CHAPTER 3

COMPONENTS OF THE RETAIL MOTOR-FUEL DISPENSING SYSTEM

**CHAPTER OBJECTIVES**

Upon completion of this chapter, you should be able to:

1\. Identify the major components of each of the following types of
retail motor-fuel dispensing systems:

> • self-contained systems;
>
> • remote pump (or dispenser) systems; and
>
> • mechanical and electronic systems.

2\. Explain the purpose and function of each of these major components.

**INTRODUCTION**

What the motorist sees of a modern fuel-dispensing system is very
impressive---a metal cabinet, extending perhaps eight feet in height,
lighted price and volume indicators, a length of hose or hoses, with
nozzles on the end, push buttons, possibly a card reader, maybe even a
small TV screen and instructions for operating the device. While the
outward appearance may be impressive, the reality of the dispenser
hydraulic, mechanical and electronic complexity is hardly recognizable.

Opening the cabinet reveals a profusion of components, tubing, wiring,
gears, and linkages, enough to daunt even the mechanically or
electronically minded amateur. In this chapter, you will learn to
recognize these parts, and gain a basic knowledge of how they work
together.

The fuel-dispensing system performs several interrelated functions. The
purpose of some components is to maintain hydraulic continuity, to
regulate the direction of flow and fluid pressure. Others are
responsible for pressurizing the fuel and moving it through the system.
Still others, of course, are involved in metering the liquid fuel,
registering accurately the quantity delivered, and computing the price
of the delivery. Finally, some components serve to control the operation
of the system, switching it on and off, resetting the volume and price
indicators, regulating the delivery, and so on. We will proceed to look
at the major components in each of these functional areas.

**HYDRAULIC CONTINUITY**

Fuel-dispensing systems are designed to measure, register, and deliver
accurately the desired quantities of fuel product. The customer is
paying for motor fuel, not air or fuel vapor. So these gases must be
prevented from entering the dispenser\'s metering device. In addition,
in order to be able to lift the fuel efficiently from an underground
storage tank to the dispensers---especially in a self-contained
system---the pipelines must be essentially free of air and vapor.

The most practical means of eliminating these gases is to keep the
entire system---from the storage tank to the delivery nozzle---filled
with liquid fuel at all times, even when the system is temporarily idle,
as it is between deliveries, or when the station is shut down overnight.
This could be accomplished in a number of ways, but the most practical
and efficient design incorporates a simple automatic valve, called a
[check valve]{.underline}. In the cutaway drawing shown in Figure 3-1,
you can see how one works. When fuel entering the valve inlet is under
sufficient pressure to push the valve off its seat against the
resistance of a spring [and]{.underline} the pressure of liquid on the
opposite side, the valve is opened, allowing liquid to flow through the
valve in the direction of the arrows. When the source of pressure
difference ceases, flow will discontinue, and the force of the spring
will reseat the valve, preventing liquid from flowing through the valve
in the opposite direction.

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  ![](media/image1.png){width="3.6777777777777776in"
  height="3.3229166666666665in"}

  FIGURE 3-1. AUTOMATIC CHECK VALVE
  -----------------------------------------------------------------------

In a fuel-dispensing system, a check valve permits liquid fuel to flow
toward the delivery nozzle, but never back toward the storage tank. A
check valve is located between the storage tank and the dispenser (as
shown in Figure 3-2), usually close to the outlet of the storage tank,
either at the angle joint where the pipeline drops vertically into the
tank (self-contained systems), or inside the discharge manifold that
sits atop a submerged pumping unit. (In some self-contained systems, a
[foot valve]{.underline}, located at the bottom of the intake pipe,
performs the function of the check valve.)

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  ![](media/image2.png){width="2.9493055555555556in"   ![](media/image3.png){width="2.8118055555555554in"
  height="1.6243055555555554in"}                       height="1.7770833333333333in"}

  FIGURE 3-2. LOCATION OF SYSTEM CHECK VALVES          
  ---------------------------------------------------- ----------------------------------------------------

As we proceed through the system, you will see other automatic valves
that operate in much the same way as these system check valves,
functioning to regulate the direction of flow or fluid pressure of the
product as it makes its way toward the meter and discharge hose. You
will also see how a similar valve, located at the opposite end of the
system, prevents fuel from draining from the discharge hose, assuring
delivery of the full metered amount of fuel.

**PRESSURIZING AND MOVING THE FUEL**

In retail fuel-dispensing systems, a motor-driven pump furnishes the
hydraulic pressure that moves fuel from the storage tank to the
dispenser, through the metering device, and to the discharge hose and
nozzle. As you know, two basic types of systems are in common
use---self-contained and remote pump systems. Most of

the systems you will encounter today will be remote pump systems. These
lend themselves to larger multiple dispenser installations. However, let
us take a look at each type.

**Self-contained Systems**

In self-contained systems, the pumping unit is located inside the
dispenser. Its major components are:

> • a pump
>
> • an electric motor
>
> • an air eliminator
>
> • flow regulating valves

Several manufacturers produce a self-contained unit, which includes the
pump, air eliminator, valves, and flow passages connecting them, all
enclosed in a single casing, as in Figure 3-3. The electric motor is
separate, and drives the pump by means of a belt and pulleys.

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  ![](media/image4.png){width="4.334722222222222in"
  height="3.466666666666667in"}

  FIGURE 3-3. SELF-CONTAINED PUMPING UNIT AND MOTOR
  -----------------------------------------------------------------------

The operation of a typical self-contained pumping unit is illustrated in
Figure 3-4. When the dispenser\'s on-off switch is placed in the "on"
position, the electric motor is activated, and begins to turn the
rotary-vane pump (other pump types may be used). As the pump turns
(clockwise in Figure 3-4), it propels fuel forward from its outlet. This
displacement of the liquid creates a partial vacuum at the pump inlet.

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  ![](media/image5.png){width="3.897222222222222in"
  height="3.5569444444444445in"}

  FIGURE 3-4. CUTAWAY OF TYPICAL SELF-CONTAINED PUMPING UNIT
  -----------------------------------------------------------------------

As long as the discharge nozzle remains closed, this vacuum is relieved
by fuel circulating continuously through the unit---you\'ll see how in a
moment. But when the nozzle is opened, suction pressure is transferred
instantaneously from the pump inlet all the way back through the
pipeline to the storage tank. There, atmospheric pressure bearing on the
surface of the fuel forces it to flow through the inlet pipe and check
valve we saw earlier, into the pipeline, and toward the dispenser. As it
enters the pumping unit, it passes through a [strainer or
filter]{.underline}, which removes any solid contaminants, and is drawn
into the pump inlet.

