I have both a Milwaukee pH controller and a Vivosun pH pen. I calibrate the Milwaukee using GH pH 7.0 and pH 4.01 standard control solutions.
Recently, the pH in my reservoir has been gradually decreasing when prior it was always gradually increasing. I try to keep it at 5.8, and recently it’s been at 5.7 (and reasonably steady). This morning it was at 5.5 and I got a bit worried (all values as displayed on the Milwaukee).
The Vivosun pen read 5.2.
So I calibrated the Milwaukee and it was dead nuts on. Added some pH up and got it to 5.7. Vivosun read 5.5.
Then went to calibrate the Vivosun, which requires (for two-point calibration) pH 4.0 and 6.89 solutions (where the Milwaukee requires pH 4 and 7). The procedure for the Vivosun is to calibrate at 6.89, then 4.0. I ran through that, then measured the pH 7.0 reference … and read 6.78. Ok, try again … pH 7.0 reference is now 6.84. Try again, 6.88.
Admittedly, I didn’t do 3-point calibration on the pen, but I’m not trying to read any values > pH 7.0. I used two-point on the Milwaukee too (and it read the 6.89 Vivosun solution at 6.9).
Maybe it’s different for soil-growers, but in my RDWC system, I really want to keep the pH correct within 0.1 (5.7 - 5.9). The Vivosun pen seems to only be +/- 0.15 or so, which makes it less accurate than I need.
I suppose it might be rude to compare a $13 pen to a $130 meter, but there are lots of people using these pens … they’re not terribly accurate. And it’s quite stupid IMO that the pen reads to the hundreths of pH.
 The MC122 is favored by freshwater aquarium gardeners to control CO2 injection - the more Co2 you add to the water, the lower the pH goes, and you control the dissolved CO2 by setting the desired pH reading below the pH the aquarium would have without Co2 injection.