Teledyne 730 Bubbler - Manual (Page 18)

730 Bubbler Module
Section 2 Programming the Module
2-6
2.4 Operation of the
Bubbler System
When measuring flow rate, the module is used with a primary
measuring device (typically a weir or a flume) or other open
channel flow arrangement where a known relationship exists
between level and flow rate (refer to Table 2-1 Flow Conversion
Types). The level measuring device is a bubbler which measures
the liquid level in the flow stream. The level reading is converted
into a properly scaled flow rate value.
The module contains microprocessor-controlled circuitry to cal-
culate level from the signals produced by the pressure trans-
ducer, and communicate with the 6712 controller.
The module’s bubbler system works as follows: A small com-
pressor pumps air into a reservoir. This air is slowly released by
an orifice into a bubble line. The other end of this tube is sub-
merged in the flow stream. Inside the module, the bubble line
also connects to one side of a differential pressure transducer. As
air is released slowly into the bubble line by the orifice, pressure
builds inside the line to force the air out of the line into the flow
stream. When there is enough pressure to counteract the hydro-
static pressure of the flow stream, a bubble will be forced from
the end of the line. The amount of pressure required to force the
bubble from the end of the line is directly dependent on the
hydrostatic pressure of the flow stream over the end of the
bubble line.
The pressure transducer inside the module senses this pressure
and converts it into an electrical signal that the module converts
into level. The 6712 controller then calculates flow rate and total
flow from the level measurement and lookup tables for the
primary device you are using.
The module produces the pressurized air supply, regulates the
rate of air to the bubbler tube, measures the pressure in the
bubbler tube, purges the air line, compensates for drift, and other
functions.
2.4.1 Purges Periodically, the module releases a high pressure, unregulated
burst of air directly into the bubble line. This is done to clear the
line and prevent any buildup of debris at the bubble line outlet.
2.4.2 Automatic Drift
Compensation
The module measures the output of the pressure transducer at
zero pressure. When the module is first turned on, and every 15
minutes after that, the processor actuates the automatic drift
compensation valve, which connects the input port of the
pressure transducer to the reference port. With the two ports
connected, it then measures the output offset.
The module stores this offset reading in memory and uses it in
level calculations. The repetition of this process causes any
pressure transducer or amplifier drift to cancel out, eliminating
the most significant cause of drift, especially when operating at
low water levels.