Third of a Series of Articles Telling Story of Successful Campaign on Broad Lines— Pitot Tube Most Valuable as Means of Measuring Pump Discharge—High Velocity Measurements Most Difficult—Data Must be Used to be of Value

AS a means of measuring pump discharge the Pitot Tube has proved itself the most valuable and acurate, also the cheapest tool available to the water works man. It can be set up, read and removed without disturbing in the least the continuous operation of the pump. It interposes no perceptible obstacle to the flow. It is easily portable, and on this account can be used where a Venturi meter would be too expensive, or even impossible.

Fig. 1. Family of Traverse Curves Taken below a double bend or offset, on the 48-inch discharge of a 40 M. G. D. turbo-centrifugal pump. The readings were taken at 24-points, which were the centers of annular rings of equal area.

In fact, it has been often used as a means of checking Venturi meters whose accuracy was questioned. As an instance in point, may be cited the use of the Venturi in connection with the acceptance test of a recently installed 25,000,000 gallon centrifugal pump. The pump was bought and installed under a guaranteed discharge rate of 25,000,000 gallons per day under the specified conditions of head, suction, and speed. The pump’s capacity, it was specified in the contract, was to be determined by pitometer, but it happened that on the two 48″ discharge lines there were already installed two recording Venturi meters. So by mutual agreement, between the manufacturer and the bureau, the clause in reference to the pitometer was waived and the Venturi meters already in place, decided upon as the means to be employed during the test for measuring the discharge.

But disappointment was in store for the pump people, for as often happens in these cases the pump failed to come up to expectations on the first few tests, and its friends blamed the Venturi meters.

Pitot Tube Establishes Venturi Meter Accuracy

At this stage, enter the pitot tube. The accuracy of Venturi meters, and the dependent results of the test, were absolutely established by the following report:

December 28, 1918.

Mr. H. R. Cady, Mechanical Engr. Bureau of Water.

Dear Sir: In accordance with your instructions, we have carefully checked the Venturi Meters at Queen Lane Pumping Station, and find that their results agree with those of thePitometer, so closely as to amount to an absolute check upon, their accuracy.

The above report shows that even in connection with a Venturi meter, the pitometer may serve a useful purpose. And further, let it be said that while the object in the above case was to check the Venturi meters, the check was a mutual one, the accuracy of the pitometers being established, as well as that of the Venturis, it being a matter of coincidence of results, through a series of determinations.

Fig. 2.

Of course it must be admitted that hydraulic conditions, in the above case were very favorable to accurate pitometer work—namely, long straight stretches of fairly smooth 48″ pipe—with no obstacles such as gate valves and check valves for a long distance up stream from the pitot orifices.

Fig. 3. Traverse Curve, Characteristic of Helical Flow in Pipe The shaded area between curve and dotted line represents effect of corkscrew motion of water.

But on the other hand, many examples of very accurate work might be described, in which the test was per force made under the most unfavorable conditions, and where a Venturi meter was out of the question. A striking instance of this kind is afforded in the acceptance test of what is now known as No. 7 pump at Lardner’s Point Pumping Station.

Another Test Under Less Favorable Conditions

In this case the location of the pitot tap was limited by practical considerations to a point just down stream from a double bend in the vertical plane of the discharge pipe. Further upstream, were also a number of 90° bends, and in spite of these unfavorable conditions, a successful determination of the pump discharge was made.

The new pump was a DeLaval turbo-centrifugal guaranteed to deliver 35,000,000 gallons a day under specified conditions and here, curiously enough, the pitot test showed a discharge of over 40,000,000 gallons per day. That this result was correct is demonstrated daily by the fact that two of the 20,000,000 gallons reciprocating pumps are always kept out of service while this pump is running, and are again thrown into service when it is shut down. Suffice it to say, that the results of that test have in the course of a year been indirectly verified in numerous ways on many occasions, and were certainly correct at the time of the test within two or at the worst 2½ per cent.

