Quoted from "Momonucleosis"

Are they both accurate? The pump could be adding heat to the loop, and the downstream sensor is picking that up?

Quoted from "cptninc"

Without knowing which (if either) is correct, I would just set my controls off of the average.

This is assuming access to an actual calibrated sensor.

Quoted from "sebastian"

You can adjust the Sensor value via an offset to match both.
Under normal conditions, the high flow next value is more accurate.

Quoted from "k2viper"

I have D5 NEXT and an inline water temp sensor installed right on D5 NEXT's top inlet, connected to the aquaero. Inline sensor was pre-calibrated by me. So I am able to compare its readings with D5 NEXT's internal temp sensor.
In my case, D5 NEXT's internal sensor has like -0,3C lower readings, but also its readings are different regarding pump's speed. I have two pumps in loop, and readings of D5 NEXT's sensor depent on its pump speed, not on a flow created by second pump.

So I wouldnt recomment D5NEXT's sensor to use for water delta T measuremens, it is too inaccurate for this. Though, it's fine enough to be a basement for e.g. water temp dependent fan speed control.

Quoted from "InfoSeeker"

Quoted from "klara"

I have installed high flow NEXT right after outlet port of D5 NEXT pump.
My high flow NEXT reports 1~2 degrees higher temperature than what D5 NEXT pump reports.
Which value is more reliable?

Or, should I do some calibration to fix temperature values?

The INTODUCTORY POST for the HIGH FLOW NEXT included the statement below:

Quoted

Water temperature - highly accurate and measured as swift as an arrow
The high flow 2 and the high flow NEXT feature a highly accurate and also extremely responsive water temperature sensor. This not only allows the water temperature to be determined but also, in combination with a second sensor, the power currently being dissipated by the cooling system.

EDIT: but then the D5 NEXT INTRODUCTORY POST included the statement below:

Quoted

Water temperature measurement: precise and fast
We are proud of the really precise water temperature sensor. This is mounted in the motor unit of the pump directly on the metal housing and delivers extremely accurate and low-delay values. It is therefore perfectly suited to provide the data to control the fans.