NATIONAL BUREAU OF STANDARDS REPORT

8640

A STUDY OF VACUUM GAGES

FOR USE IN EVACUATING REFRIGERATING SYSTEMS

by

C. W. Phillips, S. D. Cole, and P. R. Achenbach

Report to

Office of the Chief of Engineers

Bureau of Yards and Docks

Engineering Division, U,S. Air Force

<5jb§>

U. $. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS
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NATSONAL BUREAU OF STANDARDS REPORT

NBS PROJECT NBS REPORT

1003- 20-4210431 February 17, 1965 8640

A STUDY OF VACUUM GAGES

FOR USE IN EVACUATING REFRIGERATING SYSTEMS

by

C. W. Phillips, S. D. Cole, and P. R. Achenbach

Mechanical Systems Section

Building Research Division

to

Office of the Chief

Bureau of Yards

Engineering Division,

of Engineers

and Docks

U.S. Air Force

IMPORTANT NOTICE NATIONAL BUREAU OF ST

for use within the Government,

and review. For this reason, the whole or in part, is not author Bureau of Standards, Washingtc

the Report has been specifically

Approved for public release by the

Director of the National Institute of

Standards and Technology (NIST) on October 9, 2015.

;s accounting documents intended

subjected to additional evaluation

listing of this Report, either in Office of the Director, National

the Government agency for which

ipies for its own use.

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

for

A Study of Vacuum Gages

Use in Evacuating Refrigerating Systems

by

C. W. Phillips, S. D. Cole, and P. R, Achenbach

ABSTRACT

Eight different instruments for indicating the absolute pressure

of gases in a part or all of the range from 0.02 to 20 mm Hg were investigated for their suitability for field use in evacuating refrigerating systems during service operations. The test specimens included

mercury McLeod gages, thermocouple gages, one electric resistance gage,

and one absolute aneroid gage. The aneroid gage and one of the McLeod

gages were calibrated against an NBS working standard and used thereafter as reference gages for comparison with the other test specimens.

Dry nitrogen, dry air, laboratory air, saturated air, water vapor, and

the refrigerant gases Rll, R12 and R22 were used as test gases. It

wa$ found that the aneroid gage was probably best suited for field use when considering the factors of accuracy, applicability to all the

test gases, ruggedness, simplicity, and ease of use. The McLeod gages,

the electric resistance gage and some of the thermocouple gages were

found to provide acceptable accuracy for limited ranges of pressure

and for a narrower choice of gases in most cases.

1

.

INTRODUCTION

One principal method of removing water, air, or other deleterious

or undesirable gases or vapors from hermetic refrigerating systems is

the evacuation to low absolute pressures. Recommended evacuation pressures for this purpose are usually 100 microns Hg (0.1 mm Hg) absolute

or lower. However, on the production lines for commercial refrigerating

units, evacuation pressures may be lower than 10 -microns Hg absolute and

the systems for producing and measuring these pressures are frequently

quite sophisticated.

For field evacuation of refrigerating systems to low absolute

pressures, moderately-priced, portable gages capable of accurately

measuring absolute pressures in the range from 100 microns Hg to 1 mm

Hg (1000 microns Hg) or greater are needed to determine whether the

prescribed low pressures have been achieved. Further, the ability of

the several types of instruments to correctly indicate the absolute

pressure of the system under evacuation when any of several gases or vapors are present must be known.

In this study, several types of gages, including aneroid, mercury

McLeod, thermocouple, and electric resistance gages were investigated

as indicators of low absolute pressures. The gages were compared when

used with refrigerants Rll, R12, and R22, nitrogen, dry air, laboratory

air, air saturated with water vapor, and water vapor.

2.

DESCRIPTION OF PRESSURE- INDICATING INSTRUMENTS

Eight different instruments were investigated, including 1 aneroid,

2 mercury McLeods, 4 thermocouple, and 1 electric resistance gage, as follows

:

1. Wallace & Tiernan Absolute Pressure Indicator

2. Kinney Measuvac Mercury Manometer

3. Kontes Mercury Manometer

4. Arthur F. Smith Company Vacuum Gauge

5. NRC Equipment Corporation Vacuum Gauge

6. Hastings Vacuum Gauge, Model VT-6S

7. Hastings Vacuum Gauge, Model VT-4S

8. Consolidated Vacuum Company Pirani Vacuum Gauge

(1) The Wallace & Tiernan Absolute Pressure Indicator, Model FA-160,

manufactured by Wallace & Tiernan, Inc., Belleville, New Jersey, was an aneroid diaphragm type absolute pressure indicator which measured the

unknown pressure by comparison with the pressure in an evacuated capsule.

- 1 -
© ©

ABSOLUTE

PRESSURE

MILLIMETERS OF MERCURY

MODEL NO

FA 160

WALLACE a TtERNAN

BELLEVILLE, NE* JERSEY

WALLACE & TIERNAN ABSOLUTE PRESSURE INDICATOR, MODEL FA- 160

FIGURE 1

152869
This eliminated the effect of barometric pressure changes. The absolute

pressure was indicated directly by this gage without correction or adjustment in the range between 0.10 millimeter and 20 millimeters of

mercury on a six-inch dial having more than 16 inches of scale length.

As shown in Figure 1, the scale had 1-millimeter Hg numbered graduations

each divided into 10 marked intervals of 0.1 millimeter Hg.

The mechanism consisted of a pressure sensitive element, lever system, pointer and dial, all housed in a suitable pressurized case. A

metal capsule served as the pressure sensitive element which was con- nected to a geared sector through an actuating rod and flexible pivot.

The sector engaged a pinion mounted on a pointer shaft. The shaft

turned in bearings at the top and bottom. Backlash was prevented by a spring which provided uniform tension for all positions of the sector

and pinion. The pressure sensitive element had a built-in stop which

prevented damage to the mechanism from over-pressure. The instrument

had a custom-calibrated dial over its full range intended to permit direct reading without reference to correction curves.

The gage was equipped with a Wallace & Tiernan Check Valve, FU-3229,

screwed and. sealed into the inlet connection. This valve was intended

to provide a slow pressure change inside the gage if the measured pres- sure were suddenly increased, but offers little resistance when the

pressure is decreased.

The instrument should be protected from severe vibration and located

where the temperature will be as uniform as possible. To minimize hysteresis effect of the gage at the low end of the scale at the time of

initial pulldown from atmospheric pressure, the manufacturer recommends

an evacuation time of one-half to one hour.

The overall dimensions including vacuum connections are 8 1/2 inches

in diameter and 4 1/2 ± 1/4 inch deep.