In Fig. 1 are shown a family of traverse curves taken on the vertical diameter of the pipe in connection with this test. Their similarity, at different velocities testifies to the accuracy of the constant. And their curious shape is characteristic of the conditions.

Below is printed the report submitted on the determination of the pipe coefficient. It is offered chiefly because it describes briefly the operation and methods involved in a test of this kind.


Contract 305, Test, Pito.

L-65 Lardners Point.


The object of this test was to determine the rate of discharge of the new centrifugal pump, installed at Lardner’s Point Pumping Station under Contract 305.

Location of Tap.

The tap was located on the East Building line of Nos. 2 and 3 houses. Although this places it on the down stream side of a double bend in the vertical plane, this point was preferred to points further upstream because of the impeller disturbance and to any further down stream because of the increasing depth of the pipe and if taken on the header further on, because of the possible leakage through closed gates.

Two taps were drilled as shown in sketch and are hereafter designated as L-55-V for the vertical and L-55-45 for the 45° one.

Determination of Diameter.

The diameter was calipered with a special “Simplex” ⅜” brass calipering rod having a one-inch hook.

Fig. 4.

The diameter was found to be 48 1/4″ for L-55-V and 48″ even, for L-55-45, an average of 48⅛”. This value was taken as the average diameter of the pipe and used in the calculations of discharge.


The traverses were made by the 24 point method, in which the deflections are read at the centers of 12 annular rings of equal area, the velocities then calculated, regarded as the average velocity through each ring. The values of the velocities in eacn ring are then plotted to scale, a smooth curve drawn through them and the values from this curve taken as the true values of the velocities. It being assumed that the points must fall on a smooth curve and that variations from the smooth curve are due to difficulties of reading.

The 24 Point Method.

The distance iro.n the center of the pipe of the 24 points which are the centers of the 12 rings of equal area are found by the formula R = √22n-l/n wherein in this case equals 24 and R

These distances were laid off on a suitable length stick and with the rod orifices on the center of the pipe the index pointer on the rod was set at “center” on the stick. The successive setting of the index pointer of the rod on the various graduations on the stick thus sets the pitot orifice on the corresponding point in the pipe. Readings are then taken at each of the 24 points and the center is read as often as it seems necessary to detect any change of rate of discharge in the pipe, it being necessary, of course, to complete the traverse under uniform velocity conditions.

Reading the Tube.

In order to prevent violent fluctuations of the manometer liquid the tube was “choked” by “nearly closing” one of the admission pinch cocks. This also tended to give an average reading, all readings (except L-55-45 number 3 which was very rapidly read) was the mean value of an oscillation. The highest and lowest points of a surge were read and one-half their sum was taken as the value of the deflection.

Fig. 7.

Specific Gravity.

The manometer liquid was mixture of carbon tetrachloride and non-crystallizable benzol having a specific gravity of 1.50 at 15° C.

Calculation of Velocity.

The velocities were calculated from the formula V=1.179 d. Where d= deflection in inches and V = velocity in feet per second. The constant 1.179 being a product of the rod co-efficient .72, the specific gravity factor √10.50 the constant 2g=√64.4 and the factor √12, to reduce from ft. to inches.


Attached herewith are the following: Four sheets of field notes of eight traverses, four on each tap. Two sheets of traverse curves—four to each sheet.


The average value of the pipe co-efficients obtained was 0.962.


The large value of this co-efficient is due to the effect of the curved pipe upstream from the taps, and as shown in the curves it is a normal effect and characteristic of the conditions. It undoubtedly is constant condition, under all practical velocity values.

No Corkscrew Motion.

Careful rotation of the pitometer rod showed no corkscrew or helical flow in the pipe.

No Pulsations.

There was no evidence of pulsating flow.


John Coates & Thos. Lambert, Pitometer Operators.

Win. M. Crowe, Engineer in Charge.