Small quantities of trapped air and fuel vapor must be removed from the
fuel before it passes to the meter. So, as it flows from the pump
outlet, the fuel, now under pressure (typically 18-25 psi) enters the
[air separator chamber]{.underline}. The shape of this chamber forces
the rapidly flowing fuel to swirl, with the result that the air and
vapor, along with a small amount of liquid fuel, are forced through an
orifice into the [atmospheric chamber]{.underline}. Here the fuel comes
to rest at atmospheric pressure. This allows the air and vapor to rise
to the top portion of the chamber, where they leave the dispenser
through a [vent tube]{.underline}. The level of the liquid that remains
in the bottom of the chamber rises until it raises the float, thereby
opening a valve that allows it to leave the chamber and be drawn along a
passageway back to the pump inlet.

At the same time, the main body of fuel, free of air and vapor, passes
from the separating chamber to the [automatic control
valve]{.underline}. This valve is similar in design to the check valve
we looked at earlier; it permits fuel to flow only in the direction of
the meter, never back toward the pump. It also regulates the pressure of
the fuel as it leaves the pumping unit and enters the meter, by closing
when pressure on both sides has equalized (when the dispenser has been
turned on but the nozzle remains closed, for example) preventing excess
pressure from building in the meter and hose.

The control valve also has a built-in [relief valve]{.underline}, which
normally operates only when the dispenser is shut off. It relieves
excess pressure caused by expansion of fuel on the discharge side of the
valve by allowing a small amount to pass back through an orifice in the
center of the control valve into the air separator chamber, where
pressure can be relieved into the atmospheric chamber if necessary. The
relief valve is very important, especially in warm-weather climates,
because fuel expands when heated, and this expansion could raise
pressure sufficiently to cause seals to fail, resulting in fuel leaks,
or even burst the discharge hose if not relieved.

When the pump is operating (dispenser switch is in the "on" position)
but the nozzle remains closed, the control valve will also close, as
soon as pressure is equalized. The resulting increase in pressure in the
separator chamber will open the [bypass valve]{.underline}, permitting
fuel to flow back to the pump inlet, thus relieving suction pressure and
preventing additional fuel from being drawn from the storage tank.

In some self-contained systems, especially older ones, these components
may be separate, and they may be referred to by different names. (For
example, the atmospheric chamber of the self-contained unit, when
separate, is usually called the sump.) But your basic knowledge of the
interrelated functions of the pumping components should make it possible
for you to recognize them, even when separated. In newer, larger fueling
facilities the use of self-contained systems is declining. Currently
less than ten percent of dispensers in new locations are self-contained.
Remote pumping systems and remote dispensers explained in the following
sections are more of today's norm.

**Remote pump (or remote dispenser) Systems**

In this type of system, fuel is pressurized and propelled toward the
dispenser by a pumping unit located at or in the storage tank. The basic
components of the pumping system are:

> • a motor and pump assembly and discharge head,
>
> • an emergency shut-off valve, located in each dispenser, and
>
> • a control valve, also located at each dispenser.

In rare cases remote pumping systems employ a rotary vane pump like that
used in self-contained systems, which sits atop the storage tank.
However most remote systems employ a [submerged pump]{.underline}. As
its name suggests, the pump and motor assembly are completely submerged
in the storage tank. As you can see in Figure 3-5, the pump---usually a
multi-stage vertical turbine---is at the very bottom of the assembly.
The intake of the submerged pump is located approximately four inches
from the bottom of the storage tank to reduce the possibility of pumping
water or sediment into the fuel system.

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  ![](media/image6.png){width="4.731944444444444in"
  height="2.3847222222222224in"}

  FIGURE 3-5. SUBMERGED PUMP AND DISCHARGE HEAD
  -----------------------------------------------------------------------

Fuel is drawn into it through a metal [strainer]{.underline} and flows
from the turbine around the outside of the motor casing and through a
section of pipe to the [discharge manifold]{.underline} (head), which
usually sits on top of the tank. The system check valve and the air
eliminator are located inside the discharge head (manifold). The air
eliminator functions in much the same way as its counterpart in a
self-contained unit: air and vapor, along with a small quantity of fuel,
are separated from the main body of fuel and drawn through an orifice.
But in this type of system, the gases and fuel drain directly back into
the storage tank; the tank itself functions as the atmospheric chamber
or sump does in a self-contained unit.

In remote systems, several dispensers are often served by the same pump.
When the on-off switch at any of these dispensers is placed in the on
position, the pumping unit is activated, and all pipelines connected to
the pump are pressurized (to 24-28 psi for a typical unit). To prevent
the discharge nozzles of all the [dispensers]{.underline} from being
pressurized, each dispenser is equipped with its own [control
valve]{.underline}.

This control valve must not be confused with the control valve in a
self-contained system: its design and function are quite different. The
control valve in a remote system is [not]{.underline} automatic: it is
actuated by the dispenser\'s on-off switch, or is controlled by the
electronic computer system. Figure 3-6 illustrates how a typical control
valve works.

The control valve itself usually consists of a [piston]{.underline} and
[cylinder]{.underline} (or diaphragm) and a [spring]{.underline}, as
shown in the cutaway drawing. A removable [filter/strainer]{.underline}
at the inlet to the valve (or at the fuel inlet to the dispenser) traps
solid contaminants in the fuel flow before they enter the valve. When
the piston is retracted, the valve is open and product flows through it
toward the meter; when the piston is seated, product cannot pass through
the valve.

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  ![](media/image7.png){width="4.840277777777778in"
  height="3.3131944444444446in"}

  FIGURE 3-6. REMOTE-DISPENSER CONTROL VALVE
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In older systems, the control valve may be operated mechanically, by
means of a linkage between the valve and the dispenser on-off switch.
Today, however, a more sophisticated electrical operator, including a
[solenoid]{.underline} and [pilot valve]{.underline}, has been
incorporated in most designs, as depicted in Figure 3-6. In some systems
both the main valve and the pilot valve are incorporated into the same
valve body. The device operates the valve by regulating the fluid
pressure in the cylinder behind the piston or diaphragm (the area marked
"B" in the drawing). When the dispenser is not being used (that is, the
dispenser switch is in the "off" position), the solenoid is not
energized and the pilot valve rests in such a position as to connect
fluid lines (1) and (2). This maintains equal pressure on all sides of
the valve piston, allowing the spring to keep the valve tightly closed.