(2) The Kinney McLeod Gauge, Type T.D. 1, was manufactured by the Kinney

Vacuum Division of the New York Air Brake Company, 3529 Washington Street

Boston 30, Mass. It was a mercury manometer with two separate closed end

capillaries having different compression ratios enabling pressure measure

ments to be made in the range from 150 mm to 2 mm Hg on one graduated

tube and 2000 microns to 1 micron Hg on the other graduated tube. The

approximate scale lengths for the 2 to 150 mm scale and the .001 to 2 mm

scale were 4 inches and 3 3/8 inches, respectively. (See Figure 2.)

The pivoted handle on the right, when lowered, raised a rubber diaphragm

which formed the bottom of the mercury container, forcing mercury up

into the two closed-end glass capillary tubes and one open glass tube, on the right, which connects the system under pressure measurement to

the mercury container in the gage.

- 2

-
KINNEY "MEASUVAC" HG MC LEOD GAGE

FIGURE 2
The amount of mercury used in the gage was critical between 16 1/4

and 16 3/4 cc. Too little mercury would result in inability to raise

the mercury level to the set line on the open glass tube; too much mercury would result in permanent trapping of air in the high pressure

capillary tubes. The latter error could be detected by comparing the

readings on both pressure capillaries at the 2 mm Hg pressure level.

The system pressure to be measured was connected to the gage at the

top through a 1/4-inch metal tube held against the right-hand open glass

tube by means of a knurled nut washer and 0-ring, with a flexible con- nection extending from the exposed end of the 1/4-inch tube to the system.

Operation of the gage at atmospheric pressure should generally be

avoided as depression of the handle may force mercury out of the open

end tube, disturbing the critical amount of mercury in the gage.

When taking a reading, the handle should be depressed steadily.

Too quick or too jerky a movement may shoot the mercury up into the

closed-end tubes irrespective of compression and give an erroneous

reading. A steady operation of the handle will force the mercury up

the glass tubes and when the mercury in the right-hand glass tube reaches

the set mark, a reading can be taken from the mercury level in whichever

of the two closed-end glass tubes indicates on scale at the time. A

steady release of the handle clears the tubes of mercury, making the

gage ready for another reading.

The overall size of the gage with handle and legs, as used for

bench mounting, was 10 in. wide by 8 in. deep by 14 in. high, exclusive

of the 1/4-in. connection tube.

McLeod gages cannot be used to reliably measure the pressure of

gases (vapors) if the pressure in the compression tube of the gage at

the time of measurement approaches the condensing pressure of the gas

or of one constituent of a mixture of gases. For this reason, the Kinney McLeod gage was not used as a reference gage in tests with saturated

air or water vapor. The pressure in the measuring tube of a mercury

McLeod gage is equal to the system pressure plus the mercury head re- quired to compress the sample. At system pressures of 2, 1, and 0.1 mm

Hg, respectively, the mercury heads in the 0 to 2 mm Hg tube of the

Kinney McLeod gage required to compress the sample are 89, 64, and 23 mm which would produce pressures on the sample of 91, 65, and 23 mm Hg.

From Table 1 it can be seen that, of the gases tested in this investigation, water vapor is the one which approaches or would exceed its

condensing pressure in the 0 to 2 mm Hg range of the Kinney McLeod gage

at room temperature.

- 3-
TABLE 1 Condensing Pressure of Sample Gases

Critical Condensing Pressure at

Gas Temp .

, °F 7 5

° F , mm Hg

Water 706 22.2

Rll 388 765

R12 234 4740

R22 205 7660

Air -221 None, above critical Temp

Nitrogen -233 n ft ti u

(3) The Kontes Mercury Manometer was a McLeod gage manufactured by the

Kontes Glass Company, Vineland, New Jersey, and is shown in Figure 3.

The gage used a column of mercury as a piston to compress a certain

known volume of gas to a smaller known volume. Because the mathematical

product of "pressure times volume" is a constant, or PV = P'V', the pressure of this sample of gas can be indicated on a suitable scale. In this

instrument, the scale covered the range from 0 to 1000 microns Hg, and was about 4 inches long. The relationship, PV = P'V', remaining constant depends upon the temperature remaining constant and upon the gas sample

being an ideal gas. Condensable vapor in the gas sample will cause the

indicated pressure to be less than the correct pressure. This was discussed in more detail in description of the Kinney McLeod gage.

To obtain a pressure reading, the mercury-containing portion of the

Kontes gage is rotated more than 90 degrees clockwise from the position

shown in Figure 3. The rotating hub is directly behind the larger bulb

in the center of the tubing assembly. The pressure to be measured is con- nected at the back of the instrument and a connecting tube runs through

the hub into the larger bulb. In the rotated position the mercury drains

into the right hand bulb. The system pressure to be measured can then

communicate with the scale tube which is closed at the top end. Returning

the gage to the position shown in Figure 3 allows mercury to flow up into

the two left-hand tubes, compressing the gas in the scale tube and over- flowing the other, imposing a fixed mercury head on the gas compressed in

the scale tube. The tube connecting the gage to the system under measurement must be flexible to permit rotating the gage assembly.

The overall size of the instrument, including gage and mounting stand, was 10 inches high, 61/2 inches wide, and 6 inches deep when the gage is

vertical. When the gage is rotated to a horizontal position, the instrument measures 8 1/2 inches high, 9 inches wide, and 6 inches deep.

- 4-

Thermocouple- type Instruments

The four instruments described next used the heated- thermocouple

principle for determining low absolute pressures. In this type of . instrument, the change of temperature of an electrically-heated filament

in the cavity of the gage tube is measured by a thermocouple. This

change of temperature is caused by the change in heat transfer through

the gas caused by the change of pressure in the cavity and produces a change in electric current flow through a sensitive milliammeter in the

thermocouple circuit. The heater current is held constant, and the temperature of the thermocouple is in part, then, a function of the heat

conduction capacity of the gas in the gage tube or cavity. In general,

the conductivity-l/of a gas becomes almost constant above about 1 mm Hg

absolute pressure so that most instruments lose sensitivity above this

pressure. As the pressure is reduced, the temperature of the thermocouple

increases. The proportion of heat lost from the heated wire by radiation

and stem conduction increases and that lost through gas conduction de- creases. For absolute pressures below about 10” ^ mm Hg, most thermocouple

gages are not practical because a very small proportion of the total heat

transfer from the thermocouple is caused by conduction through the gas.

