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823

6.3 Hydrometers

C. H. HOEPPNER (1982) B. G. LIPTK (1969, 1995, 2003)

Types: Laboratory or industrial, can be indicating or transmitting

Design Pressure: Generally atmospheric, but the glass variable area flowmeter designs can be used up

to 100 PSIG (6.9 bars)

Design Temperature: Generally to 200F (93C)

Materials of Construction: Laboratory units are made of glass and unbreakable plastics; industrial units are

available in similar materials as are variable area flow-meters (Section 2.27).

Ranges: Minimum span is 0.05 SG; maximum span is 0.5 SG. Spans can be selected within

the specific gravity range of 0.6 and 2.1. Minimum scale divisions are 0.0005 SG.

Inaccuracy: 1% of span

Cost: Laboratory units cost between $15 and $50. Industrial indicators start at around $500,

and transmitters at $2000 in standard materials. (Costs are higher for corrosion-

resistant materials.)

Partial List of Suppliers: ABB Inc. (www.abb.com);

Brooklyn Thermometer Co. (www.brooklynthermometer.com)

Cole-Parmer Instrument Co. (www.coleparmer.com)Elite Scientific Corp. (www.ambalayellowpages.com)

ERTCO (Ever Ready Thermometer Co.) (www.ertco.com/hydrometers)

H-B Instrument Co. (www.hbinstruments.com)

Princo Instruments Inc. (www.princoinstruments.com)

INTRODUCTION

According to Archimedes principle, when a body is immersed

in a fluid, its weight drops by the same amount as the weightof the liquid that it has displaced. Therefore, all hydrometers

will sink until they displace the same mass of liquid as their

own mass. The volume of liquid displaced is indicated by the

level on the scale and density is the ratio of the hydrometers

mass divided by the displaced volume. As the density of liquidschange with temperature, most hydrometers include a ther-

mometer for temperature compensation purposes.

DESIGN VARIATIONS

The hydrometer element consists of a weighted float with a

small-diameter indicator stem attachment at the top of the

float as shown on Figure 6.3a. The stem is graduated in any

of the density units discussed in Section 6.1. The hydrometer

has a constant-weight body, which, if immersed in fluids with

differing densities, will displace differing volumes of fluid.

Therefore, the degree of stem scale submersion is an indica-

tion of the fluids density. Readings are made at the point

where the stem emerges from the liquid. The accuracy of

DI

FO

Flow Sheet Symbol

FIG. 6.3a

Hand hydrometer.

Graduations

in Any Units

(Minimum Division

is 0.0005 SpG)

Weights

6 to 15 in.

(150 to 380 mm)

2003 by Bla Liptk

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824 Density Measurement

measurement is a function of the combined effects of surface

tension, turbulence, and sample contamination.

In-Line Designs

One of the simplest in-line density indicators is illustrated inFigure 6.3b. It consists of a transparent glass tee with a hydro-

meter and a thermometer inside. Constant level is maintained

in this tee, because the process fluid sample enters from the

bottom and overflows. The sample flow rate is maintained at

less than l gal/m (3.78 l/m) to minimize the effects of velocity

and turbulence. If the process temperature varies, a thermom-

eter is added to allow for manual temperature compensation.

Industrial Designs

The hydrometer element can be mounted inside a variable-

area flowmeter (Section 2.27) housing. In such designs, as

shown in Figure 6.3c, overflow pipes are provided to maintain

the constant level inside the glass tube. Standard accessories

include needle valves for sample flow rate control at about

15 gal/h (57 l/h) and integral thermometers.

Wetted parts are available in the same range of materials

as for variable area flowmeters. These units can withstand up

to 200F (93C) and 100 PSIG (690 kPa). Specific gravity

(SG) spans of 0.1 to 0.5 are available and can be selected

within the limits of 0.6 and 2.1 SG. Reading inaccuracy is

1% of span or the smallest division on the scale, which can

range from 0.001 to 0.005 SG.

Transmitters

The variable immersion hydrometer element mounted in a

rotameter housing can also be obtained as a transmitter for

remote readout, as illustrated in Figure 6.3d three-dimensional.

This electronic transmitter can be the servo-operated imped-

ance bridge type and can be provided with automatic temper-

ature compensation. Such compensation will convert the actual

density, which is detected to the density that it would corre-

spond to at a predetermined base temperature. Ranges, accu-

racies, and design limitations are the same as given for the

local indicator version, discussed earlier.

The stem position of the hydrometer can also be detected

optically. In this design, as the stem rises and falls it changes

the amount of light that passes to a photocell. The photocell

output is calibrated in specific gravity units.

Another transmitting hydrometer design is the capaci-

tance type. Here, a stainless steel hydrometer positions a

dielectric cup inside two insulated, concentric cylinders.

The resulting change in capacitance is proportional to den-

sity. Automatic temperature compensation is provided with

this unit so that the transmitted output signal can be refer-

enced to 60F (16C). The transmission can either be analog

or digital.

FIG. 6.3b

In-line hydrometer indicator.

FIG. 6.3c

In-line hydrometer in rotameter housing.

Outlet

Glass

Tee

Mercury

Thermometer

Hydrometer

Screen Inlet

Vent or

Equalizer

Connection

OverflowHydrometer

Inlet Outlet

Thermometer

FIG. 6.3d

Hydrometer transmitter in rotameter housing.

Vent or

Equalizer

Connection

Indicating

Hydrometer

Transmitter

Coils

Temperature

Sensor

2003 by Bla Liptk

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6.3 Hydrometers 825

CONCLUSIONS

The hydrometers discussed above are basic to density mea-

surement. They are accurate, frictionless, and direct-indicating

without the need for mechanical linkages or external energy

sources. They are compatible with most corrosive fluids.

Their limitations are also multiple, because the float posi-

tion should not be affected by anything except the fluid den-sity. Effects of the velocity, friction, turbulence, and viscosity

of the process must all be minimized. In addition, material

buildup on the float cannot be tolerated, because the basis of

hydrometer operation is the assumption that the weight of

the float is constant. For these reasons, hydrometers shouldonly be considered for use on clean, nonviscous process fluids

when the sample flow rate is controlled at around 1 gal/h.

Bibliography

Cameron, D., An Instrument for Measurement of Liquid Density,Indus-

trial Electronics, March 1967.

Capano, D., The Ways and Means of Density,InTech,November 2000. Density and Specific Gravity,Measurements and Control, October1991.

Gupta, S.V., Practical Density Measurement and Hydrometry, Bristol, U.K.:

Institute of Physics, 2002.

Honeywell Instrumentation Data Sheet #10.143a, Continuous Measure-

ment and Control of Density, Morristown, NJ, latest edition.

Magaris, P., On-Line Density Measurement Is Fast and Accurate,Control

Engineering, June 1981.

November, M.H., Measuring Fluid Density and Specific Weight,Instru-

mentation Insight, Vol. 2, No. 3, August 1975.

Standard Practice for Calibration of Transmission Densitometers,ASTM

Standard,1998.

2003 by Bla Liptk