Pneumatic Operation; Gas Flow Control; Diluent And Source Gas Flow Control - Teledyne T750 User Manual

In this case, Totalflow is calculated as:
WHERE:
Again, this is a dynamic process. An optional photometer can be added the calibrator that
allows the CPU to track the chemiluminescent reaction created when the NO and O
interact to measure the decrease in NO concentration as NO
along with the other data (gas temperature and pressure, actual flow rates, etc.) is used by
the CPU to establish a very accurate NO
P O
9.3.
NEUMATIC PERATION
The portable calibrator's pneumatic system is comprised of the precision dilution system
and valve manifold consisting of one gas port valve and one diluent valve. These valves
open and close off gas flow from the zero air and bottles upstream of the MFCs.
G F
AS LOW
The precision of gas flow through the T750 Portable Calibrator is centrally critical to its
ability to mix calibration gases accurately. This control is established in several ways.

Diluent and Source Gas Flow Control

Diluent and source gas flow in the calibrator is directly and dynamically controlled by
using highly accurate Mass Flow Controllers. These MFCs include internal sensors that
determine the actual flow of gas though each and feedback control circuitry that uses this
data to adjust the flow as required. The MFCs consist of a shunt, a sensor, a solenoid valve
and the electronic circuitry required to operate them.
The shunt divides the gas flow such that the flow through the sensor is a precise percentage
of the flow through the valve. The flow through the sensor is always laminar.
The MFC's internal sensor operates on a unique thermal-electric principle. A metallic
capillary tube is heated uniformly by a resistance winding attached to the midpoint of the
capillary. Thermocouples are welded at equal distances from the midpoint of the tube. At
zero air flow the temperature of both thermocouples will be the same. When flow occurs
through the tubing, heat is transferred from the tube to the gas on the inlet side and from
the gas back to the tube on the outlet side creating an asymmetrical temperature
distribution. The thermocouples sense this decrease, increase of temperature in the
capillary tube, and produce a mVDC output signal proportional to that change that is
proportional to the rate of flow through the MFCs valve.
The electronic circuitry reads the signal output by the thermal flow sensor measured
through a capillary tube. This signal is amplified so that it is varies between 0.00 VDC and
083730750B DCN8606
= -
DIL Totalflow NO
flow
NOGAS = NO source gas flow rate (For calibrator's with multiple
flow
source gas MFC, NOGAS
all of the active cal gas MFCs)
Totalflow = total gas flow requirements of the system.
= the flow rate set for the O
O
3 flow
DIL = required diluent gas flow
flow
C
ONTROL
-
GAS O
flow 3
flow
is the sum of the flow rate for
flow
generator.
3
is produced. This information,
2
calibration mixture.
2
Equation 9-3
3
157