The reference junction for the thermocouple on the heated wire is usually

at the temperature of the case of the gage tube. Because different gases

and vapors have different conductivities and different "accommodation

coefficients", a single calibration is not applicable for all gases. The

accommodation coefficient is an index of the ability of a gas to remove

heat from a surface. Most thermocouple gages for refrigeration evacua- tion work are calibrated with dry air as the reference gas.

(4) The ASCO battery-operated thermocouple vacuum gage, shown in Figure

4, was a Type 1, Model 1013, manufactured by the Arthur F. Smith Co.,

311 Alexander Street, Rochester 4, New York. The gage operated with a gage tube, Type VTP6343, manufactured by the Vacuum Tube Products Co.,

Inc., 506 South Cleveland Street, Oceanside, California.

The gage provided a milliampere scale and current adjustment knob

to set the heater current to the milliampere rating marked on the thermocouple gage tube. The gage tube is in the lower left corner of Figure 4.

The numbered portion of the measurement range indicated on the scale

extended from zero to 1000 microns Hg absolute pressure. The non-uniformly

graduated scale was about 31/2 inches long with about 80% of the scale

length covering the range from zero to 200 microns. A 1-1/2-volt flashlight battery may be used for short periods of use, but a Mallory RM-42R

battery is recommended by the manufacturer for extended battery life.

—^Richard B. Lawrence, A Survey of Gages for the Measurement of Low

Absolute Gas Pressures, Chemical Engineering Progress, Vol. 50, No. 3,

March 1954.

- 5

-
ASCO

THERMOCOUPLE

GAGE

FIGURE

4
The instrument was 6 1/2 inches high, 7 inches wide, and 7 1/4 inches

deep. The top 5 inches of the front face was sloped so that the top was

but 41/2 inches deep.

To operate, the gage tube with six-foot lead is connected to the

vacuum system and the off-on switch turned on. After setting the heater

current to the proper milliampere rating for the particular gage tube,

the instrument is ready for use.

(5) The NRC Vacuum Gauge, manufactured by the NRC Equipment Corporation,

a subsidary of National Research Corporation, Newton Highlands 61, Massachusetts, consisted of a Type 701 Vacuum Gauge Control and a Type 501

Gauge Tube. This apparatus, which operated on 115 V, AC, is shown in Figure

5. The thermocouple gage control incorporated a vacuum indicating device

for indicating pressures over a continuous range from zero to 1000 microns

Hg absolute pressure. These pressures could be read directly on a microammeter which was calibrated in microns Hg on a 2-1/4-inch non-uniformly

graduated scale with 2 inches of the scale length covering the range from

zero to 200 microns Hg . As shown in Figure 5, the control unit provided

an A.C. ammeter (on left) for use in setting the heater current. The knob

between the two meters was used to adjust the heater current. The righthand meter indicated the pressure.

The unit used a welded metal thermocouple gage tube which enclosed

the heater and thermocouple elements. A calibration curve in microns Hg

per microampere characteristic of the gage's non-linear response was furnished on the gage tube label. Each gage tube was individually calibrated

and the heater filament current was stamped on the tube base. The gage

was reported to be capable of operation without damage at atmospheric

pressure

.

The physical specifications of the unit were:

Size - - - Length 11", Height 7 1/2", Depth 8 1/4"

Weight - - - 12 lb

Power Cord - 6 ft long, 115 volt with standard AC plug

Gage Cord - 6 ft long, 4-wire lead, octal plug

The operational specifications of the control unit and thermocouple

gage tube were:

Power supply - 115 volt, single phase, 50/60 cycles

Filament supply:

Maximum, 0.9 amperes

Minimum, 0.56 amperes

Operational, 0.60 to 0.68 amperes adjustable

- 6-
NRC

THERMOCOUPLE

GAGE,

TYPE

701

FIGURE

5
Filament current:

0.6 amperes plus hundredths of an ampere corresponding to calibration number stamped on the gage.

Maximum filament current, 1.0 amperes.

Filament resistance, 0.2 ohms (approximate).

Thermocouple output, 0-14 millivolts maximum.

Indicating range, 1 - 1000 microns Hg absolute in dry air.

(6 & 7) Two thermocouple- type Hastings Vacuum Gauges, Model VT-6S and

Model VT-4S were investigated and are shown, respectively, in Figures 6

and 7. They were manufactured by Hastings-Raydist , Inc., Hampton, Virginia, and identified as thermopile gage controls.

The two models were similar except for the working range of measurement. Model VT-6S had a principal indicating range of 0 - 1000 microns

Hg, and Model VT-4S had a principal range of 0 - 20 millimeters Hg. The

pressures were indicated directly on the microammeters of the two gages

graduated non-uniformly in microns Hg and in millimeters Hg above zero absolute pressure, respectively. Each gage was equipped with a fiveposition selector switch for reading of any one of five gage tubes. Fig- ure 7 shows the five gage tube leads, electric power cord, and one of the

five type DV-4DM gage tubes furnished with the Model VT-4S gage. Type

DV-6M gage tubes were furnished for use with the Model VT-6S gage.

According to the literature furnished with the gages, the gage tubes

for each range were matched and interchangeable without calibration adjustment. They had built-in compensation for temperature and rate of

change of temperature. It was stated that extension cables could be added

between the gage tube and the instrument with little or no effect on calibration, thus extensions for remote indications could be added, if

desired. The gage tubes were nickel plated and connected to the vacuum

system by means of a 1/8” NFT male pipe thread. The heater current input was preset at the factory and remained the same for all gage tubes used

with a particular instrument. No external means for adjusting the heater

current was provided and the gages did not have a means for indicating

the heater current.

The instruments were designed to operate on 115 volt, AC power of

any frequency and were provided with a three-wire 8-ft power cable. Power

requirement was stated as less than one watt.

The overall size of the Hastings Vacuum Gauges without gage tubes

and leads was 7 3/4 in. long by 4 3/4 in. high by 4 1/2 in. deep, with a removable carrying handle.

- 7 -
HASTINGS

THERMOCOUPLE

GAGE,

MODEL

VT-6S

FIGURE

6

(8) The Pirani Vacuum Gauge Type GP-110 was manufactured by the Consolidated Vacuum Company, Rochester 3, New York, and is shown in Figure 8.

The operation of a Pirani- type gage, which is a thermal conductivity

'vacuum gage, was based upon the changes in the temperature (and resistance)

of a heated filament. This temperature change was partly a function of

the surrounding gas pressure. Resistance was measured with a conventional

Wheatstone bridge circuit which was made insensitive to ambient temperature

changes by use of a compensating tube in one arm of the bridge. For the

gage under test, a sensitive microammeter graduated in pressure units from

zero to 50 microns Hg and zero to 2000 microns Hg on two scales was used

to measure the unbalance of the bridge. The non-uniformly graduated scale

length was about 3 5/8 inches.