After the acceptance of No. 7 pump described above, the pitot rod was left in the pipe, and connected up to a U-tube or differential manometer inside the station.

The manometer reads in inches deflection of carbon tetrachloride and is observed constantly by the pump attendant. Any falling off in the deflection serves to warn of something amiss, such as, for example, loss of suction of breaking of the water seal. It serves therefore as a valuable accessory in the operation of the pump. In addition, readings of the deflection are taken every hour and noted in the station log. They are referred to a table of capacities against deflections, and worked up into daily discharge in gallons by one of the station clerks.

The pitometer tap on the 48″ discharge main being located outside the building it was necessary to drill through the 18″ brick and concrete foundation wall of the stall wall to get the piping connections into the building. The connections are all of 1/4″ brass and substantially hung and connected.

Fig. 5.

In connection with the reading of the manometer it might be added that this is done by the attendant who happens to be on duty. No great difficulties were en countered in securing the hearty co-operation of the pump attendants in the rather tedious task of reading the height of the pulsating liquid. Only one refine ment was introduced, namely a correction for specific gravity variation due to changing temperature. A table of correction factors against temperatures is fur nished to the clerk who works up the daily reports and he takes care of this element without trouble.

The above is a typical example of what may be ac complished with an ordinary commercial pitometer rod and an ordinary U-tube. Indeed, its field of usefulness may be greatly extended if a skilled operator, acquainted with its underlying principles is available. Not only water measurements but air also may be measured with equal facility with the ordinary pitot rod. In Fig. 2, is shown part of the results of a pitometer test of a blower discharging into a 20″ pipe.

High Velocity Measurements Most Difficult

Contrary to the general belief, the most difficult pitot measurements are not those of low velocities but those of high velocities. This is so because high velocities are generally accompanied by disturbances of flow whose effect if they can be reckoned at all, can only be discounted at their true worth by a careful and searching analysis of the traverse curves and the conditions accompanying the compilation of the data from which they are platted. This is particularly true of pump discharge when the pitometer is located near the pump. A so-called “50 million gallon” pump at Torresdale, when properly tested with the pitometer showed only a 28,000,000 maximum rate. This pump had been credited with that amount for a long period on the strength of the manufacturers rating and a previous pitot test with inferior instruments. This highly important revelation of the pitometer was afterward checked absolutely and the pump is now often spoken of in the department as the “so-called 50 million pump.”

Yet this test was made under the most unfavorable conditions possible. The pump which is a double centrifugal driven by one steam turbine, discharges into a surface condenser, and thence into a 42″ cast iron pipe which, after passing through the foundations of the station, hooks into an eleven-foot header only a few feet below the condenser. Unfavorable location was, therefore, unavoidable.

Fig. 6.

The hydraulic conditions were very complicated and it seemed for a time that good results were impossible of attainment. It was found, as was to be expected, that in the pipe the water was in a greatly disturbed state. In the first place there were a helical or corkscrew motion of the water. See Fig. 3, where the shaded portion under the velocity curve on the right represents the Vector effect of this helical flow.

In addition, there were eddies and “pockets,” so that the liquid in the tube fluctuated in a remarkably violent manner. Nevertheless the data was susceptible of analysis and a coefficient was found which was correct within reasonable limits.

It is such conditions as outlined above, that when encountered by the amateur, discourage him, and discredit the pitot tube. There are many of these, and the pitfalls into which he may trip are innumerable. Horse sense in a pitometer man is much more to be prized than much technical knowledge. When the two are combined with manipulative skill the materials from which a skillful pitometer man can be made are at hand.

