The CMS50F is a full-featured pulse oximeter, useful for recording heart rate and blood oxygen saturation during periods of sleep, and also useful for periodic spot-checking any time. The recording capability of this type of pulse oximeter makes it an attractive choice for CPAP users who wish to record and analyze these two key pieces of information. The 50F has another advantage for CPAP users or anyone else who wants to use an oximeter during sleep - it's designed to be worn securely on the wrist, with a comfortable fingertip sensor attached by a cord. I'm going to give you a closer look at the 50F, highlighting some key differences between the 50F and the more popular 50D+ and 50E units.

The 50F is functionally equivalent to the CMS50E. It has all the same features, and the screen shows nearly the same information.

CMS50F vs CMS50E Screen Info
CMS50E (left) and CMS50F (right) Display Screens

The only piece of additional information on the 50F screen is the current time. On the 50D Plus and 50E models you can set the time when you begin a recording, but the time isn't displayed on the main screen like it is on the 50F. Additionally, the 50F has a dedicated date/time screen which can be accessed by a quick click of the button when viewing the main display screen.

CMS50F Compared to CMS50E Pulse Oximeter
CMS50E (left) and CMS50F (right) Pulse Oximeter Comparison

The 50E is an "alligator clip" type of oximeter; the entire device is built around the fingertip sensor and clips onto your finger. The 50F has an external fingertip sensor that attaches to the device through a short cable. An oximeter like the 50E should be taped to the finger for extended recordings, since it could fall off easily during sleep. I recommend white bandage tape that's often found in first aid kits. The 50F fingertip sensor will tend to stay in place during sleep, but I'd still recommend taping it to the finger, since gathering high quality data is dependent upon correct positioning of the fingernail directly under the sensor. While using the 50F it's possible to have your finger inserted but positioned incorrectly. Incorrect positioning will yield inaccurate and unreliable data.

At first glance, it may appear that inserting the fingertip sensor cable into the 50F case might be difficult, because proper alignment of the pins in the case and the holes in the cord is critical. To get the insertion right every time, just line up the arrow on the sensor cord to the mark on the case. This will ensure proper alignment so that you don't break any connecting pins.

CMS 50F Mistake-Proof Cord Connection
CMS 50F Mistake-Proof Cord Connection

Besides the obvious physical differences, the most notable difference between the 50E and the 50F is the menu. The CMS 50F doesn't turn off automatically like the CMS 50D Plus, CMS 50E and CMS 50H. The "Power Off" menu item must be selected in the menu to turn off the CMS 50F.

CMS 50F Menu
CMS 50F Menu (Note that the "Brightness" menu item is selected in the photo and the text isn't visible due to overexposure of the photo.)

The 50F ships with the same software that's included with both the 50D Plus and the 50E. Firmware revisions vary from one oximeter model to the next. At the time of this writing the firmware for the 50F is version 3.5, and the software is version 1.2. The firmware and software are developed independently, so it's completely normal for the two versions to be different. Check out my article highlighting the CMS50E to see exactly what you'll get from the included SpO2 and SpO2 Review software. That article includes screen shots and some other helpful information about installing and using the software. In short, the 50F will record your heart rate and blood oxygen level throughout the night, and will record and report detailed information regarding desaturation events and pulse events. The 50F also has alarm capability that will sound an alarm when you reach a pre-defined oxygen saturation level or pulse level.

Installation of the wrist strap on the 50F can be slightly confusing, and the owner's manual doesn't include adequate instructions for installing the strap. I've written what I believe to be foolproof instructions for installing the strap. Along with the pictures I've provided, this should be enough for anyone to be comfortable with attaching and removing the strap.

Wrist Strap Installation Procedure
Note to start: the side of the band with the metal ring has a little piece of velcro which allows the band to split into two parts near the metal ring. Ignore this. Keep this part of the band connected by the little patch of velcro. Also, be sure to "unvelcro" the big piece of velcro, to make the band as long as it can be.

1. Lay pulse oximeter face down, with the top of the unit facing to the right, and the bottom of the unit facing to the left

2. Grab the blue strap with the metal ring to the left, and the black velcro band to the right, and the fuzzy black velcro facing down

3. Insert the wrist band through both slots on the pulse oximeter, starting with the slot on your left (which is the bottom of the pulse oximeter)

Note 1: The fit is tight through the slots.

Note 2: Once the band is inserted through both slots you're ready to wear the oximeter. If you look at the front of the oximeter at this point, you will see the part of the band with the metal ring extending down beyond the bottom of the oximeter unit, and you will see the part of the band with the black fuzzy veclro extending beyond the top of the oximeter unit. The strap should be pulled through enough so that the entire black fuzzy velcro part is beyond the top of the oximeter. This means that most of the band will be extending out beyond the top of the unit, with just a couple of inches of the band (including the metal ring) extending beyond the bottom of the unit.

4. If you're still looking at the screen on the oximeter, flip it over so the screen is facing down toward the table again, and so the metal ring is facing away from you

5. Place your wrist on the back of the pulse oximeter

6. Pull the velcro/black part of the strap over your wrist and insert the strap through the metal ring

7. Pull the strap through the metal ring and pull the strap back over the metal ring until the long black velcro part of the strap can be attached to the fuzzy black velcro

Note: at this point, you're done. All you have to do is adjust the strap to fit your wrist.

CMS50F Wrist Strap Installation
CMS50F Wrist Strap Properly Installed

While it appears the CMS50F can only be used with proper screen orientation on the left hand due to the location of the finger sensor cord input jack, this isn't true. The display orientation can be changed so that you can wear the oximeter on your right arm while still being able to view the screen right side up. To change the screen orientation, you'll simply need to click the button once from the main screen so that you can see the date/time screen. Then, while viewing the date/time screen, you'll need to click the button again. All subsequent clicks of the button will change the display orientation. A longer press of the button will still enable the menu screen. The date/time screen can be viewed again by exiting the menu screen, and clicking the button once while viewing the main screen. If you choose to wear the pulse oximeter on your right arm, you'll probably want to install the strap in a reverse orientation as well, although doing so is not required.

Overall, the 50D+, 50E and 50H recording models are more versatile - offering stellar overnight recording functionality while being smaller and more convenient for spot-checking - but the 50F is a bit better suited for overnight use, including use with a CPAP machine.

If you're a CPAP user checking to see if your CPAP machine is working effectively for you, then any recording pulse oximeter is a great tool. Using it with common sense can give you many insights into the effectiveness of your CPAP therapy.

As always, feel free to call us at 1-888-955-2727 if you have any questions about our pulse oximeter models or any of the CPAP equipment we sell.

Andrew Senske
President is a leading online retailer of CPAP equipment. Located in Spokane, WA has been serving thousands of customers around the world since 2001. Founded on a belief that patients are their own best primary care providers, understands the importance of educating patients and customers on both the effects of and treatment for obstructive sleep apnea. For more information visit or call toll free 1-888-955-2727.

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