According to literature furnished with the gage, the power supply for

the bridge consisted of a step-down transformer, a full-wave selenium

rectifier, and an automatic current regulator tube which developed a con- stant voltage drop across the bridge voltage potentiometer.

The metal sensing tube was 7 inches long overall: 5 5/8 inches of

the tube was 1 1/4 inches in diameter, and 1 3/8 inches was a 1/2-inch

tube to receive flexible tubing for connection to the system. The tube was calibrated for use in ambient temperature between 59°F and 95°F, and

should be recalibrated for use in temperatures outside of the designated

range. The tube was connected to the gage cabinet with a 10-foot length

of cord. The cabinet was 6 inches in height, 10 3/4 inches in width, and

5 5/16 inches in depth, and its weight was 10 lb.

Gage Tubes

Figure 9 shows the shape, relative size, and means for pressure and

electrical connections for, left to right,

1. CVC Pirani resistance gage tube

2. NRC Thermocouple gage tube

3. ASCO Thermocouple gage tube

4

.

)

5.

) Hastings Thermocouple gage tubes of two types. The tubes of the

two different ranges were distinguished from each other by means of different colored marking: blue for the type BV-4DM (principal range 0 to

20 mm Hg) and brown for the type DV-6M (principal range 0 to 1000 microns

Hg).

Gage Scales

Figures 10 and 11 are actual-size photographic reproductions of the

non-uniform graduated scales for the McLeod, Thermocouple, and Pirani- type

gages investigated. It will be noted that some scales are difficult to

read because the scale intervals are not uniform in either length or value

for even the principal working portion of the full scale range. Figure 12

is an actual-size photographic reproduction of the scale of the Wallace &

Tiernan aneroid gage.

- 8-

h n n

VACUUM

GAGE

TUBES:

CVC

PIRANI

4.)

HASTINGS

NRC

THERMOCOUPLE

5.)

THERMOCOUPLE

ASCO

THERMOCOUPLE

(2

TYPES)
MEASUVAC

SET

l

2

KINNEY MC LEOD GAGE KONTES MC LEOD GAGE

SCALES, ACTUAL SIZE

FIGURE 10
o

SCALES,

ACTUAL

SIZE,

OF

FOUR

THERMOCOUPLETYPE

GAGES
x*

^88

V\\v'N \\V 9

\c 1 r

S'7

ABSOLUTE

PRESSURE

////

13

7///

I4\

MILLIMETERS Of MERCURY

m

15

"

WALLACE & TIERNAN ANEROID GAGE SCALE, ACTUAL SIZE

FIGURE 12
3. TEST APPARATUS

The test apparatus was designed to provide means for comparing the

readings obtained from the various gages under test with calibrated

reference gages, under either increasing or decreasing pressure, in the

range from below 0.1 mm Hg to 20 mm Hg , and with several different gases.

The apparatus consisted essentially of a pipe circuit incorporating a vacuum pump, a nitrogen cold trap, a gage manifold, two gas tanks, a chemical dryer and sources of the various test gases. Figure 13 is a schematic drawing of the test apparatus, with the various parts connected

with copper tube and rubber tubing joints.

The vacuum pump was a Welch Duo-Seal Vacuum Pump manufactured by the

Welch Scientific Company, 1515 Sedgwick Street, Chicago 10, Illinois;

Model 1405-B, 525 RPM, 115 V, 60 cycle, AC, 1/2 HP motor. A clutch at

the motor shaft provided for easy starting. The pump was a two-stage

unit with a rated capacity of 58 liters per min. at 1 micron absolute

pressure at the inlet and an end point of 0.1 micron. A ballast valve

was provided for the removal of gases from a system without excessive

contamination of the pump oil.

The cold trap was of glass and was refrigerated by liquid nitrogen

in a one-quart size Dewar flask. The height of the glass trap was 16

inches and the inlet and outlet connections were 3/4 inches.

Figure 14 is a slightly enlarged view of the manifold to which the

test gages were attached, as indicated in Figure 13. The body of the

manifold consisted of a 1-inch length of 2-inch copper pipe with square

copper plates hard-soldered on the ends. Eight 1 1/2 inch-long pieces of

3/8-inch copper tube were soldered with equal spacing to the circular sur- face of the manifold for use in connecting the test gages, and one section

of 3/8-inch tube 31/2 inches long was hard- soldered to one of the flat

surfaces, exactly at the center of the 2-inch pipe, for connection to the

vacuum pump. The corners of the bottom flat plate were drilled for rigid

mounting in a fixed position.

The two tanks were 2.-foot-long sections of 5-inch copper pipe and

were connected with sections of 1-inch copper pipe with rubber tube

joints. The tanks provided a volume element for the system.

The chemical dryer at the inlet opening removed unwanted moisture

from gases entering the system for some tests. Other tests required the

addition of water or vapor to the entering gases.

A six-foot diameter rubber balloon was used to hold the various test

gases, taken from high pressure cylinders, at atmospheric pressure before

and during introduction into the system. The balloon was completely

evacuated before being filled to full size by the gas being used during

a particular investigation.

- 9-
GASES UNDEft TEST

MANIFOLD

SCHEMATIC ARRANGEMENT OF TEST APPARATUS

FIGURE 13
GAGE TEST MANIFOLD

FIGURE 14
Reference Gages for Test

The Wallace & Tiernan aneroid gage (Figure 1) was used as the reference gage in the comparison tests for the pressure range from 20 mm to

0.2 mm Hg absolute. The Kinney McLeod gage (Figure 2) was used as the

reference gage for the pressure range from 0.2 mm to .001 mm Hg absolute

since this was below the calibrated range of the Wallace & Tiernan aneroid gage. The McLeod gage was not used for reference purposes when tests

with saturated air or water vapor were made.

These two reference gages were calibrated by the Pressure and Vacuum

Section of the National Bureau of Standards. The reference gages were

calibrated by means of oil and mercury hook-type differential micromanometers against an absolute pressure of approximately 1 micron (.001 mm)

Hg. The accuracy of the calibration system was within 5 microns (.005

mm) Hg

.

4. TEST PROCEDURE

In general, the test procedure was designed to compare the pressure

indications of the various test gages with one or both of the calibrated

reference gages over an absolute pressure range from about 0.1 mm Hg (100

microns Hg) to 20 mm Hg using various gases in the test system.

