I have a Bluelab Monitor (MONGUA) and the pH pen and EC pen from the Grower's Toolkit. The monitor runs continuously and I use the EC Pen for mixing a basic batch of full strength nutes. Once I get the EC of the full strength nutes, I can calculate the amounts to create nute water at the required EC.
A few weeks ago, I noticed that the EC readings that I was getting from the monitor were out of whack with the readings from the pen. This came up with autos in flower so the reading were low, as in 0.8 EC range so I long ago switched to 500 PPM readings. The reading from the monitor could be 450 and the pen would read 400. In most cases, that not too much but is seems that autos in flower are really finicky. Also, I expected that the pen and the monitor would be "real close"…maybe 10 or, perhaps, 20
I calibrated the pen and the monitor using 2.77 EC solution. With a PPM of 1385, pen came in at 1390 so that's dead on but the monitor came in at 1420 and 1430. When I contacted Bluelab, they told me that they consider +/- 50 PPM to be accurate.
While I'd like to have both devices report a closer set of values, I had to accept that the absolute value is not that important. Even with the low EC values that I was working with, the key point is that TDS is going up or down and neither the PPM itself nor the amount of change are that important.
My tuppence…