What are the possible sources of error that may affect the end-to-end temperature uncertainty of ± 0.05 °C?
The end-to-end uncertainty encompasses the entire chain of the temperature measurement process, i.e. the temperature sensor, transmitter, wiring, data acquisition system, etc. Typical sources that introduce errors in the measurement chain include:
Temperature sensors installed in thermowells are not in direct contact with the water (ie Closed-end thermowell) and this might have an impact on the accuracy. In addition, the conductive thermal compound within the thermowells tends to dry out over time, which will affect thermal conductivity.
Location of temperature sensors
Care should be taken if sensors for the chilled water supply are installed in locations exposed to direct sunlight. The metal parts of the sensor could easily reach 50°C due to direct solar radiation.
Installation of temperature sensors near pipe joints from 2 different sources might cause unstable measurements due to mixing of water temperatures.
Use of transmitters
Some systems use transmitters to convert the sensor resistance from ohms to 4~20 milliamps (mA) DC signal to allow for transmission over a long distance. There is a tendency for the transmitters to drift over time, and be affected by ambient temperature fluctuations.
A/D conversion in Data Logger
An additional source of error is introduced when an A/D convertor is used to convert the analogue signal of ohms or mA from the sensor or transmitter into digital signals for processing by a computer.
Sensor(s) affected by electromagnetic field
Most of the sensors are installed in environments with high Electromagnetic field, where a typical chiller plant room houses the chillers, pumps, fans, inverters, and sometimes, even high voltage transformers. The interference from the electromagnetic field due to these equipment may affect wireless or even improperly-shielded wired communication.