The gases used in the test series included:

1. Nitrogen

2. Dry air

3. Laboratory air (approx. 50% R.H„)

4. Air saturated with water vapor

5. Water vapor

6. Rll

7. R12

8. R22

The test apparatus was assembled as illustrated schematically in

Figure 13, when used with all gases except water vapor, laboratory air,

and saturated air. For these three fluids the dryer was removed.

Prior to making comparative pressure observations with each gas, the

system was evacuated to as low an absolute pressure as the vacuum pump

could develop and then filled to atmospheric pressure with the particular

gas being used, except for water vapor in which case the pressure was

raised to 20 mm Hg. This cycle was repeated from four to six times with

each gas.

With the hosecock downstream from the dryer closed off and the system

filled at atmospheric pressure with the test gas, evacuation by the pump was continued until the aneroid reference gage indicated a pressure between

19 mm and 20 mm Hg absolute. At this time the hosecock upstream of the

cold trap was closed, with the pump operating, and the current adjusted

for the two thermocouple gages which required manual adjustment. The indicated pressures on all gages were read and recorded at this initial

condition.

- 10-
A routine procedure was established of opening the hosecock, evacu ating the system by an increment of pressure, closing the hosecock and

recording the values indicated by all gages, until the system was evacu- ated to the limit of the pump.

The absolute pressure range from 20 mm to 0.1 mm was arbitrarily

divided into 20 intervals for comparative readings, with 5 intervals in

the range from 1 mm to less than 0.1 mm Hg , because some of the electronic

gages were designed for use principally in this range.

At the conclusion of the evacuation process, the vacuum pump was stopped, the outlet hosecock was closed and the inlet hosecock was repeatedly opened and closed, such that various levels of pressure occurred in

the system in about 20 intervals similar to those observed when the system was being evacuated.

When the tests with dry air, nitrogen, refrigerants Rll, R12 and R22 were conducted, the gas entered the system at the inlet end and passed

through a dryer to remove water vapor before entering the first volume

tank. For the test with laboratory air, the ambient room air was introduced into the system without passing through the dryer.

Water- saturated air was prepared for test by bubbling laboratory air

through a five-gallon container filled about three quarters full with

water at room temperature. The air inlet tube extended to the bottom of

the container and the entrance to the air outlet tube was located at the

top. The humidified air was drawn through the test system until the cold

trap was nearly stopped with ice; the system was then isolated from the

cold trap by stopping the pump and closing the hosecock; and a second

cold trap inserted in place of the frozen one. Following this purging

operation, the test readings were made at various pressure levels in the

system.

For the tests with water vapor, the same five-gallon container was used as for the saturated air tests, but with the water heated to about

131°F and with the air inlet tube to the container closed. The test system was alternately evacuated and allowed to fill with water vapor from

the heated container to about 20 mm Hg absolute pressure a few times,

prior to the test observations.

5. TEST RESULTS AND DISCUSSION

Comparative results of the tests of one aneroid gage, two McLeod

gages, four thermocouple gages, and one Pirani gage are shown in Figures

15 through 40. For most of the tests either the Wallace & Tiernan

aneroid gage (for tests in the absolute pressure range from 20 mm Hg to

0.2 mm Hg) or the Kinney McLeod gage (for tests below 0.2 mm Hg) was

used as a reference gage. These two gages were calibrated against an

- 11 -
NBS working standard as described under the Test Apparatus Section of

this report. The results of the calibrations of the two reference gages

by the NBS Pressure Measurement Section are shown in Tables 2 and 3, and

in Figures 15 and 16. The calibration data in these two figures for the

Wallace & Tiernan gage were plotted for decreasing pressure only since

all pressure comparisons were made under decreasing pressure conditions

during the tests.

TABLE 2

Calibration of Wallace & Tiernan Aneroid Gage

Increasing Pressure Decreasing Pressure

Average Deviation from Average Deviation from

Reading NBS Working Standard Reading NBS Working Standard mm Hg mm Hg mm Hg mm Hg

0.32 -0.12 18.62 +0.02

0.92 -0.11 16.38 +0.02

3.92 -0.11 14.03 0.00

7.90 -0.09 10.59 +0.01

13.22 -0.07 7.58 0.00

16.50 -0.06 5.10 -0.03

19.00 -0.06 2.64 -0.03

19.92 -0.04 1.13 -0.06

20.00 0.00 0.16 -0.08

TABLE 3

Calibration of Kinney McLeod Gage

0 to 2 mm Hg Range 0 to 150 mm Hg Range

Deviation from Deviation from

Reading NBS Working Standard Reading NBS Working Standard mm Hg mm Hg mm Hg mm Hg

0.026 +0.004 1.9 -0.1

0.120 +0.005 2.5 -0.3

0.190 +0.005 3.3 -0.4

0.260 +0.005 8.5 -1.2

0.400 -0.003 18.0 -2.3

0.560 -0.010

0.700 -0.023 38.0 -2.3

1.14 -0.06 80.0 -3.5

1.49 -0.03

1.86 -0.08

1.93 -0,09

- 12 -
CALIBRATION

CURVES

FOR

WALLACE

AND

TIERNAN

ANEROID

GAGE

AND

KINNEY

MC

LEOD

GAGE

§H 111111 ‘ajnssajy a^nxosqy ujoaq uoi^btasq

Absolute

Pressure

Based

on

NBS

Working

Standard
CALIBRATION

CURVES

FOR

WALLACE

AND

TIERNAN

ANEROID

GAGE

AND

KINNEY

MC

LEOD

GAGE
During the tests, all the gages were connected to the system for all

test runs, but, because the indicating ranges of the gages differed, some tests did not produce values for all of the gages investigated. The test

system was evacuated to the limit of the vacuum pump for each test, but

for the tests with saturated air and water vapor, the identification of

the reference pressure was limited to the calibrated scale range of the

Wallace & Tiernan aneroid gage, i.e. from 0.1 mm to 20 mm Hg. Pressures

below 0.1 mm Hg were indicated by the other gages and the calibrated

Kinney McLeod gage was used as a reference for absolute pressures lower

than about 0.15 mm Hg (150 microns) for all of the test gases except

saturated air and water vapor.

Table 4 lists the principal or working portion of the scale range,

and full scale range (see Figures 10-12) of the seven gages.

