Many Small Instruments Play Big Part in Operation of Today’s Fire Apparatus

Many Small Instruments Play Big Part in Operation of Today’s Fire Apparatus

Panel of 1,250-gpm pumper includes individual discharge gages as well as electric water level gage for booster tank and electric hour meter 1,250-gpm pumper discharge gages as for recording hours and minutes of pump operation Universal Fire Apparatus Corp.Pump panel of high volume engine in Gary, Ind. Bottom row of gages record pressure for each of six discharge gates Maxim Motor Co.

The modern fire engine is a complicated and costly piece of mechanical equipment. Its safe, efficient operation depends not alone upon skilled personnel but also careful utilization of scientific measuring and indicating instruments

IF YOU WERE TO ASK the student of fire fighting what has been the most noteworthy advance in the development of fire lighting facilities, he could well answer in one word—Mechanization!

Mechanization came into the fire service with the advent of the steam fire engine. The first true mechanical “engine” put more power behind fire streams and gave “horsepower” a new meaning and importance.

This event also marked the introduction of measuring and indicating instruments necessary for effective, safe operation of the engines. However, the gages employed were not so complicated that the then “engineers” could not easily master them. About all the operator had to do was to watch his boiler water level

sight glass and steam pressure gage, both usually located on the rear of the boiler. Two other gages on the sides of the steamer measured pump pressure and vacuum.

Even on the historic self-propellers the apparatus driver had no dash instruments to watch. These came later with the introduction of the internal combustion engine, which depended on indicators for proper operation. As the automotive fire engine grew in stature, instrumentation followed suit.

Most of the early motor pumpers and ladders were equipped with only the basic dash panel instruments of the day

such as a meter to measure electrical current, an oil sight indicator and a speedometer.

Few vehicles had gasoline gages— supply level was found by inserting a marked stick in the fill pipe of the fuel tank. Engine operating temperature was indicated by a thermometer sight glass fixed to the radiator cap.

Pumping instruments consisted of the same compound and pressure gages found on the old steamers, usually mounted directly on the pump in an exposed position. Little consideration was given the possibility of freezing or of lighting for night operations.

As motor fire apparatus advanced in design and construction, adding new speeds, controls and mechanisms, the necessity for indicating operating conditions became more imperative. This spurred the introduction of new devices and improvements on the original instruments.

Inset panel of modern volunteer fire company pumper including tachometer, individual gate pressure, water level and temperature gages well lighted for night operation —Young Fire Equipment Co.Front-mount pumps usually are provided with a minimum of instrumentation. Apparatus shown includes tachometer in addition to pressure and compound gages—American Fire Apparatus Co.

Night illumination of instruments, their flush mounting and solution of the cold weather freezing problem followed.

It has always been recognized by the fire service that because of the critical nature of their functions, every piece of automotive fire equipment and its component parts must be of the highest quality. This applies to all operational instruments used on apparatus; it requires that they be accurate, reliable and easily read; that individual gages and indicators be clearly identified.

Bourdon gage consists of hollow tube, oval in cross section, and closed at one end. Pressure admitted at open ends tends to straighten tube which is in 270-degree arc when at rest; vacuum tends to increase curvature. Link at closed end of tube is connected to sector which transmits minute movements through gear to dial pointer. Principle is used on pressure and compound pumping gages as well as in mechanical oil pressure gages on engine Marsh Instrument Co.


Perhaps the most underrated instrument on the dash panel is the ammeter. It indicates current in the electrical system which to the common gasoline engine is of equal importance with the fuel. With the present trend to greater use of radio and increased signal and warning light installations on apparatus, the ammeter is becoming more important to the operator.

It is placed in the circuit connecting the vehicle battery and generator and indicates when the generator is charging and how much electricity is consumed when the battery is supplying the requirements of the system. It also serves as a rough indicator of the condition of the battery.

If the charging rate increases gradually over a period of several runs, it is a warning that one or more of the battery cells are shorting. If the time required for the charging rate to drop back to normal is unusually long, it indicates that the battery has been discharged more than usual.

When the apparatus engine is stopped and the ignition turned off, the pointer should return to zero. If it continues to show a discharge it is a warning that electrical equipment has been left on or a short exists somewhere in the system. The driver should immediately attempt to locate the trouble and if he cannot, a mechanic should check the apparatus. If this is not done quickly the battery may be exhausted and the engine will not run.

