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7
SHOP CALIBRATION
 
A. TEST AND CALIBRATION IN SHOP
 
7A1. General. The components of the pitometer underwater log that are most likely to need overhauling are the rotary pump on the rotary distance transmitter, and the control unit. The procedure generally followed is to remove these two units from the ship and overhaul them in a tender or base shop, in the manner described previously. The most practical means of checking the units before installation in the ship is to operate them in the shop under the same conditions as those encountered in service. This entails speeds up to 10 knots, under water pressures up to 200 psi. The pump should be tested for leakage at   pressures up to 200 psi, and for freeness of operation, at speeds as low as 1 knot. This means that the pump should be tight enough not to leak, yet free enough to operate at low speeds. The control unit contact arm must be centered, the contacts and stop rods properly set, and the unit operated in conjunction with the pump, so that it will regulate the pump speed down to 1 knot under static pressures up to 200 psi. Any leakage or binding of the seal assembly will be evident during this operation.

7A2. Equipment required. The log equipment listed below is the minimum required:

Number of Units Equipment Pit. Log No.
1 Pump drive motor, complete 45-3
4 Spacer rods S645-3
1 Motor mounting bracket (optional) S686B
1 Motor pump coupling No number
1 Terminal block S1144M-1
1 Transtat assembly, complete 21-0
1 Limit switch assembly, 2 switches S1133-1
1 Follow-up motor with driving pinion 37A
1 Follow-up motor base K40A-1
1 Transtat mounting plate S1140-1
1 Gear, 60 T, 24 P S1134
2 Pinions, 16 T, 24 P S1135
1 Gear, 72 T, 24 P S1136
1 Transtat gear, 12 T, 24 P S1137-1
1 Gear bracket S1138-1
2 Pivots S1139
1 Set (2 stacks) armature rectifier No number
2 Rectifier brackets S1141
1 Field rectifier No number
 
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Figure 7-1. Shop calibration equipment.
1. WATER STORAGE TANK
2. KNOT SCALE
3. STATIC PRESSURE TANK
4. WATER LEVEL GAGES
5. MASTER SPEED INDICATOR
6. ROTARY DISTANCE TRANSMITTER
7. CONTROL UNIT
8. AIR COMPRESSOR LINE (OPTIONAL)
9. DYNAMIC PRESSURE TANK
10. COUNTERWEIGHT
11. AIR LINE TO STATIC PRESSURE TANK
12. PRESSURE GAGE
13. PRESSURE STORAGE TANK
14. VENT LINE
15. HAND PUMP PRESSURE LINE

Figure 7-1. Shop calibration equipment.
 
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for properly testing the log. It can be assembled and installed by personnel of tender and base shops from readily available material. Because the entire rotary distance transmitter rarely needs to be removed from the ship for overhaul, for shop use it will be necessary to provide a duplicate pump drive motor with the speed control connected to operate from the control unit.

Any or all of the above parts may be ordered from the Pitometer Log Corporation. The following hydraulic equipment is necessary: a water storage tank, or water line with connection, a static pressure tank, a dynamic pressure tank, a pressure storage tank, a pressure gage, and air compressor, water level gages, a knot scale, hydraulic hoses, and fittings and connections. This equipment is described in the following paragraphs.

7A3. Pump drive motor. The pump drive motor is the same model as that used on the ship. It may be mounted on a bench so that the pumps under test can be easily attached. If it is desired that the master speed indicator be tested in the shop in conjunction with the pump and control unit, the slow speed end of the motor may be geared in a 6 to 1 ratio to a self-synchronous transmitter, while the mileage counter shaft is geared in a ratio of 36 to 1 from the slow speed shaft of the pump drive motor.

7A4. Transtat and rectifiers. The transtat assembly and rectifiers may be mounted in an out-of-the-way place with the three wires brought out for connection to the control unit. Power is obtained from 115-volt, 60-cycle a.c. and is connected to terminals 1 and 3 of the transtat.

7A5. Knot scale. (See Figure 7-1.) The knot scale is laid off from the values listed below and is preferably made of metal. The marks are cut with a milling machine so that the distances from the zero line can be measured accurately. This scale is suitable only for use with fresh water in tanks and system, and should be used only for units using the Pitometer rodmeter. Pressure values from rodmeters of other manufacturers may differ from the pressures prescribed here and should

  be verified before using this equipment. However, with a correct scale, this equipment may be used on other types of logs operating on the principle of pressure differences from a rodmeter. The scale should be mounted so that it can be shifted up and own approximately 6 inches. The distances to be marked above zero, and the corresponding knot speeds are as follows:

Knot
Speed
Distance
Above
Zero
10.58 in.
22.32 in.
35.22 in.
49.29 in.
514.52 in.
620.88 in.
728.42 in.
839.17 in.
946.98 in.
1058.00 in.

In any case: distance (inches) equals 0.58 x (knots)2.