TABLE 4

Gage

Absolute Pressure Scale Ranges

Tvne Working Scale

of Test: Gages

Full Scale

Wallace &

Tiernan Aneroid 0 .. 1 to 20 mm Hg 0 . 1 to 20 mm Hg

Kinney McLeod 0 to 2 mm Hg 0 to 2 mm Hg

0 to 150 mm Hg 0 to 150 mm Hg

Kontes McLeod 0 to 1000 microns Hg 0 to 1000 microns Hg

ASCO Thermocouple 0 to 1000 microns Hg 0 to Atm

NRC Thermocouple 0 to 1000 microns Hg 0 to >1000 microns ' Hastings Thermocouple 0 to 1000 microns Hg 0 to Atm

(Mod. VT-6S)

Hastings Thermocouple 0 to 20 mm Hg 0 to Atm

(Mod. VT-4S)

CVC Pirani 0 to 50 microns Hg 0 to 50 microns Hg

0 to 2000 microns Hg 0 to 2000 microns Hg

Figures 17 through 23 show deviations of the pressures indicated by

the three gages having useful scale ranges greater than 1 mm Hg absolute

from the absolute pressure as determined with the calibrated Wallace &

Tiernan aneroid gage for the several gases used in the test series. The

absolute pressures determined with the calibration data for the Wallace

& Tiernan gage are shown on the abscissae of these curves and the deviations of the Kinney McLeod gage, the Hastings thermocouple gage, (Model

VT-4S), and the CVC Pirani gage are shown on the ordinates.

Although the applicable scale of the CVC Pirani gage had a marked

range of only 2 mm Hg (2000 microns) deviations are plotted for absolute

pressures far above 2 mm Hg because the gage continued to indicate on

- 13 -
scale. Results obtained with the Pirani gage are omitted from Figures

21 and 22 because the indicated pressures did not exceed 0.8 mm Hg (800

microns) for absolute test pressures approaching 20 mm Hg.

Figures 17 through 23 show that the deviations for the CVC Pirani

gage and the Hastings thermocouple gage were excessively large for all

test pressures greater than about 1 mm Hg and for all gases used for

test. Furthermore, the Kinney McLeod gage was of no practical value

for measuring low absolute pressures of saturated air or water vapor,

as indicated in Figures 19 and 20.

Figures 24 through 31 show deviations of the pressures indicated

by some or all of the gage specimens from the absolute pressures determined with one or both of the calibrated references gages for the range

of absolute pressure from approximately 20 to 900 microns Hg and for all

of the test gases. The Kinney McLeod gage was used as the reference

gage for absolute pressures below about 150 microns and the Wallace &

Tiernan aneroid gage was used as the reference gage for absolute pressures above about 200 microns. No results are shown in Figures 27 and

28 for pressures below 200 microns Hg when saturated air and water vapor

were used as the test gases because of the probability of error in the

McLeod reference gage due to condensed water vapor.

In Figure 24, the deviations of the Wallace & Tiernan aneroid gage

are shown for the test using nitrogen. These deviations are the calibration values for this gage, and would be the same for all tests in

this pressure range, since the pressure indication of the Wallace &

Tiernan aneroid gage would be unaffected by the composition or nature

of the test gas.

Comparison of Figure 16 with Figures 24 through 31 indicates that

on the average, in the absolute pressure range from 200 to 900 microns

Hg, the lowest deviations from absolute pressure would be observed with

the Wallace & Tiernan aneroid gage for refrigerant R12 and for saturated

air and water vapor, whereas slightly lower average deviations were observed with Kinney McLeod gage for laboratory air and refrigerant Rll.

The average deviations of the ASCO thermocouple gage and the Kinney McLeod

gage were lower than those of the Wallace & Tiernan aneroid gage for dry

air and nitrogen. The average deviations of the ASCO thermocouple gage

and the Wallace & Tiernan gage were approximately equal in this pressure

range for refrigerant R22. None of the gages except the Wallace & Tier- nan aneroid gage was very satisfactory throughout this pressure range

when used with saturated air or water vapor.

Since the pressure indication of the Kinney McLeod gage was probably unaffected by any constituent of the test gas except water vapor,

a comparison of Figure 16 with Figures 24 through 26, and Figures 29

through 31 indicates that, in the pressure range below 150 microns Hg

,

the lowest deviations from absolute pressure would be observed with the

Kinney McLeod gage. However, the deviations of the NRC thermocouple

gage were quite low for refrigerants Rll, R12 and R22 and the deviations

of the ASCO thermocouple gage and the CVC Pirani gage were quite low for

nitrogen and dry air in this same pressure range.

- 14-
In Figures 32 through 34 the deviations of the observed pressures

from the absolute pressures determined by the Wallace & Tiernan reference

gage are shown for the Kinney McLeod gage, the CVC Pirani gage, and the

Hastings thermocouple gage, Model VT-4S, respectively. In these figures,

the deviations for all gases used for the test of a single gage, are shown

in one graph for comparison. These three gages all had working scale

ranges greater than 1 mm Hg

.

Figure 32 shows that the Kinney McLeod gage had a much different

error characteristic for saturated air and for water vapor than for any

other test gas. The deviations were approximately the same for nitrogen,

dry air, laboratory air, and the three refrigerant gases, and ranged from

about 10 to 15 percent of the absolute test pressure.

The results observed with the CVC Pirani gage for water vapor, Rll

and R12 are shown in Figure 33 for test pressures up to about 14 mm Hg

because the instrument continued to indicate on scale even though the

maximum scale range was only 2 mm Hg . Other gases used for the tests

produced similar results to those shown in Figure 33. The data in the

figure show that the deviations ranged from about 90 to 95 percent of

the absolute pressure.

The results in Figure 34 for the Hastings thermocouple gage show

that the gage indicated too high a pressure for nitrogen, dry air, and

laboratory air whereas it indicated considerably below the absolute pressure for the three refrigerant gases and for water vapor and saturated

air for the upper portion of the range of test pressures used. This gage

read too high for all gases in the absolute pressure range from 0 to about

3 mm Hg

.

Figures 35 through 40 show the results of tests of a particular gage

when tested with seme or all of the test gases at absolute pressures up

to 1000 microns Hg (1 mm Hg) . In these figures the calibrated Wallace &

Tiernan aneroid gage was used as the reference gage for pressures above

200 microns Hg and the calibrated Kinney McLeod gage was used as a reference gage for pressures below 150 microns Hg. No data are shown for the

0 to 150 micron Hg pressure range for tests with water vapor or saturated

air because of the limitations of the McLeod gage with respect to condensable vapors. The data shown in Figures 35 through 40 are the same data

shown in Figures 24 through 31, but with all data on a single gage shown

on the same graph in Figures 35 through 40 whereas all data with a single

gas were grouped together in the other figures.

