Away with the solenoid valve!
A significant advantage of live switching should be that
the sample does not come into contact with valves. We
must therefore find another way of controlling the
pressure difference via the coupling capillary.
We need another two connections on the live T-piece
with associated needle valves, and also the needle valve
NV
Straight
.
Straight
We connect P
M
on the left of the T-piece, and restricted
via NV
Straight
on the right. The pressure on the right is
approx. 20 mbar lower than on the left. The pressure
profile already appears as at the top on the previous
page for Straight: new pressure conditions, i.e. switch-
ing also functions!
Cut
If we connect P
M
on the right of the T-piece and re-
stricted via NV
Cut
on the left, the pressure on the left is
approx. 20 mbar lower than on the right. The pressure
profile then appears as on the previous page under What
we don’t like, i.e. this switching also functions!
The solenoid valve must return!
But at a position where no sample passes by. We use
this solenoid valve to switch back and forth between
straight and cut by bypassing one of the needle valves
in each case. The same pressure profiles result again as
shown on the right.
P
A
P
M
NV
STRAIGHT
NV
CUT
OUTLET
NV
PURGE
OUTLET
Column A Column B
Live T-piece
P
A
P
M
NV
CUT
OUTLET
NV
PURGE
OUTLET
NV
CUT
Column A Column B
Live T-piece
P
A
Atm.
P
M
P
P
A
M
Atm.
P
A
P
M
NV
CUT
OUTLET
NV
PURGE
OUTLET
NV
STRAIGHT
NV
CUT
MV
CUT
Column A Column B
Live T-piece
Four pages ... - and you know how live switching functions!
6 - 6
live.chp, Issue August 17, 1999
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