Important to Remember Everyday Facts

It is important to remember the common everyday facts when analyzing pitot data and avoid getting lost in a maze of mathematical formulae. For instance, it is easy to forget that traverse curve, which is merely the calculated velocity plotted against the pipe diameter, does not by any means represent actual conditions within the pipe. It is merely a method of plotting data and must not be regarded literally. Water in pipes, except under rare conditions does not flow in straight lines. In fact, just how it gets through the pipe is a good bit of a mystery. We do know, however, that it doesn’t follow the straight line or the annula tube flow upon which Weisbach’s formulae and, indeed, most formulae are based. This is the first and greatest menace to the amateur pitometer man, that he will accumulate such a mass of mathematical rubbish that he will forget how to apply common sense—the greatest of aids to the problem before him.

As an instrument of precision the pitometer takes high rank. Given good conditions, and good instruments, results may be obtained of any required degree of precision.

In Figs. 4 and 5 are shown pitot data obtained in straight 48″ pipe, under reservoir flow. The traverses were carefully made, with calibrated rods, and the results are such as to leave no doubt that the pitometer is a precision instrument of the highest degree.

Only One Unavoidable Failure

Its application in the Philadelphia Bureau of Water has been very broad, yet it has never failed to give results, except in one case, namely, a 48″ discharge main at the Mingo Creek Drainage Station, in which air was so churned up with the water in the pipe as to be practically nothing but foam and bubbles. This was a physical condition having no reference to the instrument. In all other cases, it has proved a great and often an indespensable aid to the operation of the plant.

For instance, the chart shown in Fig. 5, is compiled from ink line records, taken from two Simplex pitot recorders installed on each of the two 48″ effluents from the clear water basin of Queen Lane Filters, Philadelphia.

These recorders are of the integrating, recording and indicating type. The integrators calculate the total quantities on each of the instruments in gallons, the rate of flow in gallons per day is recorded on a chart in red ink as shown in Fig. 6 and a needle pointer moving across a scale indicates the rate of flow at any instant. The charts are changed every three or four days, then carefully averaged up with a planimeter, and the results platted as shown in Fig. 5. This is a monthly record of the daily performance of the plant. The figures in the first column marked “Pumpage” are taken from the Venturi meters whose reliability was established, as described at the beginning of this article. Any great discrepancy between the figures in this column and those in the third column entitled, “Total,’’ which is the sum of the quantity registered on the two pitot recorders on the filter effluent, is immediately investigated. For instance a break, even a small one, in the approximately four miles of 48″ pipe between the “pumps” and “filters” shows up on comparing these two columns.

(Continued on Page 1001)

Philadelphia’s Fight Against Water Waste

(Continued from Page 971)




Second of a Series of Articles Telling the Story of a Successful Campaign on Broad Lines—Indispensability of Pitot Recorders—Their Elasticity—High Voltage Difficulties Experienced—Annual Cost Insignificant and Value of Data Great

THE Pitot Recorders in connection with the previously described Water Waste Campaign were indespensible. They served as a guide in laying out the work, and gave a true measure of accomplishment. In connection with the night surveys the pitot recorders were used as follows:

A district to be surveyed was first “isolated”: that is all the mains leading into the district were shut down, except one or more through which the entire district was fed. Consequently Pitot Recorders “set up” on these latter mains, measured all the water going into the district, assuming, of course, that none of the boundary valves were leaking. It was possible, to make sure of their tightness by testing each valve as it was closed. This was done by including the valve in question, in a smaller “shut-off” consisting of two or three nearby valves, and testing for water by opening a fire plug within this smaller shut off. As a final check, on the completion of the big “shut-off” the entire district was shut down, for a minute or two, just long enough to open a fire hydrant and test for water. With the district thus isolated and the total supply feeding through the recorders, a daily and hourly record of flow was obtained, from the charts as shown in Fig. 1.

From these charts it was possible to obtain the following data:

1. The daily consumption of the district. 2. The per capita daily consumption of the district. 3. The ratio of the “night rate” to day rate.

The second is obtained by dividing the estimated population of the district into the first, but the third and equally important factor can be learned only from an inspection of the charts, when properly used this fact or ratio of night rate to day rate is of great importance, for it is generally a good indication of the conditions in a district from a water waste standpoint.