When the dispenser switch is turned "on" or when the electronic computer
sends power to the valve, the solenoid is energized, opening the pilot
valve. This has the effect of opening a passageway between lines (2) and
(3), and simultaneously closing off line (1). As long as the discharge
nozzle remains closed, pressure remains the same in regions (A), (B),
and (C), and the control valve remains closed. However, when the nozzle
is opened, pressure at the outlet (C) falls, bleeding pressure from the
space behind the cylinder (B). Pressure is now higher at (A) than at
(B), and pressure on the head of the valve piston pushes it off its
seat. When the dispenser is shut off, the solenoid is once again
de-energized, closing the pilot valve and thereby shutting off the
connection between (2) and (3) and opening the connection between (1)
and (2). When pressure is once again equal at (A), (B), and (C), the
spring (or diaphragm) forces the piston onto its seat, closing the
valve.

The control valve prevents a remote dispenser from delivering product
unless it has been switched on, and thus prevents accidental discharge
under normal conditions. However, if a remote dispenser were to be
struck with sufficient force, as might happen if a moving vehicle
collided with it, even at relatively slow speed, piping in the dispenser
could quite easily be ruptured. Since the fuel entering the dispenser is
pressurized, this situation could cause fuel to flow uncontrollably from
the ruptured pipe, creating an extreme safety hazard. To prevent this,
every remote dispenser is equipped with an automatic [emergency shut-off
valve]{.underline}, also called an [impact valve or shear
valve]{.underline} because of its function, or a [fire valve (as shown
in Figure 3-7)]{.underline}.

+-----------------------------------------------------------------------+
| ![](media/image8.png){width="2.8895833333333334in"                    |
| height="3.001388888888889in"}                                         |
+-----------------------------------------------------------------------+
| FIGURE 3-7. REMOTE SYSTEM EMERGENCY SHUT-OFF                          |
|                                                                       |
| (IMPACT OR FIRE) VALVE                                                |
+-----------------------------------------------------------------------+

This valve is located at the bottom of the dispenser, connected to the
pipeline at the point where it enters the unit. The operation of this
valve is very simple. In the event of damaging impact, the top portion
of the valve shears away, isolating the damaged dispenser, and a spring
valve automatically closes off the pipeline, preventing any further flow
of fuel from that source. If it has not been too severely damaged by the
collision, the control valve in the dispenser should then respond to the
sudden drop in system pressure and close automatically, keeping fuel
loss from the dispenser also to a minimum.

Self-contained systems do not require an emergency shut-off valve
because fuel is pressurized [inside]{.underline} the dispenser. So, a
damaging collision will not result in uncontrolled flow from the storage
tank pipeline, since suction pressure will cease immediately when a
severe rupture occurs.

**Blended-product Dispensers (Blenders)**

The components that pressurize and move product in a blended-product
system are essentially the same as those described above. However, the
control valves in a blended-product system serve an additional function:
they must be capable of controlling the volume flow rate of product, so
that the blend will contain the correct proportions of component
products. Such devices are commonly called [metering]{.underline} or
[proportioning]{.underline} valves, and are somewhat more complex in
operation than the basic on/off control valve described above.
Blended-product dispensers have become more common in recent years and
their proliferation is expected in the future due to environmental
concerns with leaking underground storage tanks and associated piping.
With blenders a retail outlet can provide three or more products to the
consumer from only two underground storage tanks. Thus, the
environmental hazard is reduced by having fewer underground tanks.
Generally the blended product is delivered to the customer through a
single product hose. When taking a sample for fuel quality (octane),
special notice needs to be given to the flushing recommendations made in
EPO 22.

**Multi-product Dispensers**

Prior discussion on pressurizing and moving fuel has concentrated on
single and dual product dispensers. Many, if not most, dispensers used
in fuel outlets today are of the multi-product design. Here three or
more fuel products are available from a single dispenser. Usually only
one product at a time is available for delivery from each side of the
dispenser. Like the blended-product dispenser only one computer is
provided on each face of the multi-product dispenser. The multi-product
dispenser may have a hose and nozzle assembly for each product, or there
may be only one hose per dispenser side (face). In the case of the
single hose multi-product dispenser the three (or more) product
discharge lines are manifolded into one casting at or near the hose
outlet fitting. Special notice should be made of the requirements in EPO
21 for flushing the line before taking samples for fuel quality (octane)
testing.

**METERING AND INDICATING FUEL DELIVERY, AND COMPUTING PRICE**

Two major components are involved in metering and indicating the
quantity of fuel delivered and computing the price of the delivery. In
both self-contained and remote pump (or dispenser) systems, pressurized
fuel flows from the pumping unit (self-contained systems) or control
valve (remote systems) through a [meter]{.underline}, which measures the
fuel as it is being delivered. The meter is connected to the
[computer]{.underline}, which gets its name from one of its primary
functions, that of computing the price of the delivery. In the case of
mechanical, analog computing systems the meter is connected directly to,
and drives, the computer through by mechanical linkage. In the case of
electronic digital computers the computer receives its information in
form of an electrical signal (pulse) from the pulsing mechanism (pulser)
which is mechanically driven by the meter. The computer indicates the
amount of product delivered, the price per gallon, and may perform other
functions as well, as you will see. The meter is typically a mechanical
device, but the computer may be mechanical, electronic, or both.

**The Meter**

Metering devices in most gasoline and diesel retail fuel-dispensing
systems are [positive-displacement]{.underline} meters, so called after
the basic principle of their operation. In a positive displacement meter
finite quantities of fuel are separated into compartments of known
volume. These compartments may be cylinders within a piston meter,
segments between two vanes in a vane type meter, or the space between
rotors in other meters. The most popular type meter in retail
fuel-dispensing systems is the piston meter. This discussion will
concentrate on piston meter type of positive displacement meters.

A piston moving through a cylinder filled with liquid will displace a
quantity of liquid ahead of it. The amount of this displacement is
determined by the [bore]{.underline} of the cylinder (its inside
diameter) and the [stroke]{.underline} of the piston (the maximum
distance that it travels in one direction). The positive displacement
method of metering is employed in fuel dispensers because it is capable
of highly accurate measurement.

The meter itself consists of two or more (but usually no more than four)
reciprocating pistons (each in its own cylinder), intake and outlet
ports, and fluid channels. The pistons are connected to a crankshaft, or
other stroke-regulating mechanism, so that one cylinder is discharging
fuel during its piston\'s forward (discharge) stroke while another is
being filled during the backward (intake) stroke of its piston. This
provides a continuous flow of fuel through the meter. Figure 3-8 depicts
meters from several manufacturers . In some piston type meters the
pistons operate in the horizontal plane. In some systems the pistons are
in the vertical plane and convert their to reciprocating action to a
rotary shaft output to drive either the pulser or the mechanical
computer.

