Pressure Systems 9046 - Manual (Page 9)

Pressure Systems, Inc. Model 9046 User’s Manual
Page 3 www.PressureSystems.com
For operation, the Model 9046 only requires 18 to 36 VDC (i=200mA), and the appropriate
Ethernet interface.
1.1.3 Theory of Operation
The Model 9046 is a 32 channel analog multiplexer, controlled by a microprocessor. It
communicates over an Ethernet TCP/IP or UDP interface. The module utilizes 16 small Signal
Conditioning Modules (SCMs), which plug into a motherboard inside the module. There are two
types:
• universal (thermocouple or resistance, set by jumper)
• thermocouple without open circuit detection
1.1.3.1 Thermocouple Cold Junction Compensation
To accomplish cold junction compensation, the thermocouple SCMs incorporate a precision
temperature device located between the thermocouple input sockets. It is in close proximity
(1/16” or 1.5 mm) to the actual contacts. This gives very precise junction temperature
measurement, which contributes to the overall accuracy of the module.
Sixteen of the multiplexer channels are used to scan the embedded precision temperature
devices. The processor then calculates the junction temperature. The temperature this precision
temperature device reports (a voltage that will be referred to as Vb) is limit checked, and stored
for use in the Engineering Unit (EU) calculation to compensate for junction temperature.
Junction temperatures which deviate by a preset amount from the average of the other junction
temperatures will issue an alarm to the external host. An alarm will also be issued if the absolute
temperature of any junction exceeds the rated operating temperature of the 9046.
The thermocouple channels are then scanned, and the voltages used in the EU calculation. The
EU calculation uses previously defined coefficients to determine thermocouple type, engineering
unit, and calibration offset. The universal SCM incorporates the thermocouple and resistance
circuitry in the same package. A jumper can be installed to tell the SCM to operate in the
thermocouple mode. When the jumper is not installed, it will function in the resistance mode. In
this mode, the Vb channel is internally connected to measure the voltage drop (Vb) across an
internal precision resistor. The processor then solves for line current which is a result of an
internal reference voltage being passed through the unknown resistance (RTD Thermistor,
Resistor, etc.) Now that the current is known, and the Voltage (Va) is known, the resistance is
calculated. By performing a table-lookup, or polynomial calculation, the proper engineering unit
value is obtained and returned in a packet via the Ethernet connection.