Figures 35, 37, and 40 show that the Kinney McLeod gage, the NRC

thermocouple gage, and the CVC Pirani gage were not very sensitive to the

type of gas in the pressure range below 1000 microns except that the

Kinney McLeod gage was significantly in error when water vapor or saturated air was used as the test gas. Figures 36, 38 and 39 show that three

of the thermocouple gages were significantly more sensitive to the type of

- 15 -
gas used for the test. In Figure 36 it is shown that the deviations of

the ASCO thermocouple gage were quite low throughout the pressure range

from 0 to 1000 microns Hg for dry air and nitrogen, and somewhat

greater for refrigerants Rll and R22. In Figure 37 the NRC thermocouple

gage revealed low deviations for the three refrigerant gases for pres- sures up to about 200 microns Hg. By comparing Figures 35 and 40, it is seen that the indications of the GVC Pirani gage were somewhat more con- sistent for the different gases than the indications of the Kinney McLeod

gage in the pressure range from 200 to 1000 microns Hg. The deviations

of the Pirani gage were a little higher at the upper end of this pressure

range except for water vapor and saturated air.

Figure 41 shows the repeatability of observed deviations of each of

two gages during two different test runs with nitrogen. The values

plotted are the deviations from the corrected absolute pressures determined by the aneroid reference gage.

Figure 42 shows the observed deviation, in mm Hg, of the Kinney

McLeod gage from the corrected Wallace & Tiernan absolute pressure ref- erence values during a test with dry air as the gas, and may be compared

with Figure 41 (with nitrogen) and with Table 3 (calibration by NBS Pres- sure Measurement Section)

.

In all the graphs shown in Figures 15 through 42, the deviation above

the reference line is the amount the test gage indicated higher than the

reference absolute pressure and should be subtracted from the observed

value to obtain the absolute pressure, likewise, the deviation value below

the reference line should be added to the observed pressure to obtain the

absolute pressure.

6. DISCUSSION AND CONCLUSIONS

Refrigerating systems that need evacuation under service conditions

are likely to contain refrigerant gas and room air mixed in any proportions. Since room air always contains some moisture, there is the possibility that some free moisture would exist in the refrigerant circuit

especially if air entered the system while it was cold. When free moisture exists in a system under evacuation, water vapor will probably be

the last gas to leave the system because water has a lower vapor pressure

than any other fluid in the system except the oil. In order to remove water from a system by evaporation and evacuation, the pressure in the

system must be decreased to the range from 22.2 to 5.2 nsn Hg for ambient

temperatures in the range from 75°F to 35°F in order to evaporate the

water. Thus, it is evident that the gages used to indicate evacuation

pressures in the field must provide reasonable accuracy for saturated air

and water vapor in addition to the refrigerant gas employed in the system

for pressures considerably below 20 mm Hg. Furthermore, gages used for

field service must be reasonably rugged in construction and simple to use.

- 16-
The comparisons made in this study of several gages indicate that

the Wallace & Tiernan aneroid gage is probably best suited to the needs

of the Defense Department for field and shop evacuation service of re- frigerating systems because its indication is not affected by the type

or composition of the gas, its accuracy is quite good down to absolute

pressures of 1 mm Hg or lower, it does not require a battery or other source of electric power, it does not require any manipulations or adjustment by the operator, and it is quite rugged in construction. It

might be desirable to request a wire-reinforced glass face for the gage

to decrease the hazards of breakage.

The McLeod gages would have usefulness where water vapor or saturated

gases were not involved and where the tilting or lever movements required

could be safely and effectively accomplished. The Pirani gage, and the

NRC and ASCO thermocouple gages could be used for reasonably accurate

pressure indications for some gases and for selected ranges of pressure

below 1000 microns Hg absolute.

Although the scales for Pirani and thermocouple- type gages, such as those included in this study, are typically graduated for use with dry

air as the reference gas, these instruments could probably be furnished

with scales graduated for some other gas, e.g. refrigerant R12 or R22,

if desired.

USCOMM-NBS-DC

- 17 -
DEVIATIONS

IN

OBSERVED

PRESSURE

FOR

THREE

GAGES

USING

DRY

AIR

T

OH HH *3HflSS3M aUllOSHV WOHi NOLLVIA3G

ABSOLUTE

PRESSURE

BASED

ON

CALIBRATED

WALLACE

AND

TIERNAN

ANEROID

REFERENCE

GAGE,

MM

HG
OH WW * 3HQSS3HH 3imOS9V WOM NOLLYIAHd

ABSOLUTE

PRESSURE

RASED

ON

CALIBRATED

WALLACE

AND

TXERNAN

ANEROID

REFERENCE

GAGE,

MM

HG
DEVIATIONS

IN

OBSERVED

PRESSURE

FOR

THREE

GAGES

USING

SATURATED

AIR
DEVIATIONS

IN

OBSERVED

PRESSURE

FOR

THREE

GAGES

USING

WATER

VAPOR

7I

6 T“ T

J o04

sth. ww ‘aanssaaa aimosav wom NOiivi'Aaa

ABSOLUTE

PRESSURE

BASED

ON

CALIBRATED

WALLACE

AND

TXERNAN

ANEROID

REFERENCE

GAGE,

MM

HG
DEVIATIONS

IN

OBSERVED

PRESSURE

FOR

TWO

GAGES

USING

Rll 1

II

11 ±

CNJ + wowozwW

fcW2QMowz<z

<1wM

EhOz

<cwu3gQWH

ss

CQM

hJgQW

CO

COCO§

Cu

13

!=> Zlo

CO

CQc

CSI w

gfc

l-l Cu

OH WW * 3HflSS3HH SLMOSaV WOHJ SMOUVIAaQ
DEVIATIONS

IN

OBSERVED

PRESSURE

FOR

TWO

GAGES

USING

R12

I

CM

CM

oh ww

c amiss huh aimossv wouh snoiiyiasu

ABSOLUTE

PRESSURE

BASED

ON

CALIBRATED

WALLACE

AND

TIERNAN

ANEROID

REFERENCE

GAGE,

FIGURE

22
DEVIATIONS

IN

OBSERVED

PRESSURE

FOR

THREE

GAGES

USING

R22
DEVIATIONS

IN

OBSERVED

PRESSURE

FOR

VARIOUS

GAGES

USING

NITROGEN

I I "T T r

o O o

o o o

vO <* CN+

o o

or\l

t

o

co§

Vi

C_> H

a

o

o

o

w

l-a CM w o o

hJ o o

Cm o w

ta s J

O

w o

O p s

H H

§ 2! o

CO <5 K

S O

H a w CO H CM W

O H

U CO a g CO < E> O

<5 3 o i*; <

M

CM

!=> o

u

o

«H

O

Viw

!S<

w1—

i

H

W

o

3

Q o o

O o o

cO CO

o

o

30

e>

rc

CO

Pdu

M

£

COW

e>3

o w

o

w

Piw

O H

paM

t-5

>h

PQ O QW

a

w

o

COC

cO W

pc! P

coCO§CM o

.~o

ta

>-a o

CO

pa<C

<r

CN

PCV

C

ifx

oh snohoiw ‘annssaHH aimossv hohj NOiiviAaa
600 L

DEVIATIONS

IN

OBSERVED

PRESSURE

FOR

VARIOUS

GAGES

USING

DRY

AIR

oh snoh£)iw ‘annssaM axmosHV Mdm ifloiiv'iAaa

T

±

8

200

300

400

500

600"