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  ![](media/image9.png){width="4.488194444444445in" height="4.20625in"}

  FIGURE 3-8. TYPICAL POSITIVE DISPLACEMENT METERS
  -----------------------------------------------------------------------

The pistons are driven by fluid pressure supplied by the system pump.
The cutouts on the piston heads function as valves, alternately opening
and closing channels in such a way as to allow fuel to enter the
cylinder during its intake stroke only from the inlet (pump side) and
allowing it to exit the meter on the discharge stroke only through the
meter outlet (nozzle side).

Other positive displacement metering concepts, as mentioned previously,
are used, primarily for high speed dispensers in truck stop
applications. In these metering systems a precise quantity of fuel is
isolated between vanes or rotor blades in the metering chamber. The
result is the same, highly accurate measurement is accomplished and the
dynamic movement of the fuel converts the fluid motion to a rotary shaft
motion to drive the computer, either mechanically or electronically.

The meter units are calibrated at the factory, and are designed to meter
fuel accurately and reliably. However, recalibration will be necessary
if the meter is found to be the source of over- or under-registration,
so an adjustment mechanism is built into the unit. Adjustment may be
accomplished by increasing or decreasing the [throw]{.underline} of the
pistons or by changing the size of the meter chamber. Changing the throw
has the effect of increasing or decreasing the pistons\' stroke, and
thereby increasing or decreasing their displacement. These adjustments
can be made in very small increments, changing the meter\'s discharge by
as little as 1/3 cubic inch per 5 gallons indicated (or an average of
less than 3/10,000 of a gallon per gallon indicated).

The adjusting mechanism may be located on the top of the meter or on one
of the piston caps. It may be a knurled knob, keyed disk, or calibrated
wheel (as in Figure 3-9), or have some different design. But it should
be immediately identifiable by one feature: the adjusting mechanism must
be designed to be protected with a security seal. This seal, if broken,
may indicate someone has tampered with the meter, or has made a needed
calibration adjustment. Without this assurance, an unscrupulous retailer
could adjust the meter to deliver [less]{.underline} than a gallon for
each gallon registered, and thereby charge customers for fuel that they
are not actually receiving. So, as part of your inspection procedure,
you will check this seal. With some of the high capacity systems, there
may be no adjustment mechanism on the meter. In these cases the
adjustment may be made electronically, you will need to refer to the
Certificate of Conformance for the specific model to determine how to
access the adjustment mechanism and the method of sealing used to
protect the adjustment mechanism. Figure 3-10 shows the adjuster portion
of several meters, which will be encountered in the course of field
inspections.

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  ![](media/image10.png){width="2.5902777777777777in"   ![](media/image11.png){width="2.4208333333333334in"
  height="2.94375in"}                                   height="1.7694444444444444in"}

                                                        ![](media/image12.png){width="2.2784722222222222in"
                                                        height="1.2743055555555556in"}

  FIGURE 3-9. TYPICAL METER ADJUSTERS                   
  ----------------------------------------------------- -----------------------------------------------------

  ----------------------------------------------------- -----------------------------------------------------
  ![](media/image13.png){width="3.0618055555555554in"   ![](media/image14.png){width="2.9590277777777776in"
  height="2.307638888888889in"}                         height="2.248611111111111in"}

  FIGURE 3-10. METER ADJUSTERS                          
  ----------------------------------------------------- -----------------------------------------------------

**The Computer**

Each time the meter\'s crankshaft makes a complete revolution, an exact
quantity of product has been metered. The quantity per revolution of the
meter crankshaft varies from manufacturer to manufacturer, and with the
meter design. So, the revolution of this shaft provides the most direct
and accurate indication of the amount delivered. Whether the computer is
mechanical or electronic, the computer or pulsing mechanism is driven
directly---or through a universal coupling---by the meter shaft.
Mechanical and electronic computers are quite different in design and
operation, so we will take a brief look at each separately.

**Mechanical Computers**

Only a small percentage of new dispensers placed into service
domestically have mechanical computers. Some of the dispenser
manufacturers have discontinued mechanical dispensers as part of their
product line.

What the motorist sees of a computer like the one shown in Figure 3-11
certainly does not look very complicated. Three sets of number wheels
are visible: one set indicates the price of the delivery, another the
amount delivered, and the third the price per gallon (unit price). Of
these, of course, only the first two sets actually move during a
delivery.

In fact, each computer has two identical groups of number wheels, one on
each side, so that the attendant and the customer can see how much fuel
is being delivered regardless of which side of the service island they
are using.

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  ![](media/image15.png){width="3.3291666666666666in"   FIGURE 3-11. MECHANICAL COMPUTER
  height="4.548611111111111in"}                         

                                                        
  ----------------------------------------------------- ---------------------------------

Each individual wheel turns independently, and the numbers inscribed on
it represent one digit of a decimal number. For example, the wheel
furthest to the right in the price set represents cents, the next number
represents multiples of 10 cents, the next multiples of 100 cents
(dollars), and so on. As the wheels revolve, a fixed pointer indicates
the values.

What the motorist does not see, the "insides" of the computer, does not
look at all simple: a multitude of gear wheels, drive shafts, chains,
and spring mechanisms. The design is indeed complicated, as you might
expect of a precision instrument, but the principle is really quite
straightforward.

The mechanical fuel-dispensing computer is basically a clockwork
mechanism. In fact, the computer is sometimes referred to as the clock,
as according to a technical definition, it is. As in a mechanical
grandfathers clock, trains of gears transmit and modify mechanical
action in stages. A mechanical computer works in a very similar way: one
gear train, deriving its movement originally from the meter crankshaft,
will operate the 1/10th gallon wheel (compare the second hand in the
grandfather clock). Another, deriving its movement from the [same
source]{.underline}, will operate the gallons wheel (compare the
clock\'s minute hand). And so on.

This is how the computer operates both price and volume wheels. However,
there is an additional element in the price computation, because---as we
all know the price of fuel changes frequently. So the system of gear
trains for the price wheels must be variable: otherwise, the station
owner would have to install a new computer each time prices changed!
This is accomplished by a component called the [variator]{.underline},
which is usually located in the bottom portion of the computer assembly.
The variator consists of a [cone gear]{.underline} and a series of
[range arms]{.underline}, one for each price number wheel.