Fig. 2

Significance of the Charts

For instance Fig. 1. shows three Pitot Recorder charts—Two (b and c) having a high night rate and one (a) having a low night rate, (a) represents a residential section, in good condition, consisting of new houses, many of which are metered, and is nearer the ideal condition than is generally obtained in practice. Charts (b) and (e), however much they may resemble each other, in respect to night conditions, have nothing in common. The high night rate shown on (b) represents an enormous wastage of water, while that shown on (e) represents nothing of the kind. This is because the former (b) is from residential districts, while the latter (e) is from a manufacturing district in which a number of large industrial establishments operate all night.

As a general thing, it may be said that a high night rate represents leakage, and because of the higher night pressures, a higher rate of leakage occurs at night than in the day time. For instance, a considerable economy was effected in West Philadelphia, by operating the outlet valves from the George’s Hill Reservoir so as to cut down the pressure at night to a point just above that at which the complaints began to come in. In the district in question, the daytime pressure was formerly about 15 lbs. and the night time pressure about 35 lbs. The latter was reduced to 251 lbs., by operating the three outlet valves at the reservoir twice a day at small expense, and immediately, the daily consumption fell from an average 39,000,000 gallons a day to 34,000,000 gallons per day.

In studying a district, the records were usually taken and the data platted only long enought to determine the necessary facts concerning the district. It was found that a week or a ten days run was sufficient, especially if good records for two Sundays were included. See Fig. 2. If the data thus obtained justified it, the night survey corps and house inspectors were then turned into the district to locate and eliminate the sources of waste, the recorders being usually moved on to a new position.

Fig. 1. Pitot Recorder Charts of Three Different Districts Showing Night Conditions and Their Relation to Waste. A good district (a) has a low draft between mid night and 5 A.M. while a high leakage district (b) has a night rate not grealty dissimilar from a manufacturing district (c) with high legitimate night consumption.

The progress of the inspection and night work could be readily measured at any time thereafter, by again setting up the recorders in their former position and noting how much improvement, if any, had been effected.

Elasticity of the Recorders

Shifted from place to place, the Pitot Recorders, furnished much valuable and interesting data. The striking feature of these instruments is their elasticity; placed on a four inch main, they measure the supply of a few thousand gallons per day to single establishment, with the same degree of accuracy as when placed on the eleven foot effluent at Torresdale, they measure its 200 million gallons daily discharge. Set up on the Rex Avenue 30 inch line on either side of its Wissahicken Creek crossing, they showed a difference of 1,000,000 gallons a day. The leak was found and repaired, and the recorders again set up showed no difference. On the 48 inch effluent from Upper Roxboro Filters, (see Fig. 3) a recorder checked the filter wiers day after day, within less than ½ of one per cent.

At Lardner’s Point, pumping station for the past two years, five pitot recorders have measured daily discharge, with a high degree of accuracy. This installation, the largest in the Philadelphia Water Bureau is perhaps the most interesting.

At a point about 800 ft. below the pumping station, its total discharge of approximately 200 millions per day, passes through five mains running parallel and close together. By digging a trench directly across the street at this point, a narrow concrete under ground chamber including the five lines was constructed at a reasonable cost. See Fig. 4.

The mains, range in diameter from 30 to 60 inches, there being one thirty inch, one forty eight inch, and four sixty inch mains, only three of the latter however, being in use at this point.

Difficulties from High Velocity

In calipering, the 60 inch pipes, to determine their diameter, unexpected difficulties were encountered. The ordinary calipering rod, of ⅜ inch brass rod could not be used, because the existing combination of high velocities and large diameters, caused it to be greatly deflected down stream. Furthermore, for operating reasons a shut down was entirely out of the question. So by means of a special gland made for the purpose, the sixties were calipered with a ⅞ inch cold rolled steel rod, which because of its greater stiffness, was not bent by the velocity pressure, and thus made possible measurement of the diameters to the nearest 1/32 of an inch.