TOO

1

ABSOLUTE

PRESSURE

AS

DETERMINED

BY

CALIBRATED

REFERENCE

GAGES,

MICRONS

HG
vO

CN

O
600 (-

deviations

in

observed

pressure

for

various

gages

using

saturated

air

oh snohoiw ‘ annssHHH aimosav woHH NOiiviAaa
o

w

C/3 B

ftu

M

£

ftw

o

$

w

o

z

w

piw

fa

fa

Piq

w

ft£

ftM

fa

>-< m

q

w

aM

CdftW

ftft<

w

ft

ftw

fa

faw

faft

fao

ft

ft<

00

CN

o

fa

fa

oh snohoiw ‘aanssana aimosav woaa MOiiviAaa
DEVIATIONS

IN

OBSERVED

PRESSURE

FOR

VARIOUS

GAGES

USING

Rll

oh snohdiw ‘amssHM aiiriosav mohh HoxivxAaa

100

~

200

300

400

500

600

700

800

ABSOLUTE

PRESSURE

AS

DETERMINED

BY

CALIBRATED

REFERENCE

GAGES,

MICRONS

HG
DEVIATIONS

IN

OBSERVED

PRESSURE

FOR

VARIOUS

GAGES

USING

R

12

+ 1 oh snouoir ‘aunssHM aunosav ijouh NouviAaa
DEVIATIONS

IN

OBSERVED

PRESSURE

FOR

VARIOUS

GAGES

USING

R

22

OH SNOROIW ‘aHTlSSSHJ aiMOSaV WDHJ H0IIYIA3CI
DEVIATIONS

IN

OBSERVED

PRESSURE

FOR

KINNEY

MC

LEOD

GAGE

USING

VARIOUS

GASES

OH WH ‘BDUlSSaRJ 3LM0S9V WOM NOIXVIAaa

ABSOLUTE

PRESSURE

BASED

ON

CALIBRATED

WALLACE

AND

TIERNAN

ANEROID

REFERENCE

GAGE,

MM

IIG

FIGURE

32
DEVIATIONS

IN

OBSERVED

PRESSURE

FOR

CVC

PIRANI

GAGE

USING

VARIOUS

GASES

ABSOLUTE

PRESSURE

BASED

ON

CALIBRATED

WALLACE

AND

TIERNAN

ANEROID

REFERENCE

GAGE,

MM

HG
DEVIATIONS

IN

OBSERVED

PRESSURE

FOR

HASTINGS

0-20

MM

HG

THERMOCOUPLE

GAGE

USING

VARIOUS

GASES

oh ww ‘asnssaaa aimosav woaa NOLiviAaa

ABSOLUTE

PRESSURE

BASED

ON

CALIBRATED

WALLACE

AND

TIERNAN

ANEROID

REFERENCE

GAGE,

MM

HC
DEVIATIONS

IN

OBSERVED

PRESSURE

FOR

KINNEY

MC

LEOD

GAGE

USING

VARIOUS

GASES

—

200

300

400

500

600

700

““

800

ABSOLUTE

PRESSURE

AS

DETERMINED

BY

CALIBRATED

REFERENCE

GAGES,

MICRONS

HG
600 1

DEVIATIONS

IN

OBSERVED

PRESSURE

FOR

ASCO

THERMOCOUPLE

GAGE

USING

VARIOUS

GASES

100

200

300

400

500

600

700 “

800

ABSOLUTE

PRESSURE

AS

DETERMINED

BY

CALIBRATED

REFERENCE

GAGES,

MICRONS

HG
DEVIATIONS

IN

OBSERVED

PRESSURE

FOR

NRG

THERMOCOUPLE

GAGE

USING

VARIOUS

GASES

T r

oh snohoxh ‘amssaM aunosav mm soximaa

SATURATED

AIR
DEVIATIONS

IN

OBSERVED

PRESSURE

FOR

HASTINGS

0-20

MM

HG

THERMOCOUPLE

GAGE

USING

VARIOUS

GASES

oh sNOHDiw ‘aanssaaa aamosav icraa NonviAaa

100

200

300

400

300

600

700

800

900

ABSOLUTE

PRESSURE

AS

DETERMINED

BY

CALIBRATED

REEERENCE

GAGES,

MICRONS

HG

FIGURE

38
DEVIATIONS

IN

OBSERVED

PRESSURE

FOR

HASTINGS

0-1000

MICRONS

HG

THERMOCOUPLE

GAGE

USING

VARIOUS

GASES 1

CNi CN 1 M<£

e>

100

200

300

400

500

600

700

800

#00

ABSOLUTE

PRESSURE

AS

DETERMINED

BY

CALIBRATED

REFERENCE

GAGES„

MICRONS

HG
DEVIATIONS

IN

OBSERVED

PRESSURE

FOR

CVC

PIRANI

GAGE

USING

VARIOUS

GASES

T

1

vn ~i ' r~“ T

+

OH SNOHOIW ‘3HDSS3RH 3J,mOS9V WDHi NOLIVIAaQ

1 1 __

1

"200

300

~

400

500

600

700

800

ABSOLUTE

PRESSURE

AS

DETERMINED

BY

CALIBRATED

REFERENCE

GAGES,

MICRONS

HG

FIGURE

40
oh m ‘sransssHd aimosav woaa NOiiviAaa

ARSOT.TTTF.

PRF.SSTTRF

AS

PF.TFRMTNFn

RY

PAT

TRRATFn

PRFSSTIRF

PA

PF

MM

HP
9H m ‘annsssaa simosav whi mouyiasci

ABSOLUTE

PRESSURE

AS

DETERMINED

BY

CALIBRATED

PRESSURE

GAGE,

MM

HG