The cone gear is actually nine separate gears, one for each decimal
value from 1 to 9. All nine revolve at the same rate---usually that of
the meter crankshaft. A single gear on the [range arm]{.underline} can
be raised or lowered to key to any one of the gears in the cone.
Changing the gear to which the range arm gear is keyed will result in a
higher or lower gear ratio, so the range gear will revolve more or fewer
times for each rotation of the cone gear shaft, and this rate will be
transmitted by the [range shafts]{.underline} to the gear trains that
operate the price number wheels. The range shaft is also connected
directly to the price-per-gallon wheels, so that when a change in price
is made by raising or lowering one or more of the range arms, that
change will be indicated automatically.

The volume and price indicator wheels must be reset to zero before each
delivery. We will discuss this function later. But in addition to the
indicators that the motorist sees, the computer also operates sale and
volume [totalizers]{.underline}. These are [not]{.underline}
automatically reset between deliveries. So the totalizers, as their name
suggests, indicate running totals, which are useful for inventory
control, and also protect the station owner from theft by employees,
since sales totals can be expected to match sales receipts. The
totalizers are required to be nonresettable. Some manual reset
mechanisms may be sealed so that tampering can not be done without
mutilating the seal and, thus, leaving detectable evidence. Again, this
is to protect the [owner]{.underline}, since totalizers are generally
not used as indicators for individual sales.

**Electronic Computers**

As you know, the modular design of most electronic systems permits the
incorporation of a variety of extended functions, so their functional
capabilities usually are considerably more extensive than those of
mechanical systems: they go beyond simply indicating the quantity of
fuel delivered and computing and indicating its price, as mechanical
computers generally do not. We will discuss some of these extended
features later in this chapter. For now, though, let us simply consider
the functions that correspond to those of a mechanical computer.

Understanding the technical details of electronic systems requires some
knowledge of various high-technology components---integrated circuits,
microprocessors, logic components, interfaces, and so on. Presenting
this basic knowledge is beyond the scope of this course. So keep in mind
that our discussion will be focusing on a [functional]{.underline}
description. The basic functional components we are concerned with are
illustrated in Figure 3-12.

+-----------------------------------------------------------------------+
| ![](media/image16.png){width="4.86875in"                              |
| height="3.890277777777778in"}                                         |
+-----------------------------------------------------------------------+
| FIGURE 3-12. FUNCTIONAL COMPONENTS OF AN                              |
|                                                                       |
| ELECTRONIC FUEL DISPENSING COMPUTER                                   |
+-----------------------------------------------------------------------+

The [transducer]{.underline} (or [pulser]{.underline}, as it is commonly
called) is coupled directly to the meter shaft, just as in a mechanical
computer. The function of the pulser is to transform the mechanical
action of the revolving shaft into digital signals. It does this by
generating a fixed number of discrete electrical pulses per revolution
of the meter shaft. (A discrete number of pulses are generated per
gallon delivered; while several dispenser manufacturers use 1000 pulses
per gallon, or 250 pulses per revolution of the meter shaft, that is not
so with all systems.)

The pulser is essentially a switch, which is actuated periodically by
the meter shaft (usually via a gear train). The terminals of the switch
are connected to an external power source. Each time the switch is
closed, voltage is applied; when the switch is open, voltage ceases to
be applied. In most systems the low voltage pulse is generated by a
metallic element making and breaking a magnetic field. In some cases the
magnets are imbedded in a disc and the magnet poles rotating in front of
a sensor generate the electrical pulse. The result, diagramed in Figure
3-13, is a single discrete pulse.

  -----------------------------------------------------------------------
  ![](media/image17.png){width="4.917361111111111in"
  height="3.482638888888889in"}

  FIGURE 3-13. GENERATING DISCRETE PULSES
  -----------------------------------------------------------------------

The [duration]{.underline} of individual pulses created in this way, and
the [intervals]{.underline} between them will vary with the rate of
rotation of the meter shaft, but their [value]{.underline} (voltage)
will be the same and, for the duration of each pulse, constant.

These pulses are transmitted as input to the [central processing unit
(CPU)]{.underline}. The first thing the CPU must do is recognize the
pulses as signals from the pulser and not from another input device; as
Figure 3-12 illustrates, even a very simple system like the one we are
looking at has various sources of input (zero reset devices for the
totalizer and pulser, and the price-per-gallon adjustment device), and
these must be distinguished from one another so that they can be
processed correctly. So the CPU checks the characteristics of the
incoming signal against information stored in its memory. Once it has
determined that the source of the signals is the pulser, the CPU is able
to process the information, "counting" the discrete pulses and computing
the volume being delivered and its price with each pulse received.
Again, its memory provides the necessary information for these
computations (pulses per gallon, price per gallon, number of pulses
already received, accumulated price).

The final step for the CPU is to convert the "results" of its
computations into signals that will actuate the indicating devices,
usually light-emitting diodes (LEDs) or liquid crystal displays (LCDs).
This [output]{.underline} is transmitted to the appropriate indicator
(dispenser price/volume indicator or totalizer) where the display is
generated automatically.

This all may seem complicated and even cumbersome when compared with the
relatively simple clockwork operation of a mechanical computer. But
remember that the functions we have just considered can be performed by
the electronic computer virtually instantaneously, employing very few
moving parts (only in the pulser), and with great accuracy and
reliability. At the beginning of this section, it was said that
computers can be both mechanical [and]{.underline} electronic. This is
often the case when an older system is adapted for use with an
electronic control console or other electronic devices.

Usually this involves linking a pulser to one of the shafts of the
existing mechanical computer (for example, the one-cent wheel shaft).
The important thing to keep in mind when inspecting such a device is
that the dispenser indicator is an analog device while the console
readout is a digital device. Handbook 44 includes special guidelines for
inspecting such installations.

**CONTROLLING THE OPERATION OF THE FUEL-DISPENSING SYSTEM**

All of the components of a fuel-dispensing system we have discussed so
far in this chapter operate automatically or like the remote dispenser
control valve---are activated by other components during fuel
deliveries. Of course, many of these components are adjustable, like the
meter and computer, but these adjustments are not part of the operating
function of the dispenser, and are normally made while the dispenser is
shut down.

Operating a gasoline pump is one of the few truly simple procedures that
most of us have to master in our adult lifetimes. One reason for this is
that there are relatively few controls to manipulate, and these master
controls are operated in a definite and invariable sequence. We will
conclude this chapter with a look at these system controls---some of
which are not as simple as they may seem.

**The Discharge Nozzle**

The customer receives exactly the quantity of fuel he or she wishes to
purchase from a properly installed and adjusted fuel-dispensing system.
Yet the customer does [not]{.underline} receive all of the fuel that has
passed through the meter between the time when the dispenser was
switched on and the time when it was switched off. These two statements
seem contradictory, but they are really not, and this puzzle---if you
have not already guessed the solution---provides a good introduction to
one of the fuel-dispensing system\'s primary control devices, the
discharge nozzle.

