The simple answer: a lot! There is no direct relationship between the two. While TDR is affected by changes in the soil (compaction, soil type, EC levels and more), the POGO is not. So you are measuring an absolute precise and accurate measurement with POGO every time. Where TDR may report 20% moisture one day but that same 20% moisture (for instance) may not be 20% at all another day if you fertilized, irrigated with non-neutral water, compacted your turf or something else changed the dynamics of the turf system as happens every day in turf management.
The margin of error for the POGO is less than 1% so you are certain you can remain confident that 20% moisture (or whatever the moisture is) today is the same 20% you see at other times, or some level of EC in dS/m (mmhos/cm) is the same as other times when you see those numbers. Nobody else can make this claim.
Maybe you are used to using TDR devices and have a number that you are comfortable with on the unit and that is why you are trying to make a correlation. POGO is measuring the most dominant region of the turf system that is responsible for moisture exchange and salinity exchange with the turf. Even if you have very shallow or very deep roots, or whether you have a clay loam or a sandy loam, this holds true. We target our sensor for that region and measure that bulk activity perfectly. That alone will cause differences with TDR measurements although sometimes they may be close.
If you are used to TDR and want to find a workable range for your POGO, find your average across all greens (or other target zones) on your property using the POGO. Take 9 to 12 readings per green (24 or more on fairways). On a day or days that you feel your turf is right where you want it, set that number as your mid point between the low and high warning settings. I would give yourself 3 to 4% on each side of that number. So if your number is 22%, I would et my low at 18-19% and my high at 25 to 26%. I would set your critical low and critical high values 3 to 4% below/above these numbers so you would have settings as follows as an example:
Critical Low (14%), Low (18%), High (26%) and Critical High (30%). This will get you very much in your ball park for moving forward with POGO. You can then use trends and visual analysis to know when you are outside of your target ranges and understand if your turf is draining more rapidly or holding water, distributing EC uniformly from fertilizer applications or not, and a bunch more insight that the system offers. You’ll learn that trends are far more important than numbers. POGO is about those values, not a number and once you get used to that, it will serve you very well.
Most users will get into this range and then dial the numbers in further after they correlate what happens after irrigation, rain or other weather events, fertilizer applications and more. You can make notes and pictures in the app to analyze those events…even cultural practices like spiking, vertical mowing, core cultivation, deep tine and more. From that point, you can dial those numbers in and really get as efficient as possible. You’ll probably find as others do that the number you were used to with other technology wasn’t your ideal number at all.