This past week there were few discussions on the Polar forums about the calibration of the foot pod with Polar watches. It does not really matter which watch you have S625X, S725X, RS400sd, RS200sd, RS800sd, RS800cx or the new FT60 or FT80. If you use foot pod you will get better results if you calibrate it. With all foot pods in the industry it is true that they are sensitive to your running style, running form, surface you run on, type of the course you run on, etc. Over the years I found that for the best results it is good to keep track of most common calibration factors for combination of surfaces you run on and shoes you run in. I keep a running tally of calibration factors for all pairs of shoes I run in and generally keep at least two calibration factors (CF) - one for running on my treadmill at home and one for running outside on the road. I do not do much of trail running, but if I did I would keep that as a separate CF.
All depends on how much accuracy you strive for. The S1 and S3 foot pods from Polar will give you result within the 5% range of accuracy out of the box. That may be sufficient for some folks, but if you want to be more accurate I suggest to calibrate. All Polar HRMs I used so far let you calibrate the unit by either running pre-determined distance and then correct the lap distance manually or by manually adjusting the CF. I prefer the manual calibration. In order to determine the new CF I run known distance of at least 1 mile. Sometimes I take the measurement multiple times to get better feel for the accuracy. I record each calibration run in a spreadsheet and look at the new CF from the run (or multiple runs). In case I have multiple runs I may take average of the CFs or eliminate the obvious error CFs and take average of the CFs that are quite close. Sometimes you may hit the lap button too early or too late and that may mess-up the CF calculation.
Actually I have a mile long distance measured from the corner of my street to another corner and I use that for all my outside calibration runs. That makes it easy to repeat the calibration when I need it - even like the day before the race if I want to be 100% sure the calibration is OK.
If you want to be super accurate - which may be important for some track workouts you will want to run mile repeats at constant pace and record the laps at different speeds - like 8 minute mile, 7:30, 7:00, 6:30 and 6:00 or faster. Your CF may be slightly different for different speeds, but this is too anal even for me. I usually run with just the CF for combination of shoes and surface. Most of my calibration is done at pace between 8 minute mile and 7 minute mile and they work quite well even for running intervals at 6 min mile. Especially the S3 foot pod is less susceptible to pace changes and accelerations / decelerations that the S1 does not handle as well. The S1 may not give you 100% great distance in interval run - it is best to calibrate it well for the interval part of the workout and just ignore the pace / distance recorded in the recovery sections of the workout.
If you are calibrating on treadmill it is best to use the exactly the same treadmill every time. If you go to the gym you may need to repeat this for few machines as they tend to be quite abused and each may produce different results. You also need to understand that all you do in this process is that you are calibrating your watch to show the same numbers as the treadmill - no guarantee that the treadmill itself is accurate. They quite often are not. If you run for a week in a hotel - don't even bother to calibrate. It is pretty much waste of your workout time. The calibration itself on the treadmill is easy - just hit the lap button at some nice round number, keep running until you ran 1 mile (or longer) and then plug-in the number to the spreadsheet and you know the new CF.
I use the spreadsheet I was referring to in the text above - you can download it from my website. It is very simple and all it does is that it takes your input of the actual distance, old CF and watch displayed distance. I then tells you the new CF and % accuracy of the previous CF. I use it to fine tune my CF over time. If you do this over time you may achieve quite good accuracy. I ran 5 mi race last December in which I had recorded 5.03 miles over quite varied terrain - hills and flats on the road. Now we can discuss whether it was the CF fine tuning or just dumb luck. I'll let you judge it.
Remember that the CF is quite important if you use the software like WKO+ to calculate your intensity factor and TSS as for running. The software uses normalized graded pace to determine how hard the workout was - if your calibration is off you may be working at lower or higher intensity than what the software tells you.
OK so you have determined new CF - what do you do with it? Look at your manual and follow the instructions to change the manual calibration factor. Most Polar HRMs support only one CF at the time so you will need to remember to modify it for each workout. If you have the new RS800CX it allows you to maintain up to 3 pairs of shoes each with different CF. I currently use two pairs of shoes for my running - one is calibration for running outside and the other one for running inside on the treadmill at my house. In other cases like the RS800sd and RS400sd you can define different sports and assign the CF to the sport. When you then define guided workout remember to set the 'Sport Profile' field to Sport-specific settings which will pick-up the pre-set calibration factor from the PPT5.
I hope this helps people deal with the calibration topics. Let me know if you have any questions.