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The Use of a Barometer in Cave Surveying

By R.D. Stenner

For quickly checking the altitude at any point in a cave and for producing a survey of the surface contours above a cave system and the value of a sufficiently accurate and portable barometer is obvious.  The instrument has a place in rapidly surveying a system which is essentially vertical, in nature, used with a clinometers and compass.  The combination of instruments can be used to survey continental shafts that may be hundreds of feet deep, and the resulting survey can be as accurate as one produced using a tape, providing that the precautions outlined in the following article are followed.

There are two types of instrument that can be used.  The first is a capsule barometer calibrated to measure air pressure in millibars. One can read this instrument to an estimated 0.01mb. measuring differences of heights to ±0.3ft. representing a precision of one part in 105.  Such instruments are expensive.  The second type of instrument is an ex. W.D. altimeter, which is an aneroid barometer calibrated to measure altitudes directly.  The instrument most commonly available is calibrated at intervals of 20ft. and readings can be taken to be estimated to ±5ft.  Altimeter readings need to be corrected for temperature and relative humidity.  Both instruments have the same method of operation.

Air pressure varies in a complex way.  In some conditions an unstable inversion of layers can make any barometric survey unreliable.  In settled weather with a steady blight wind, pressure variations will be approximately linear.  In such conditions it is permissible to use a single instrument, duplicating readings at one station to obtain the correction graph.  The time of reading should be noted, and the air temperature should be measured.  In stormy weather and especially in thundery weather rapid and irregular changes in air pressure can take place.  If it is necessary to make a barometric survey in these conditions a second instrument should be used on the surface to obtain a correction graph.  If a strong draught such as may be met in a narrow constriction or at a pitch where a considerable volume of water is falling in a narrow shaft, and is strong enough to extinguish a carbide flame that may be a pressure difference big enough to cause considerable error.

All aneroid barometers must be tapped lightly before a reading is made.

The Calculation of the Results.

The change of height between two stations is related to the change of air pressure.  The relationship can be expressed mathematically by the following equation:

H =   x   x

h:  increase in height from station 1 to station 2

t:  mean temperature in 0oF

po:  air pressure in millibars at station 1

p:  air pressure in millibars at station 2

go: local value of gravity in C.G.S. units

gm:  mean value of gravity, 970.67c./sec2

wm = water vapour pressure at t0F x relative humidity divided by pm.

Pm:  mean pressure between stations 1 and 2

The local value of gravity can be calculated.

g(seal level) = 978.05 (1 + 0.005288sin2Ø – 0.000006sin2Ø)

where Ø is the latitude.

gn = g(sea level) – 0.000309H + 0.000042 ÓH

where Ó is the density of the mountain and H is the altitude.

If a Fortin barometer is used to calibrate a barometer or an altitude the readings must be corrected as follows:

Lc = Lt 1 - 

Lt: height of mercury column in cm at toC

Lc:  corrected height

 C: 0.0001818 per oC

 :  0.0000184 per oC

L.o.g. = pressure in dynes cm-2

Gn: mean value of gravity, 980.67 cm/sec-2

This is the pressure at standard gravity, and with go as before, and the mercury barometer calibrated for standard gravity.  The readings can be corrected for the local value of gravity as follows;

hn = ho x

hn: corrected reading                              ho:  observed reading

 

The gravity corrections are usually negligible, and leaving them out gives an error usually less than 1%. Neglecting the humidity gives an error of 0.5% at 50oF and 100% relative humidity.  An error of 5oF gives an error of 0.9%.  An error of 0.1mb gives an error of 3ft.

Examples of the Use of Barometers in Cave Surveying.

1. A sensitive capsule was loaned to the Bristol Exploration Club by the makers, Mechanism Ltd., of Croydon, for evaluation of the instrument.  The micrometer could be read to 0.05mb, and the electrical contact was sensitive to vertical movements of 6 inches.  The instrument was mounted in a wooden box measuring 9” x 9” x 7”.

A barometric survey was made by Roger Stenner and George Honey in St. Cuthbert’s Swallet, on 2nd August 1959, using the instrument.  The trip was deliberately lengthened to almost eleven hours.  Readings were taken at the entrance at the beginning and the end of the trip to enable the readings to be corrected (assuming a linear change of air pressure at the surface) and to test the correction, readings were duplicated at one station (High Chamber) with a time interval of five hours.  When the barometric survey was made the cave was un-surveyed, but it is now possible to evaluate the measurements.

Air temperature measurements in the cave have shown the air temperature to be fairly constant at about 10.5oC and the Relative Humidity 100% (Burt and Petty, 1958, pp -5; Petty, 1957, pp.3-4).

The equation becomes:  h = 54790 x 

The stations: -

1 – top of the Old Entrance Shaft.

 

2 – floor of passage from Entrance Pitch to Arête Pitch.

 

3 – Arête Chamber to top of Arête Boulder.

 

4 – floor of passage to wire rift.

 

5 – Mud Hall, bottom rung of ladder.

 

6 – Boulder Chamber, near Kanchenjunga.

 

7 – Upper Traverse Chamber.

 

8 – High Chamber floor.

