, the cap is filled with electrolyte and then screwed onto the sensor as opposed to filling the sensor and then stretching the membrane over the solution. This design minimizes the chances of an air bubble getting trapped underneath the membrane which could result in inaccurate readings.

The Model 55 featured a stretch membrane that required the user to stretch a membrane over the sensor. This can be challenging for anyone. Even experienced users sometimes have to install a membrane several times, as air bubbles can easily become trapped under the membrane. Excess membrane material has to be carefully cut away after the membrane is stretched over the sensor. Seem challenging? It is!

Installing a cap membrane can literally be done in seconds. 

2) Less Stirring and Faster Response Time

All electrochemical sensors require some sample movement over the probe, but flow dependence varies based on the type of membrane material used.

Not only can the membranes on the Model 55 be difficult to install, but the membrane material has a high flow dependence that ultimately requires a lot of sample movement across the probe (12 inches per second). If the user isn’t measuring a rapidly moving stream or river, this requires the user to physically move the probe through the water.

Membrane caps for the Pro20i and Pro20 are constructed of polyethylene, a material with lower flow dependence (6 inches per second for yellow cap; 3 inches per second for blue cap). This makes it easier to collect repeatable results and makes the instrument much easier to use!

The response time of an electrochemical sensor is also dependent upon the type of membrane material used. The membranes on the Model 55 feature a response time of 18 seconds, while membranes for the Pro20i and Pro20 feature a response time as low as 8 seconds.

Ever hear of optical DO sensors? There’s no stirring required! YSI instruments with an optical sensor include the ProSolo, ODO200, and ProDSS.


Response Time (T95)

​0 to 100% Oxygen Saturation

Flow Dependence
Model 55 18 seconds 12 in/sec
Pro Series with Yellow Cap 8 seconds 6 in/sec
Pro Series with Blue Cap 17 seconds 3 in/sec

3) Easier to Operate

The Pro20i and Pro20 are much easier to calibrate since there is a CAL key right on the keypad – no need to remember complex button sequences!  

Calibrating is quick and easy with the Quick Cal feature. Once the calibration environment is ready, all the user needs to do is press and hold the CAL key for 3 seconds in order to calibrate (see Comparison video above for a demonstration).

The Pro20i and Pro20 also feature a built-in barometer for automatic barometric pressure compensation, ensuring accurate measurements and quick calibration!

In addition to improvements made to instrument operation and calibration, the Pro20i and Pro20 are also easier to see in the field and easier to hold compared to the Model 55.

4) More Sensor and Cable Options

Sensor options for the Pro20i and Pro 20 include an instant-on galvanic sensor and a polarographic sensor, both of which are user-replaceable. The Model 55 featured a polarographic sensor that wasn’t user-replaceable.

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

Unlike the Model 55 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 55 (approximately 15 meters) are much shorter. Additionally, a BOD probe and cable assembly are available for the Pro20, allowing the instrument to be used in the lab.

5) Higher Accuracy and Wider Measurement Range

Most users need an instrument that is very accurate, so one of the many improvements on the Pro20i and Pro20 is improved accuracy (±0.2 mg/L from 0 to 20 mg/L) compared to the Model 55 (±0.3 mg/L).

A wide measurement range is important for many customers, so we have also made the range MUCH wider for the Pro20i and Pro20 (0 to 50 mg/L) when compared to the Model 55 (0 to 20 mg/L).

6) More Rugged Design – Longer Warranty!

There is a reason why so many people trust the Model 55 – it is reliable! This was not overlooked when we designed our rugged and reliable Pro Series line of instruments (Learn more, Rugged, Field-Proven Water Quality Meters | 3 Real Life Examples). These field-worthy instruments (e.g. Pro20i and Pro20) can put up with a lot!

The Model 55 featured a 2-year handheld and 1-year cable warranty, while the Pro20i and Pro20 feature a 3-year handheld and 2-year cable warranty. We protect your investment!

These are just some of the reasons to upgrade your Model 55 to a new YSI Pro Series instrument!

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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