The beforementioned combination of high velocities and large diameters also made traversing the pipes impossible, with the standard ⅝ inch brass pitot rod then in use. The first rod inserted in a-sixty inch pipe broke off in the pipe, and the second one on withdrawal was found to be badly bent. So four special flat rods (See Fig. 5) having approximately 6 times the stiffness and offering but 6/10 area of resistance to the flow of water were designed, and constructed for this work by the Simplex Valve and Meter Company of Philadelphia. Their coefficient, was determined at the Hydraulic Laboratory of Cornell University, by Dr. Turner of that institution and the author. At the end of the test it was then believed that the coefficient had been determined within 1 per cent of its true value. Subsequent determinations as well as results obtained in practice, seem to bear out this belief.

Previously to the construction of this special flat form of rod, an unsuccessful attempt to traverse was made by strengthening the inch round rod in the following manner: A ⅞ inch 11 guage, cold rolled steel tubing was slipped over and sweated to the ⅝ inch rod. It was found however that the vibration resulting from the increased area of resistance offered to the flow, not only gave unreliable results, but at some points on the diameters, became so violent as to threaten to tear the corporation cock off at its junction with the pipe wall. The flat rods, however, having no vibration, gave excellent results, and with them elaborate traverses were made, five on each of two diameters (90 degrees apart) on each sixty inch main, the results checking each other within one per cent.

The under ground construction, piping and connections of this installation are all of the most permanent and substantial kind. The piping (see Fig. 2) was ¼ inch lead pipe 12 oz. to the foot, and it was fastened in place with substantial brass straps and screws. The instruments were set up on a one ton concrete block (shown in Fig. 6) necessitated by the vibration from heavy traffic passing over the manhole covers in the street.

The instruments are portable Simplex Pitot Recorders which have been in constant service for seven years, the first five years of which were on the street. Every one of them has been subjected to exceedingly rough usage, particularly at those times when the collapsible houses in which they were operating were struck by automobiles. Yet they are still giving fairly good service, but it must be admitted, at a somewhat higher expense in both repairs and supervision, than new instruments would require.

Some idea of the data furnished by this installation can be gathered from a study of two monthly charts shown in Figs. 7 and 8 entitled “Distribution” and “Pumpage.

These two charts are published monthly. The one “Distribution” shown in Fig. 7 and the other “Pumpage” Fig. 8. On the right hand margin of Fig. 7 are platted the monthly averages of filtration from the wiers, of pumpage from the revolution counters on the pumps, and also of the monthly average discharge through each of the discharge mains.

Fig. 5. Underground Connections of Lardner’s Point Pitometer Station The Pitot Rod Meters are Shown in Operation. The Mains are Covered with Water and Therefore not Visible.Fig. 6. Lardner’s Point Pitot Recorders Showng Concrete Block to Cut Down Vibration and Method of Bringing Piping Through Floor.Fig. 3. An Abandoned Drain Valve Vault, Utilized as a Pitot Recorder Station

A special condition at the Lardner’s Point pumping station makes the daily slippage as shown in Fig. 8 a very important quantity. The station foundations were carried down to rock through a seam of quicksand, which ocasionally leaks into the pump wells and enters the valve chambers of some of the pumps. A sudden increase in slippage due to this cause may occur at any time, and might continue undetected if it were not for the pitot recorders.

The recorders are rated weekly, b y the application of water columns, and the undergi round piping inspected for air and leaks semi-weekly . The charts are changed daily, and taken immediat ely to the office where the average velocity is determined by a radial planimeter. This work and the plat ting of the data, occupies only, about half the time of a young woman, trained in the Bureau during the War. The total annual cost of the pitot division is insignificant, compared with the annual budget of the bureau and the value of the data not only as a matter of record, but as an aid in the daily operation of the plant, is far out of proportion to its cost.

(To be continued)

Fig. 4. Underground Pitot Chamber at Lardner’s Point