To summarize, the discharge nozzle performs three basic control
functions. It:

> • controls fuel delivery,
>
> • prevents vehicle tank over-fill (when the nozzle is so-equipped),
> and
>
> • prevents the discharge hose from draining after completion of a
> delivery.

Additionally the nozzle may be equipped with mechanisms which prevent
flow of fuel unless the nozzle is inserted into the fuel filler pipe of
the automobile. Some nozzles are equipped with pressure sensitive
devices which prevent the main poppet valve of the nozzle from opening
unless the fuel system is pressurized. In pre-pay situations nozzles are
sometimes replaced into their holders with the nozzle lever still cocked
or in the open position. This feature causes the nozzle poppet to close
when the dispenser is turned off and will not allow flow until the
system is again pressurized (by turning the dispenser on) and the nozzle
lever released and then reopened.

Of these three functions, the last two are fully automatic, and require
no action on the part of the operator (either the station attendant or
the self-service customer). However, the [nozzle outlet
valve]{.underline} (see Figure 3-14) must be opened before fuel can flow
into the customer\'s vehicle. In fact, fuel does not flow through
[any]{.underline} part of the system (except the self-contained air
eliminator and by-pass circuit and the submersible pump riser tube and
air eliminator) until the discharge nozzle is open. The operation of the
nozzle outlet valve, by means of the [control lever]{.underline}, causes
fuel to flow---or cease flowing through the entire system; it also
regulates the rate of delivery, so the valve must have a continuous
range of openings.

  -----------------------------------------------------------------------
  ![](media/image18.png){width="5.142361111111111in"
  height="4.615277777777778in"}

  FIGURE 3-14. DISCHARGE NOZZLE
  -----------------------------------------------------------------------

In most locations retail dispensers are required to be equipped with
[automatic shut-off devices]{.underline}, which prevent fuel from being
delivered once the vehicle tank is full, thus reducing the risk of
over-flow and resulting spillage, which is always hazardous.

There are several different designs of automatic shut-offs in common
use. One design (shown in Figure 3-14) incorporates a small air tube,
called a [venturi]{.underline}, which runs down the length of the
nozzle. The shut-off mechanism is designed to "breathe" through this
tube. As long as air can pass through the tube, the delivery is
regulated only by the outlet valve. But when the entrance to this tube
is blocked---by touching liquid fuel, or anything else---even
momentarily, the shut-off mechanism automatically trips the outlet valve
shut, preventing delivery until the venturi is again unobstructed.

When the discharge nozzle is opened at the beginning of a delivery, fuel
flows from the nozzle outlet immediately. This is because the discharge
hose, like the rest of the dispensing system, is filled with liquid fuel
at all times. (For this reason, retail motor-fuel dispensers are
referred to as "wet" hose devices.)

Here, of course, is the answer to the puzzle that introduced this
section: the fuel that fills the hose at the beginning of a delivery
actually passed through the meter during the [previous]{.underline}
delivery. The assurance that the customer always receives exactly the
quantity of fuel he or she is paying for is the principle of
displacement. Fuel metered during a delivery will initially displace
exactly the quantity contained in the discharge hose.

Fuel usually flows [with]{.underline} gravity through at least part of
the discharge hose. So, in order to keep the hose full at all times, and
assure accurate deliveries, the nozzle must have some means of
preventing fuel from being drained from the hose after the dispenser has
been shut off. If this was not done, one customer might receive more
fuel than he or she has paid for by draining some of the fuel in the
hose, while the next customer could receive [less]{.underline} than he
or she has paid for, because at the beginning of the delivery, fuel
flowing through the meter is displacing air, not liquid fuel.

The device that prevents this is called the [antidrain
means]{.underline}. In retail systems its operation is automatic. It
works by permitting fuel to flow [only]{.underline} while the system is
pressurized by the pump. Many newer nozzles will not allow the main
poppet to open unless the system is pressurized by the pump.

The pressure-regulating valves in the pumping unit (or control valve in
a remote system) will maintain stable pressure as long as the dispenser
is turned on. System pressure drops immediately when the dispenser is
turned off, and this drop in pressure activates the antidrain means,
preventing further delivery or draining of fuel from the nozzle and
hose.

As mentioned in the last chapter, many States now require that gasoline
pumps be equipped with vapor recovery systems. Over the past few years
there has been a proliferation of states or local jurisdictions
requiring vapor recovery. Use of these systems is expected to grow in
the future. Since the design and operation of the discharge nozzle is
substantially affected by the incorporation of vapor recovery, this is
an appropriate place to describe this feature briefly.

Technology in vapor recovery equipment is expanding as rapidly, keeping
pace with the expanding requirements for use of vapor recovery. At one
time the most apparent component of the [balanced vapor recovery
system]{.underline} was the "boot," a semi-rigid corrugated tube that
surrounds all but the very tip of the discharge spout. In order to pump
gas, the end of the nozzle must be pushed firmly into the vehicle fill
pipe and held in this position, so that the boot is compressed, with its
lower end seated tightly against the rim of the fill pipe, as shown in
Figure 3-15a. Additionally Figure 3-15b gives more detail of the
internal parts of a [balanced system]{.underline} vapor recovery nozzle.
If insufficient pressure is applied, or if the seal is broken, the
automatic shutoff mechanism remains disengaged, and the nozzle can not
be operated.

  -----------------------------------------------------------------------
  ![](media/image19.png){width="4.529861111111111in"
  height="4.529166666666667in"}

  FIGURE 3-15a. VAPOR RECOVERY NOZZLE
  -----------------------------------------------------------------------

  -----------------------------------------------------------------------
  ![](media/image20.png){width="6.004861111111111in"
  height="3.2284722222222224in"}

  FIGURE 3-15b. VAPOR RECOVERY NOZZLE
  -----------------------------------------------------------------------

As can be seen in Figure 3-15a, when product is being delivered, air and
vapor inside the receiving tank, which are displaced by the rising
liquid level, pass up the fill pipe and through the clearance between
the boot and the discharge spout (this clearance is only open when the
boot is compressed). The vapor is kept separate from the liquid flow
inside the nozzle, and enters a hose at the dispenser end, which leads
back to the air space of the facility\'s product storage tank for that
product.