7A6. Pressure tanks. (See Figure 7-1.) Two pressure tanks are mounted one on each side of the knot scale. The tank on the right side supplies dynamic water pressure. The left-hand tank supplies static water pressure. Provision is made for raising and lowering the right-hand (dynamic) tank, while the left-hand (static) tank is stationary. When the dynamic tank is elevated above the static tank, the head, or difference of level of water, creates a pressure difference which is equal to dynamic pressure at some known speed. Water level gages are mounted in front of the pressure tanks and show the exact level of water in each tank. The knot scale mounted between these pressure tanks is calibrated in knots, and enables the water level to be read closely, even at low speeds. The tanks are made of 3- or 4-inch heavy-duty pipe, approximately 18 inches long with heavy threaded caps at each end. The glass tubing and gage fittings should be able to withstand a pressure of 400 psi for safety. Shut-off cocks should be mounted on the lower ends, ahead of the nipples. Nipples only are required at the top. When mounted as shown in Figure 7-1, the left-hand (static) tank is connected to the

 
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static line. The right-hand (dynamic) tank is suspended on a chain, or cord, that is connected to a counterweight so that it may be moved up or down. The tanks should be mounted so that the zero water level is at least 3 feet above the control unit bellows and the rotary pump. However, the shop ceiling may be the determining factor, and may not permit raising the dynamic tank to the 10-knot mark on the scale. In this case the distance of the zero water level above the bellows will have to be less than 3 feet.

7A7. Water and air connections. (See Figure 7-1.) The hose connecting the dynamic tank to the lower nipple of the pump must be long enough to permit the dynamic tank to be raised to the extreme upper position of 10 knots. The nipples on the upper ends of the tanks are connected together, and the hoses are so arranged that they can be connected to a water supply line, or to a water storage tank, mounted from 8 to 12 feet above the tanks. It should also be possible to connect these hoses to a pressure line capable of delivering a pressure of at least 100 pounds per square inch. A pressure storage tank mounted as shown in Figure 7-1 permits the use of higher pressures which are developed in the top of the tank when water is pumped up into it from below by a hand pump, or when air is pumped into the top of it from an air compressor. An air pressure gage should be mounted on either the dynamic or static tanks, on the common air line, or on the pressure storage tank. Connect the left-hand nipple of the control unit to the center nipple on the pump; the outer pump nipple to the bottom of the dynamic tank, and the right-hand nipple of the control unit to the bottom of the static tank.

7A8. Preparing equipment prior to testing; Fill both tanks nearly to the top and vent the control unit and pump thoroughly as described in Section 3A3. Close the left-hand valve of the control unit, open the bypass valve, and proceed to center the external contact arm, set contacts, and set the stop rods as described in Chapter 5. It is advisable to have at least 3 feet of water above the control unit while making adjustments. Less head than

  this may make it necessary to reset the contacts later.

7A9. Testing equipment at surface pressure. (See Figures 7-2 and 7-3.) To operate the units at normal surface pressure, set the valves and vent cocks of the control unit to the operating position as shown in Figure 3-3. Vent both the static and dynamic tanks to the same level at the zero point on the knot scale. The top connections of the tanks are open to the atmosphere. Energize the pump motor power supply. Raise the dynamic tank to the knot mark desired. If adjusted properly, the control unit should control the pump speed and cause the transtat arm to regulate, even at speeds of 1 and 2 knots. Lowering the dynamic tank to the zero mark should cause the pump to stop, and the transtat arm to return to the zero voltage position.

7A10. Testing equipment, submerged condition. (See Figure 7-1.) Connect the common pressure hose at the top of the tanks to a controllable supply of air pressure, preferably from the pressure storage tank (Figure

Figure 7-2. Pressure tanks at zero position.
Figure 7-2. Pressure tanks at zero position.

 
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7-1). Apply air slowly, and test at steps of 50 psi. Because of hose expansion with pressure, the water level in the tanks may change and the zero of the scale may have to be slightly lowered. Raise the dynamic tank to the desired knot mark on the scale and note the operation of the units. Pressures beyond 200 psi are unnecessary if the parts operate satisfactorily up to this pressure. The pump seal bellows should tighten and not leak at 200 psi, while no leakage should be evident at any part of the control unit. The pump should not be so tight that it stalls at 1 or 2 knots, but should be turning over smoothly at 3/4 of a knot without excess hunting of the transtat arm. If the test under pressure (submerged condition) is satisfactory, release the air pressure from the top of the tanks. Do not release the air by opening the vent cocks on the

Figure 7-3. Dynamic pressure tank of 4-knot position.
Figure 7-3. Dynamic pressure tank of 4-knot position.

  control unit. Again test the equipment with only atmospheric pressure on the water in the tanks. The units should start and stop freely as before. However, if the bellows has not been properly aged as described in Section 5K23, it may be necessary to reset the contacts and stops, and perhaps to recenter the contact arm. If this has to be done, recheck for proper operation. When the performance is satisfactory, the equipment under test may be installed in a ship. It should not be necessary to make any further adjustments on the ship.