Oil pressure

This gage tells the operator whether or not the engine lubricating system is operating. It is mounted on the dash panel in all vehicles but it may be duplicated on many pump panels.

Oil gages are of mechanical or electrical type. The mechanical unit consists of a “bourdon” tube with a pointer attached. Pressure is introduced by means of a small tubing connected to the engine oil system.

The electrical gage consists of a sending unit mounted on the engine and connected to a dash-mounted receiving unit by means of a wire. Oil under pressure is admitted to the sending unit where it exerts force on a diaphragm. Connected to the diaphragm is a sliding contactor which moves across a resistor according to the force exerted and in turn regulates the flow of current through the wire to two electromagnets in the receiving unit.

With no oil pressure in the engine, all of the electric current passes through one of the magnets holding the dial pointer, which is pivoted between both, at zero. As the pressure builds up, the current is divided by the resistor between both magnets and the dial pointer moves according to the greatest attraction. This movement is calibrated at the factory for the pressure required by the engine design.

With the engine running the pressure indication should be at or about that stated in the manufacturer’s instruction book. It is possible that a lower pressure may be recorded if the engine speed is less than that necessary to maintain 20 mph when the vehicle is moving.

Instruments are clustered in two locations on this pump panel. Standard group at left; pressure gages at top right are for individual discharge gatesIn addition to individual gate gages, panel of this pumper includes electric tank gage. Hale hydraulicoperated stage valve with position indicator, and hour meter included in same case with tachometer

Rapid fluctuations of the pointer should be investigated as they are usually caused by low oil level. No pressure indication is a warning to stop the engine immediately and investigate the failure.

Fuel gage

The same principle of electrical sending and receiving units is used for gasoline gages, with the exception that the sending unit consists of a tank float connected by an arm to the resistor contactor. The position of the tank float on the fuel level is reflected in the calibrated magnetic attraction exerted on the pivoted pointer of the gage.

The foregoing are basic to all apparatus. Many vehicles, in addition, are equipped with air brakes and the dash panels of these units will include an air cylinder pressure indicator and a warning light or audible signal device which gives notice of low pressure in the brake system.

The modern apparatus panel also includes a number of pilot lights which show when the ignition is on, that lights are displayed and that radio or other accessories are being used. These are important sight indicators and should be considered in the same category with gages, although their use is self-evident and little or no explanation is necessary.

Tachometers and speedometers

Tachometers and speedometers are essentially the same instrument and frequently duplicated on both the dash and pump operator’s panel.

The tachometer is valuable to the operator for indicating pumping conditions especially in regard to the proper use of a multi-stage centrifugal pump. The operating manual supplied by the manufacturer is the correct guide for each individual case, but the fire service has developed a “rule-of-thumb” which works very well under most conditions.

It states that the proper stage in which to operate at any given time is that which results in the desired pressure being developed at the lowest engine speed. Without a tachometer the rule would be difficult to follow.

Familiarity with the instrument as it applies to each pumper or ladder should be a primary objective of fire department operators as observations of its indications will frequently give warning of improper conditions.

If the speed of the engine is higher than normal, it may indicate a slipping clutch. This is possible during driving or pumping. A high engine speed when pumping may indicate that the transmission is in the wrong gear.

The difference between tachometers and speedometers is the point of connection on the vehicle and the resulting dial calibration. The tachometer is calibrated to measure engine revolutions and is driven by a rotating part of the motor such as the cam shaft, generator shaft, distributor, etc. It registers the rotations per minute of these parts and frequently is connected at a point which rotates onehalf the true engine speed. When such is the case the dial is calibrated to read twice the shaft speed.

Speedometers operate from a drive gear built into the transmission. This gear is designed by the manufacturer to give correct revolutions per mile to the drive cable connected to the speedometer head. For example, it may be so designed that 1,001 revolutions occurring in one minute will indicate 60 mph.


A total register connected to and built in the speedometer case, it visibly displays the number of miles the vehicle has traveled and is driven through a series of gears originating at a worm gear cut on the speedometer shaft. It consists of five figure wheels numbered from one through zero and so constructed that as one wheel completes a revolution, it turns the next figure wheel to the left one-tenth of a revolution.