Since product is being drained from the storage tank at the same rate as
vapor is discharged from the receiving tank, pressure is continuously
equalized, and because the system remains closed, vapor does not escape
to the atmosphere. Most vapor recovery systems now incorporate a
concentric "hose-within-a hose" design, like that shown in Figure 3-17,
with the product hose running inside the vapor hose. This makes the hose
less unwieldy. With the [balance system]{.underline} the outer of the
two hoses is the vapor return line and the inter hose the fuel line.

More recently introduced into the market place are systems which have
[vacuum assist]{.underline}. A vacuum pump or venturi within the system
develops a vacuum which helps to "pull" the gasoline vapors from the
automobile's fuel tank and "push" them into the underground storage
tank. Some of these systems have a boot which is much smaller than the
balanced system boot as shown in Figure 3-16a. In these systems there is
not a requirement for the seal at the interface of the nozzle bellows
and the automobile fill pipe as with the balance system. Here the
bellows end serves more as a funnel to direct the vapors into the
nozzle, and thus back to the underground tank.

  -----------------------------------------------------------------------
  ![](media/image21.png){width="5.864583333333333in"
  height="4.491666666666666in"}

  FIGURE 3-16a. VACUUM ASSIST VAPOR RECOVERY NOZZLES (WITH BOOT)
  -----------------------------------------------------------------------

Still other nozzles used in [vacuum assist]{.underline} systems have no
boot as shown in Figure 3-16b. In these systems the vapor is drawn from
the automobile fill pipe through small holes near the nozzle tip. Vacuum
assist nozzles of this type are hardly distinguishable from a
conventional automatic shutoff nozzle. The hose is slightly larger than
a conventional hose since it must contain both vapor and fuel hoses in a
coaxial arrangement. With vacuum assist systems, since there is an
"assist" to get the vapors to the underground tank, the vapor line
generally is the smaller inter hose. Fuel flows through the outer hose
in the concentric hose design. This arrangement allows for a smaller
hose than the balanced system hose.

  -----------------------------------------------------------------------
  ![](media/image22.png){width="5.779861111111111in"
  height="4.759722222222222in"}

  FIGURE 3-16b. VACUUM ASSIST VAPOR RECOVERY NOZZLES (BOOTLESS)
  -----------------------------------------------------------------------

The vapor recovery system is quite effective in reducing vapor emission
during delivery. However, the weights and measures inspector must be
aware of factors that can affect measurement accuracy during testing.
One of these is the tendency for some product to become trapped inside
the boot or within the vapor return portion of the hose at the end of a
delivery, especially if the operator attempts repeatedly to over-fill
(top-off) the tank. Although the amount of liquid trapped after any
single delivery is likely to be small, it can accumulate in the boot and
back into the vapor recovery hose. When the nozzle is held in a nearly
vertical position (as it is when filling a test measure) the trapped
liquid will drain from boot and/or hose. It is therefore recommended
that when testing vapor recovery equipment any trapped liquid be
carefully drained before commencing a test draft (for procedure, see
Chapter 4).

Figure 3-17 shows two types of hose used with vapor recovery systems.
The larger hose on the left is used in balanced vapor recovery systems,
where the larger outside hose is the vapor hose. The smaller hose on the
right is used in assist vapor recovery systems. In these systems the
vapor hose is the smaller inside hose.

  -----------------------------------------------------------------------
  ![](media/image23.png){width="6.426388888888889in"
  height="4.450694444444444in"}

  FIGURE 3-17. VAPOR RECOVERY HOSE SYSTEMS
  -----------------------------------------------------------------------

**Other Manual Controls**

When the dispenser is turned on, two things happen:

> • The dispenser\'s computer is reset to zero.
>
> • The system pump is activated, in a self-contained dispenser; in
> remote systems, power is applied to the submersible pump, and the
> control valve is opened (a short delay, 2-4 seconds, may be
> experienced between the time the pump started and the valve is
> opened).

The sequence must happen in this order to assure accurate measurement
and indication of quantity and price. Systems vary widely in the method
used to perform the dual function to reset the computer and start fuel
flow. For mechanical computer systems and some electronic computer
systems the nozzle is lifted from its normal hanging position and the
on-off lever (as Figure 3-18) is rotated or lifted to cause these
functions.

  -----------------------------------------------------------------------
  ![](media/image24.png){width="4.485416666666667in"
  height="3.3583333333333334in"}

  FIGURE 3-18 DISPENSER CONTROL LEVER
  -----------------------------------------------------------------------

Many electronic systems use selection of a grade, price, or pressing a
"push to start" button or icon to perform the start function. Safety
regulations prohibit lifting of the nozzle from being the only action
required to set the pump or dispenser into its fuel delivery mode.
Regardless of the operating method, the dispenser computer is reset to
zero and then pumps are turned on and valves opened to allow fuel
delivery from the nozzle.

In some systems, the pump control switch and zero reset are operated by
separate controls. In this case, an interlocking device prevents the
pump control switch from being activated first. This design is most
common in self-service facilities, with the zero reset operable only by
means of a key held by the attendant. This feature prevents unsupervised
self-service deliveries.

Dispensers are designed in such a way that the nozzle cannot be
reinserted in its hanger until the on-off control has been returned to
the off position. This feature is intended to prevent a dispenser from
being inadvertently left on after a delivery has been completed,
permitting the dispenser to be used again before the computer has been
reset. In some cases merely replacing the nozzle into the nozzle
receptacle (boot) performs the action to shut-off the dispenser. A
shut-off switch linked to a "flap" within the nozzle boot deactivates
the dispenser when the nozzle tip strikes the "flap." Some dispensers
with the flap type actuators also have a stop button located on the face
of the dispenser, either pushing the stop button or replacing the nozzle
will deactivate the dispenser. Some systems require that the start-stop
lever be rotated to the off position (as discussed previously) or part
of the nozzle hanger mechanism to be pushed downward to shut the device
off and allow replacement of the nozzle.

**Service, Payment, and Product Selector Controls**

Until the mid 1980s, the only controls on most gas pumps were the on/off
control lever and the discharge nozzle control lever; all other
operations were automatic. However, advances in technology, especially
in the area of electronic computers, and new marketing
strategies---including self-service, blending, single hose multi-product
dispensers, multi level pricing, and card readers or cash acceptors in
the dispenser or on the island---have given the consumer choices to
make. This variety of choices and operating characteristics also make
the task of pumping gas somewhat more complicated.

The late 1980s and early 1990s brought a surge of multi-tier
pricing---discount for cash, cash/credit, and full-serve/self-serve.
With the exception of full-serve/self-serve, the other multi level
pricing marketing strategies have pretty much gone by the way side. In
most installations full-serve/self-serve are handled with dedicated
islands according to the type of service. Seldom are both offered
through the same dispenser.