7A11. Additional tests. An operation test under at least 100 psi, at 4 or 5 knots, should be run for 10 to 15 hours if possible. The dynamic tank should be lowered and raised at intervals during this run to determine whether the units stop and start correctly. When the pump alone is being repaired, it can be run at various speeds under pressure to determine whether or not it leaks. However, it will not be possible to determine whether it is sufficiently free at very low speeds, and it is recommended that it be run in conjunction with the control unit at these speeds whenever possible. A static test of 100 to 200 psi of water pressure may be placed on the control unit alone to test for leaks; but this shows up leaks only, it will not test for proper operation under pressure.

7A12. Preparation for pump calibration. The procedure outlined previously for testing the control unit and pump together, with a water column as a standard can also be used to check the accuracy of the log speed and distance indications. If the control unit contacts have been adjusted as previously outlined, and the pump impeller clearances maintained, the accuracy of the log should not be changed from the previous calibration. However, there are times when an over-all check for accuracy is desirable. This is particularly true when a new impeller and shaft or a new thrower disk have been installed in the pump, causing a possible change in clearances between the impeller blades and the pump cover. The incorrect setting of contacts on the control unit will cause considerable error at low speeds. If the pump only,

 
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but not the entire rotary distance transmitter, is removed from the ship at this time, it will be necessary to gear the slow speed end of the pump drive motor with a 6 to 1 ratio to a self-synchronous transmitter, to transmit rotary distance to the master speed indicator. The interior unit of the master speed indicator is removed from the ship and connected to the previously mentioned transmitter. The 115-volt supply connected to the 3Y terminals must be of exactly 60 cycles frequency.

7A13. Pump calibration. The system is thoroughly vented and is run at speeds of 2, 5, and 10 knots, with atmospheric pressure on the water in the pressure tanks. There is no need to check calibration under pressure. To make a check at 10 knots, for example, the following operations are performed: Note the position of the pump orifice plug (Figure 61). Loosen the orifice plate and set it on the zero percent mark, and tighten the plate retaining screws. Using an accurate stop watch, time the mileage counter on the master speed indicator for exactly 1 mile, noting the reading of the speed pointer every 30 seconds. The time should be close to 360 seconds. Calculate the percentage of error from the following formula:

(Seconds deviation from 360 x 100)/360 = percent of error

If the time is less than 360 seconds, the pump is high, or fast. If the time is more than 360 seconds, the pump is low, or slow. The true speed reading is more or less than 10 knots by the percentage calculated. Average the speed readings taken above. They should indicate the same percentage deviation, if any, from 10 knots as was calculated for the pump. At 10 knots, 0.05 knot is 0.5 percent, 0.1 knot is 1 percent. If the master speed indicator indications do not agree with the calculated speed, the error is either due to the 3Y constant frequency supply not being exactly 60 cycles, or to the master speed indicator not being in exact calibration. This test is made primarily to check the pump accuracy and if the master does not

  agree within 0.1 knot, it should be first checked as described in Section 5M52, before an attempt is made to change the pointer setting. Using the average speed indication from above as the standard reference point, shift the orifice plug to the other marks, plus and minus 2, 4, and 6 percent. Tighten the orifice plate each time before making a test. It will not be necessary to time each setting if the master pointer is read carefully. For example: The speed reading may be 10.05 when the orifice is on zero, and should be 10.25 for +2 percent, and 10.45 for +4 percent, and 10.65 for +6 percent. Shift the orifice plug to the original setting as found on the ship when the above tests are completed. The error at zero pump setting should be within plus or minus 1/2 percent, and within 3/4 percent at the other points.

If the error is greater than this, it maybe necessary to re-mark the orifice plate. To remark the place, first file off, or cut off, the old lines. With the static water level carefully set at zero knots and the dynamic water level at 10 knots, shift the orifice plug to get a timed accuracy of plus or minus 2 percent. Be certain that the orifice plate screws are tight. This setting will be the new "zero" point, and a thin line should be described on the plate opposite the index line of the plug. Shift the plug to obtain plus and minus marks of 2, 4, and 6 percent from indications of the master speed indicator.

Unless the pump error has been found to be considerable, it will be evident that it is unnecessary to recalibrate the pump. The correct registration of speed and distance by the instrument as installed in the ship and in actual service is the criterion of the log accuracy. Because so many factors other than the pump calibration affect the over-all accuracy, it is preferable to attempt to calibrate the log on a measured mile course as described in Chapter 6, rather than spend considerable time on the equipment in the shop. If the pump and control unit have met the requirements mentioned previously in this chapter, little trouble may be expected from the log in actual service.

 
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