Like all mechanical flowmeters, the Neptune Type S meter is essentially 8 a fluid motor driving a gear train whose output is a number roller bank displaying the output shaft speed as flow rate (volume).

The motor provides a known number of revolutions per gallon, however, like all fluid motors, this number will vary somewhat with changes in fluid viscosity.

A typical Type S, 5/8 meter has about 47 revolutions per gallon, the same design in a 2" size meter has 2.56 rev/gal. when calibrated with water. If you use a fluid with a higher viscosity, the number of revolutions per gallon will increase.

The motor produces revolutions based on fluid velocity and it's characteristics, but the register must display a number in engineering units of Gallons, Pounds, Cubic Feet, etc. To accomplish this, two gear trains are used. The first gear train is located directly on the output shaft of the motor and provides a "course" reduction to reduce the number of revolutions per gallon to roughly what the register is going to display. A second gear train provides a "fine" adjustment through what are called "change gears". The change gears are selected at the factory to make the meter read correctly on water. If the customer knows what his fluid viscosity is, this change gear ratio is then adjusted per a set of curves, to the expected operating condition.

A PD meter can accommodate a wide range of viscosity variations, for example, a 2" meter will exhibit about a 1% shift in going from 500 to 2000 Cp, after undergoing a initial shift of about 7% in going from water to 500 Cp. Accordingly, adjusting the change gear ratio is not a precise calculation.

If the customer reports that the meter register is incorrect, it can be field adjusted fairly easily. We need to know:

a. The exact error, i.e. "meter is reading high by 27# in a batch of 200#".

Key points:  - is the register reading high or low?
                    - what is the error in what size volume?

b. The change gear ratio. The change gears are stamped with the number of teeth and they are always located next to a stamped "S" or "R". The "S" means stuffing box gear. The "R" means register. We need to know the number of teeth on both the "R" and "S" gears.

Location of these gears:

On a 157 non-resettable register, you can see them through the lens.

On a 600 resettable register, remove the small plate on the top and you
will see them just inside the cover.

On a 800 batching or resettable register, it is a bit more involved:

a. Remove the valve linkage ... easy way, pull out the cotter pin at the valve arm.

b. Loosen the two square head bolts at the two lower corners on the front of the register face. Don't try to remove them, just back them off 8-10 turns.

c. Lift off the register and turn it over. You will see a plate with the two gears, again next to the stamped "R" and "S".

Call us and give us this information. We will:

a. Calculate the error as a percent
b. Apply this % as a correction to the existing change gear ratio.
c. Send you gears marked "R" and "S" to meet this new ratio.


The new change gears are only as accurate as the customers determination of his error. Press the customer for an accurate measurement.

Why the change gears have a little plate across the middle:

The obvious answer is that the plate slides into the split shaft and locks it into place.

The less obvious answer is that the plate serves as a "mechanical fuse". The working chamber of a positive displacement meter with direct shaft coupling, like the Neptune Type S, produces an amazing amount of force. Should something jam the register gear train, this plate will shear, minimizing damage to the register. The customer will complain that the meter seems to be working but the register is not turning. A sheared off plate is easy to spot ... the ends of the plate are crimped into the face of the gear. If the crimp marks do not line up with the split in the shaft, the plate is broken off.

Sometimes the plate will shear and the register is perfectly all right. The only time this will happen is when the meter is asked to accelerate rapidly and the inertial mass of the gear train in the register cannot respond fast enough. The "fuse" blows. We have seen this happen at unloading stations where a large hose is connected to a tanker and the operator does not allow some time for gravity to fill the hose. He turns on his pump and the resulting high velocity air coming out of the hose hits the meter, the working chamber responds, but the register can't move fast enough and the "fuse" pops.

In this case, he will also sometimes crack the disk ... when the solid fluid hits the rapidly spinning disk, the shock is tremendous.

Excessive speed of the disk or piston with air can also cause "nose burn". This relates to the wear placed on the mating surface of the disk or piston on the divider plate.