In most cases selection of the cash or credit payment mode,
[today]{.underline}, relates to "in pump" cash and/or card acceptors and
has no impact on the price the consumer pays for the product. The few
facilities that offer discount for cash (cash/credit) generally have
controls on the dispenser for selection of the payment method. An
example of such a dispenser is shown in Figure 3-19.

  -----------------------------------------------------------------------
  ![](media/image25.png){width="5.852083333333334in"
  height="2.515277777777778in"}

  ![](media/image26.png){width="2.6659722222222224in"
  height="2.5944444444444446in"}

  FIGURE 3-19. MULTI-PRICING DISPENSER
  -----------------------------------------------------------------------

In 1988, the National Conference on Weights and Measures adopted the
policy that "the use of a single-price-computing dispenser for sale of
motor fuel at multiple unit prices is inappropriate, facilitates fraud,
and should be eliminated." As you will learn in Chapter 5,
nonretroactive changes were later made to Handbook 44 in support of this
policy.

The cash and credit unit prices for each product offered at this
multi-product dispenser are displayed beneath the markings that identify
the product dispensed from each hose (premium unleaded, etc.). As the
instructions in the upper left corner of the display panel indicate, the
operator first selects the method of payment by pressing either the cash
or credit pushbutton. When the product is selected, by raising the
nozzle rest (the control lever on this dispenser), the price-per-gallon
display for the current sale (on the right hand side of the panel in
this example) is automatically updated to display the selected price for
the selected product. For example, if  "credit" and "unleaded plus" were
selected, this dispenser would display a price per gallon of \$1.149.
The computer would also be set automatically to compute the total sale
at the selected price per gallon.

Equipment like that shown in Figure 3-19 could be adapted to provide
multi-tier pricing and computing capability for an additional type of
service (full- or self-serve). This would require the display of twice
as many unit prices, and additional selector buttons. Or, a more likely
scenario is that manufacturers would probably design equipment to
"scroll" the unit price for the various modes since Handbook 44 does not
require all the unit prices to be simultaneously displayed.

As described in the last chapter, blended-product dispensers are capable
of delivering several different products from the same dispenser. In the
case of multi-hose blended-product dispensers, like the one shown in
Figures 3-19 and 2-11, the operator selects the desired product by
operating the control lever for the corresponding hose (which is marked
to indicate the product dispensed). For single-nozzle blended-product
dispensers, the operator usually selects the product by means of a
separate control on the dispenser (selector dial, pushbuttons, etc.).
When this is done, the computer and display are automatically updated
for the unit price of the selected product.

As you will learn shortly, Handbook 44 includes a number of regulations
relating specifically to selector type controls and multi-pricing
displays. These regulations are intended to minimize confusion for the
customer and discourage abuse or fraud by the owners or operators of
such sophisticated fuel-dispensing equipment.

**Electronic Control Consoles**

Electronic control consoles and point of sale (cash registers) systems
are being installed in more and more fueling facilities, because of the
popularity of self-service, and because of the many additional features
and efficiency they provide. The modern console is more than a display
that duplicates the current sale information on the dispenser. It is a
valuable business tool which aides in inventory control, shift
performance, and the like.

As was the case with electronic computers, we can only undertake a
simplified, functional description in this course. However, you should
also have some familiarity with the basic "external" features of these
devices, since remote consoles are, at least in part, indicating
devices, and so must be checked in the course of your inspection.

You will recall from our discussion of electronic computers that two
types of devices were linked to the CPU: input devices (the pulser,
price adjustment, zero resets) and output devices (dispenser and
totalizer displays). Control consoles incorporate functions of
[both]{.underline} input and output devices, as illustrated in Figure
3-20 on the following page.

  -----------------------------------------------------------------------
  ![](media/image27.png){width="4.926388888888889in"
  height="4.071527777777778in"}

  FIGURE 3-20. CONTROL CONSOLE AS A FUNCTIONAL ELEMENT
  -----------------------------------------------------------------------

As [output]{.underline} devices, they:

> • show the status of each dispenser (on or off);
>
> • provide readouts of the same delivery information (quantities
> delivered, price and price per gallon) as shown on the dispenser; and
>
> • display sale/transaction information sent as input to the CPU, and
> the results of any computations made (e.g., change due, credit OK).

As [input]{.underline} devices, control consoles may be used:

> • to pre-set the quantities or total price of fuel to be delivered
> (pre-pay);
>
> • to control individual dispensers, allowing deliveries only upon
> "authorization" by the console operator;
>
> • to provide emergency shut-off control for one or all dispensers;
>
> • as a cash register (point of sale terminal), to enter non-fuel
> purchases and payment received;
>
> • to transmit credit card numbers for verification and authorization;
> or
>
> • to program the CPU (to change the price per gallon, or to collect,
> process, and store or communicate data from other input devices).

The external features of the control console are its displays, keypad,
and control buttons. To get a better sense of these "external" features,
let us take a look at a typical control console, like the one shown in
Figure 3-21.

  -----------------------------------------------------------------------
  ![](media/image28.png){width="4.822222222222222in"
  height="4.423611111111111in"}

  ![](media/image29.png){width="2.7222222222222223in"
  height="2.5770833333333334in"}

  FIGURE 3-21. FEATURES OF TYPICAL CONTROL CONSOLES
  -----------------------------------------------------------------------

As you can see, the display area placement varies with different
manufacturers, but generally the consoles have similar display and
keyboard layouts. The upper area displays volume, price, and price per
gallon, and also a numerical identification of the dispenser (hose) for
which this delivery information is being displayed. The lower display
shows the status of each dispenser. Information for these displays is
"called up" by depressing one of the dispenser identity buttons (there
are 12 on this model) These two displays represent the output device
functions of the control console.

The keypad in the center provides the operator with the means of
entering information or commands from the console in a variety of
"modes."

The buttons on the right of the keypad are used to send information to
the CPU, while the buttons on the left are used to send particular
commands, via the CPU, to the individual dispensers (authorizing them or
halting them), or to request transmission of data to another output
device (a printer). In addition, an emergency switch controls the entire
dispensing system.

The console is designed in this way to facilitate the monitoring and
control of the operation of an entire fuel-dispensing system by a single
operator.

# SUMMARY

A number of separate elements work together to assure the safe,
efficient, and accurate operation of a retail motor-fuel dispensing
system. Because of their functional interdependence, the failure or
malfunction of one element can inhibit the effectiveness of others and
impair the correct operation of the entire system. Specific requirements
relating to the selection, installation, maintenance, and use of these
elements are described in detail in Chapters 5 and 6.