 

9 – High Chamber floor.

 

10 – Main Stream, Plantation Junction.

 

11 – Main Stream, Beehive Chamber.

 

12 – Gour Hall, lip of Great Gour

 

13 – Main Stream, bottom of the gours.

 

14 – Main Stream, the Duck.

 

Station

Time hours

Air Press. mb

Corr’d Press mb

Depth feet

Height ab. O.D. feet

O.D. ht. from survey ft.

Difference feet

1

13.25

992.85

992.85

0

783

783

0

2

13.45

994.80

994.85

55

728

725.5

-2.5

3

13.50

996.25

996.30

94

689

691.5

2.5

4

14.00

998.45

998.55

156

627

631.0

4.0

5

14.15

1000.70

1000.80

217

566

571.5

5.5

6

15.15

1001.05

1005.25

230

553

558

5.0

7

16.15

1001.40

1001.80

244

539

545

6.0

8

16.22

999.85

1000.25

202

581

587

6.0

9

21.25

999.25

1000.25

202

581

587

6.0

10

21.45

1000.10

1007.20

391

388

394

6.0

11

21.50

1006.10

1007.20

391

388

392

4.0

12

21.53

1005.50

1006.60

374

409

412

3.0

13

21.55

1006.30

1007.40

396

387

391

4.0

14

22.00

1006.60

1007.70

404

379

382

3.0

1

11.10

991.45

992.85

0

783

783

0.0

The agreement is good enough to show the useful ness of a single accurate barometer, used in stable weather conditions, and the value of such an instrument (use in conjunction with a clinometer and compass on a combined head) in an essentially vertical pothole, is obvious.  The variation between stations 1 & 4 may be due to a variation in air temperature near the cave entrance, warmer surface air being drawn into the cave.

2.  A. Single ex W.D. altimeter used in stable weather to survey surface features.

The surface features in the neighbourhood of St. Cuthbert’s Swallet were surveyed by Roger Stenner, Dave Irwin and Alan Thomas.  On April 2nd, 1966 a single line survey was taken across the valley from the cave entrance to Plantation Swallet by conventional cave surveying techniques, and a number of stations were marked.  Using this line as a base line, the depression was surveyed on April 3rd. 1966 using a compass and the altimeter.  The plan position of features in the depression were fixed by triangulation, using the compass.  The altitude of the features were determined by moving around the features in an anticlockwise direction estimating the altimeter readings to two feet.  Readings were repeated at the starting point, the cave entrance, and the rest of the readings were corrected.  The whole set of readings (and the corrections) were repeated, moving in a clockwise direction.  The corrected altitudes were compared.  In most cases the readings differed by less than two feet, in which case the mean was used. If corrected values differed by more than two feet the determination was repeated.

By this method the surface depression above St. Cuthbert’s Swallet was surveyed quickly and to the required accuracy.  The altimeter was free from the common fault of sticking and thus being unresponsive to small altitude changes.  Altitude measurements made that day were quoted as having a precision of ± 1ft.

3.  The use of two ex-W.D. altimeters in unstable weather conditions.

During August 1967 an expedition was made to the Ahnenschacht, in Totesgebirge, Austria, by members of the B.E.C.  During the expedition a series of altimeter readings was made at the surface by the writer, while Alan Thomas made a series of altimeter readings in the Ahnenschacht, over a period of 32½ hours.

The measurements were started during a fine hot spell and during the first night a thunderstorm swept the mountain.  The following day was dry and windy, but the night brought another thunderstorm, and ten hours of heavy rain that put an end to the assault on the final unexplored shaft. The results of the measurements made in the cave are shown in figure 1.  Results of the measurements made in the cave are shown in the following table: -

Stations

1  Entrance.

 

2  Top of Pitch 5.

 

3  Top of Pitch 9

 

4  Foot of Pitch 10.

 

FIGURE 1.

Station

Date

Time (hrs)

Altitude Ft.

Correction (ft.)

Corr. Alt. (ft.)

Depth (ft.)

1

23.8

18.15

4942

0

4942

0

2

23.8

20.00

4760

-20

4740

255

2

23.8

20.20

4750

-22

4728

267

3

23.8

21.30

4680

-40

4640

355

3

23.8

22.00

4680

030

4650

345

4

23.8

23.45

4590

-45

4545

450

4

24.8

09.40

4640

-115

4525

470

4

25.8

02.50

3695

-165

4530

465

The altitudes quoted are relative to an arbitrary value of 5,000ft. for the base camp.  Apart from the second reading at station 4, the mean will agree to ±6ft., even in these unsuitable weather conditions.  At the time of the second reading at station 4 was made the air pressure at the surface was varying rapidly, and an error in time measurement of only ten minutes at the surface would bring the value of the depth to 460ft.  Figure 1 demonstrates the need for the second instrument if the weather is unsettled, or if the measurements have to be made over a long period of time.  Both of the altimeters were sticking at the time.

The Ahnenschacht is a cave which lends itself ideally to survey by the combination of accurate barometer, compass and clinometer, provided that the clinometer mounting allows for readings of ± 90o (degrees).