On many pumpers the speedometer gear is included in that part of the transmission which drives the pump. This permits the odometer to register at all times and gives an indication of the actual over-all work of the pumper, whether it is moving over the road or pumping.

Tachometer and speedometer are essentially the same instrument. Magnet is connected to rotating shaft; non-magnetic speed cup, influenced by eddy currents set up by revolving magnet, moves in proportion to speed of magnet. Pointer registers rpm's or mph depending on point of connection to vehicle. Mechanical unit is connected to rotating part by flexible shaft. Electrical style uses sending unit which supplies current through wire to motor attached to magnet

Some tachometers include an odometer to record the equivalent miles the engine has worked. This is usually included on vehicles where an odometer is not provided in the speedometer or if the drive gear for the latter is not in the gear train which powers the pump.

Other manufacturers supply total recorders termed “hourmeters. These are odometers connected to the tachometer and calibrated for the average engine operating speed. One manufacturer supplies a style which is based upon an average of 100,000 rotations per hour.

Heat indicators

An important indicator to all drivers, but in particular to pump operators, is the engine temperature gage. In addition to its standard dash mounting, it is frequently duplicated on the pump panel where it is used to give warning of the necessity of using auxiliary cooling during long pumping stands.

This is an important consideration upon which not only proper performance depends but in addition, the operating life of the engine may be prolonged by its proper application. Continuing to operate an overheated engine may cause permanent damage to its working parts.

Pump pressures

Two of the most important gages are used only for pumping; these are the pump compound and the pressure gages. Both are identical in construction, only the calibration and point of connection to the pump being different.

Compound gages are used on the suction side of pumps and are designed to measure in inches of mercury (sometimes referred to in the fire service as “inches of vacuum”) any pressure below atmospheric pressure which may be developed by the pump. (The zero reference point on a pump gage actually represents atmospheric pressure.) The same gage is also calibrated in pounds per square inch above the zero and is useful in determining residual pressure when pumping from hydrant or in relay.

Some fire officers have expressed a preference for the use of compound gages on both the intake and discharge side of a centrifugal pump with the contention that priming action may possibly damage a pressure gage during this operation. However, all manufacturers will provide pressure gages for use on fire apparatus which will not be damaged by priming. The present National Board of Fire Underwriters specifications, contained in Pamphlet 19, include a provision that gages of this type shall be supplied with centrifugal pumps.

The inclusion of individual pressure gages for each pumper discharge gate has become very popular in the fire service. When used in conjunction with modern controlling gates they permit a pump operator to overcome the problem of supplying various hose lengths and nozzle tips at one time. Instead of averaging the line requirements and setting the pump pressure to meet the average, the individual gage and controlling gate permit each line to be supplied at optimum pressure as desired. This results in increased operating efficiency, as well as greater safety for the nozzlemen.

Dash of modern pumper has cean uncluttered look. Dial face at right behind wheel includes ammeter, oil pressure, fuel and temperature gages clustered around the speedometer. All engine controls are conveniently mounted for easy use by the driver Seagrave Corp.Principle of electric gas gage operation. 1. Battery current energizes left magnet coil and goes to ground through path of least resistance via tank float connection. 2. Contactor position on sending unit provides equal resistance path for current which divides. Energized right coil causes pointer to assume balanced position between magnetic field of both coils. 3. Sending unit resistance is greater than right magnet coil. Most current flows to ground through coil. Greater magnetic flux at right coil causes armature to move pointer to full position

AC Spark Plug Div.

Special panel gages

The increased popularity of large booster tanks has caused a demand for inclusion of tank water level gages. This has resulted in the adaptation of the electrical fuel gage to the purpose by some manufacturers, while others have designed a “sight glass” for visibly displaying the level of the water. The latter is an adaptation of a boiler “water glass” and works on the same principle. Special precautions are taken to prevent possible freezing in cold weather.

A second type of sight gage of a somewhat unusual style is the indicating proportioner incorporated in some large industrial foam and wetting agent pumpers. These are used to secure proper and accurate proportioning of foam liquid and 1 and 2 per cent wetting agents with water; they are useful above 100-gpm flows.

