Input Impedance; Thermocouple Measurement Accuracy - Measurement Computing WebDAQ 316 User Manual

Open thermocouple detec on
Each channel has an open thermocouple detec on (OTD) circuit which consists of a current source between the
TC+ and TC– terminals. If an open thermocouple is connected to the channel, the current source forces a full-
scale nega ve voltage across the terminals.

Input Impedance

Each channel has a resistor that produces an input impedance between the TC and COM terminals. The gain
and offset errors resul ng from the source impedance of connected thermocouples are negligible for most
applica ons. Thermocouples with a longer lead length can introduce more significant errors.
Overvoltage protec on
WebDAQ provides 30 V overvoltage protec on between any two inputs.

Thermocouple measurement accuracy

Thermocouple measurement errors depend partly on the type and accuracy of the thermocouple, the
temperature being measured, the resistance of the thermocouple wires, and the cold-junc on temperature.
For increased accuracy, follow the guidelines in the
channel to compensate for offset errors.
Cold-junc on accuracy
Heat dissipated by nearby heat sources can cause errors in thermocouple measurements by hea ng the
WebDAQ 316 terminals to a different temperature than the cold-junc on compensa on sensor. Thermal
gradient across the terminals can cause the terminals of different thermocouple channels to be at different
temperatures, which creates accuracy errors and affects the rela ve accuracy between channels.
The temperature measurement accuracy specifica ons include errors caused by the thermal gradient across
the WebDAQ 316 terminals for configura ons with the terminals facing forward or upward.
Auto zero channel
The WebDAQ 316 has an internal auto zero channel to compensate for offset errors. When enabled, the
WebDAQ measures the autozero channel and subtracts that measurement from the measurement of each
thermocouple channel. Use the web interface to enable auto zero. WebDAQ 316 specifica ons assume that
auto zero is enabled and applied to every sample.
Rear panel components
1 External power
connector
2 Power bu on
3 Func on bu on
WebDAQ 316 User's Guide
Minimizing thermal gradients
Figure 8. Rear panel
4 LED indicators (4)
5 SD card slot
6 Factory reset bu on
(recessed)
page 12
sec on, and use the auto zero
7 Ethernet connector
8 Ground connector
9 USB connectors (2)