The different types of level instruments presented in this chapter are summarized below.
Level Instrumentation Summary
In the gauge glass method, a transparent tube is attached to the bottom and top
(top connection not needed in a tank open to atmosphere) of the tank that is
monitored. The height of the liquid in the tube will be equal to the height of
water in the tank.
The operation of the ball float is simple. The ball floats on top of the liquid in
the tank. If the liquid level changes, the float will follow and change the
position of the pointer attached to the rotating shaft.
The operation of the chain float is similar to the ball float except in its method
of positioning the pointer and its connection to the position indication. The float
is connected to a rotating element by a chain with a weight attached to the other
end to provide a means of keeping the chain taut during changes in level.
The magnetic bond mechanism consists of a magnetic float that rises and falls
with changes in level. The float travels outside of a non-magnetic tube which
houses an inner magnet connected to a level indicator. When the float rises and
falls, the outer magnet will attract the inner magnet, causing the inner magnet to
follow the level within the vessel.
The conductivity probe consists of one or more level detectors, an operating
relay, and a controller. When the liquid makes contact with any of the
electrodes, an electric current will flow between the electrode and ground. The
current energizes a relay which causes the relay contacts to open or close
depending on the state of the process involved. The relay in turn will actuate an
alarm, a pump, a control valve, or all three.
The differential pressure (DP) detector uses a DP detector connected to the
bottom of the tank that is being monitored. The higher pressure in the tank is
compared to a lower reference pressure (usually atmospheric). This comparison
takes place in the DP detector.