Booster water level is shown in sight glass type of indicator on this rural pumper. Special engineering prevents freezing of gage during cold water operation. Recessed lighting illuminates the column for excellent visibility under all conditionsAerial ladder operating pedestal mounts hydraulic oil pressure gage and extension indicator (in window behind rotation control). Inclinometer with safety scale is located on bed ladder adjacent to operation position

Two gages are used, one for water and one for the solution being proportioned. If both 1 and 2 per cent wetting agent are to be supplied by the same pumper then three flow meters are necessary.

The meters are of the “armored” type, consisting of a tapered stainless steel tube in which is placed a magnetic float. An indicator ring is fitted around the metering tube on the exterior and by magnetic attraction follows the float. Only the indicator is viewed through the sight glass, which is calibrated by graduations; the liquid being proportioned does not actually contact the glass.

In operation the desired discharge flow is adjusted by regulating the pump performance. A portion of the water enters the small end of the tapered metering tube, rises and contacts the float. The float creates a variable ring orifice or annulus between its own diameter and the tapered inner wall of the metering tube.

The area available for flow past the float depends upon the position which the float seeks in the tube.

As the flow rate increases, the upward fluid pressure plus the fluid buoyant effect exceeds the downward pressure due to the weight of the float. This differential raises the float, increasing the annular area between the tube and the float, until the upward and downward forces are equalized in dynamic balance; at this point the float elevation in the tapered tube is proportioned to the fluid rate flow.

The operator then admits the agent to be proportioned to the second flow meter. This is accomplished by an auxiliary pump with the volume controlled by means of a bypass valve. The second float indicator is simply brought up to the same point as that of the water level indicator; during the operation the indicators are kept in horizontal line which assures that the proper predetermined proportions of the agent to the water is maintained.

This type of proportioned has found favor in the heavy industrial service as it is simple in operation, has a wide performance range above the minimum level and requires the manipulation of only one valve while watching the indicator level.

The increase in the application of power to aerial ladder operations has brought with it a new scries of gages and indicators which seldom, if ever, appeared on the early apparatus. Many of these have been added for the safety and operating convenience upon the insistence of the fire service, while others were necessitated by the need for accurate information without which it would be nearly impossible to fully utilize the equipment.

Electric discharge valve includes gage to show exact position during operation

—Wat Waterous Co.

The gages supplied are usually mounted on the control pedestal located on or near the turntable of hydraulic ladders and in general, indicate the oil pressure in the hydraulic system.

In addition, aerial ladders are frequently equipped with several sight indicators not normally found on other apparatus. The most common of these is the “inclinometer” which is in effect a mounted protractor to indicate the angle at which the ladder is raised. As a rule, the same device also includes a safety scale which contains information on the limits of operation at the desired angle and extension.

Some manufacturers include the safe operating data in a visual display on the control pedestal. A mechanical computer transfers the information gained from the angle of inclination and the extension of the ladder to tapes which are viewed by the operator through small windows on the pedestal. As ladder corrections and movements are made, the changing safe conditions are automatically displayed.

A more recent safety limit indicator uses a color-coded gage face to give a visual display of ladder stress. The operator may tell at a glance the conditions by noting if the dial pointer is in the green field for safe operation, or yellow or red for increasing danger.

Operating panels complex

The inclusion of all gages necessary to accomplish proper operation has led to a seeming clutter of present-day apparatus panels. Fire departments have been demanding greater convenience and the manufacturers have found it increasingly difficult to include all the necessary instruments in the limited space available.

The popularity of individual discharge pressure gages, electric gate valve position indicators and similar accessories has given the design engineers serious food for thought. In addition, the necessary training required to properly use and service these instruments has posed additional problems for the fire service and the manufacturer as well. At present many manufacturers are attempting to solve the problem of increased operating efficiency and convenience without overcrowding the operator’s panel. It must be admitted that the modern fire apparatus operator’s panel may confuse the neophyte at first glance. However, as each operation, and the part its indicator plays, is explained, the purpose becomes clear and as the operator becomes more familiar with his instruments his efficiency increases in proportion. □□

In addition to the companies credited with illustrations, the author acknowledges assistance of the Howe Fire Apparatus Co.; Wooster Brass Div., The FyrFyter Co.; and Stewart Warner Corp., for text material.

No posts to display