MKS Baratron 631D, Инструкция по эксплуатации, Страница: 37 / 64

Chapter Three: Overview
29
Chapter Three: Overview
Sensor
The Type 631D transducer contains an absolute sensor that is referenced to high vacuum (less
than 10
-7
Torr). This pressure sensor utilizes a single-sided, dual-electrode/AC bridge circuit
design. In this design, two capacitance electrodes are deposited upon a ceramic disk in a
concentric “bull’s-eye” arrangement. The disk is positioned close to an Inconel
®
tensioned
diaphragm to form two capacitors in an AC bridge circuit. The other side of the diaphragm is
exposed to the process gas.
The diaphragm deflects with changing pressure—force per unit area—independent of the
composition of the measured gas. This deflection causes a capacitance change between the
diaphragm and the adjacent electrode assembly. When pressure is equal on both sides of the
diaphragm, the bridge is balanced. As pressure deflects the diaphragm toward the electrode, the
center capacitance changes more than the outer capacitance, causing the bridge to become
unbalanced and an AC voltage to be generated.
Signal Conditioner
The signal conditioner contains the state-of-the-art, low impedance balanced bridge circuitry,
self-compensated for thermal stability with ambient temperature changes. The output is linear
with pressure and provides a 0 VDC output at Zero pressure and a 10 VDC output at sensor Full
Scale.
Instrument Components and Dimensions
The 631D transducer has two adjustments for zero, a potentiometer and a coarse zero switch. The
location of both are the same for the 631D with and without the trip point relay system and are
shown in Figure 2: Top View of the Type 631D Transducer Without Optional Relay System and
Figure 3: Top View of the Type 631D Transducer With Optional Relay System, pages 30 and 31,
respectively.
The top of the 631D transducer without the optional relay system has two LEDs to indicate the
status of the sensor heater. At power up, the “At Temp Status” LED will be yellow. Once the
sensor temperature reaches the normal operating temperature, this LED will turn green. The “Htr
Fail Status” LED is off unless the heater unit fails. Should the heater unit fail, “Htr Fail Status”
LED will illuminate red and the “At Temp Status” LED will turn yellow. A “heater alarm relay”
will become de-energized and change state providing a remote indication of a heater circuit
failure using dedicated pins in the interface connector. Refer to Figure 2: Top View of the Type
631D Transducer Without Optional Relay System, page 30, to identify the location of the “At
Temp Status” and “Htr Fail Status” LEDs. Refer to Heater Status LEDs and Heater Alarm Relay,
page 40, for a detailed discussion of the operation of the heating system LEDs and relays.