A compact self-contained ground temperature recorder

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A compact self-contained ground temperature recorder

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NATIONAL RESEARCH COUNCXL

CANADA

DIVISION OF BUILDING NESEARCH

A COMPACT SELF-CONTAINED GROUND TEMPERATT'RE

RECORDER

G. H. JonNsroN

; Aruarvzro

Reprinted from C anadinn Eeotechnical I oumal Vol. III, no, 4, November 1966

pp.24G-250

NRC 9316

s3319

Technical Paper No. 288 of the

Division of Building Research

OTTAWA

November 1966

Price l0 cents

til

9

600

RESBARCH NOTE

A Compact Self-Contained Ground Temperature Recorder G. H. JonNsroN*

frequent observations, manual methods are time-consuming, costly, and- some-tim6s unreliable due to observer error, resulting primarily from difficult ffeld conditions such as biting insects, marshy ground, and severe weather. Elec-tronic recording equipment, which is usually rather complex, generally requires frequent and regular maintenance and servicing. Many northe_rn locations are isolated or quite remote and access is dificult, if not impossible, at certain periods of the year. In addition, observers trained in the use and servicing of manually or electronically operated equipment are not usually available.

There was a real need, therefore, for a simple temperature-measuring device of reasonable accurary which would run unattended for a period of at least one year. Such an instrument, which is relatively small, fairly rugged, self-contained, and which will operate at reasonably low temperatures,

DsscRrPTroN oF INSTRUT,GNT

The instrument consists of two major components: a temperature-sensitive bimetallic strip and a battery-powered electric clock. Two L1A-volt flashlight-size mercury batteries connected in series are used as the power supply. The coil-wound bimetallic strip contracts or expands in response to temperature fluctuations and moves a pointer attached to one end. The pointer leaves a trace on a pressure-sensitive (wax-coated) paper ffxed to a metal disk (31i in. dia.) which makes one complete revolution in one year. The disk is rotated, through an arrangement of gears, by the battery-operated electric clock motor. The complete assembly is placed in a moisture-proof metal canistet (33/a in. dia' *Research Officer, Northern Research Group, Division of Builfing Research, National Research Council, Ottawa, Canada.

246

RESEARCIINOTE 247

by ll in.long) for installation in the ground. Total weight of the instrument is

about 7 lb. Two views of the device are shown in Figure I.

Temperature fuctuations within a range of 20' F. can be measured with

the instrument described here. Temperatures encountered in permafrost at

depths of from 3 to 30 ft. may lie between 20o and 40' F. The

temperature-sensing element is therefore positioned in the instrument so that the pointer

will mark the dial accordingly. Stops are provided to prevent the pointer

from running over the edge of the chart should the temperature range be

exceeded.

After fabrication each instrument is adjusted and calibrated in the laboratory

for the temperature range to which it will be exposed at a particular location.

A special transparent template (lucite), on which 1o F. temperature intervals

( a ) ( b )

Frcunr 1. Ground tenrperafure recorder, Left, front view; right, side view

are inscribed, is made up following calibration. When a record has been

obtained temperatures can then be taken from the chart by placing the

template over the waxed dial. A typical chart and template are shown in

Figure 2.

Although the instrument has been designed to record relatively slow

temperature fluctuations, it will respond fairly quickly to rapid changes in

temperature. It should be noted that the time scale is compressed,

i.e. quite

short in length, and therefore this particular instrument is best adapted for

situations where temperature changes take place rather slowly. For example,

Figure 2(a) is a one-year record (actually 386 days) of ground temperature

at a depth of 20 ft. A temperature change of about 0.5" F. during this period

is recorded on a trace about 4.2 in. long. Laboratory tests and ffeld checks

indi-cate that the accuracy obtained is better than 1o F., probably about t 0.5" F.

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248 CANADTAN GEIOTECHNICAL JOURNAL

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Frcrmr 2. Ground temperafure.recorder, (a) One-y.ear ground temperature record, (b) Temperature template

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To install the instrument, a 5 in. dia. hole is drilled in the ground and cased

with plastic pipe, tightly sealed by a plastic cap at the lower end to prevent

entrance of water. The canister containing the instrument is then lowered

inside the casing to the depth at which temperatures are to be measured. To

minimize or eliminate temperature fluctuations within the casing as a result

of air movement by convection, lengths of standard pipe insulation, separated

by rubber washers, are slipped over the stainless steel cable used to lower and

retrieve the canister. The ,casing is filled with insulation from the canister to

the ground surface; insulation is also placed below the canister if required.

The casing extends to within 2 to 3 in. of the ground surface and is closed

by a snug-fitting plastic-pipe cap. The top of the casing is covered with turf,

moss or soil following installation of the instrument. A typical installation is

shown in Figure 3.

Orspn Appr-rcerrows

Lake bottom water temperatures have also been measured using a slightly

modiffed version of this instrument. All components of the water temperature

device are virtually identical with the exception that a larger wax dial (5 in.

dia. ) is used to increase the range of temperature measurement to 40 F deg.

The protective canister is correspondingly

larger (6 in. dia X 12 in.).Th"

canister ( weighted if necessary

_{and standing on three legs ) is lowered to the}

NESEARCH NOTE

i'11

250 cANADTAN cEorEcHNrcAL JoURNAL

bottom of the lake by a stainless steel cable. To the top end of the cable is

attached a set of brightly painted copper foats which is positioned below the

bottom of the ice cover (usually < 5 ft. thick). To retrieve the instrument

(the location of which is established from on-shore reference points and

visual identiffcation of the float) a looped cable is lowered from a boat, or

through a hole in the ice, and secured around the foats. The assembly is then

pulled to the surface. A typical unit prior to installation in an ice-covered

Iake is shown in Figure 4.

This instrument can also be used, of course, to measure temperafures in

other environments,

e.9., in buildings-attics, crawl spaces,

etc. It is particularly

useful whenever long-term records are desired at locations that are remote or

where access

is diffcult. By simply changing the gear train the recording time

can be lengthened or shortened. The recording range of temperature can also

be increased or decreased

by appropriately changing the size of the dial.

This combination of two standard components, a battery-powered clock and

a bimetallic element, results in a very useful, compact temperature recorder

that can be placed in a relatively small drill hole and will run unattended

for a period of at least a year. It is entirely self-contained-that is, no external

power source, read-out apparatus, or accessory equipment of any kind is

required; nor does a shelter have to be provided on the gound surface.

Temperatures measured can be read directly on the chart which is preserved

for a permanent record. Details of the instnrment can be obtained by writing

to the Division of Building Research, National Research Council, Ottawa,

Canada.

Acrxowr-rncnrsNrs

The idea for the device resulted from discussions within the Division of Building Research. Mr. C. St. Tacques and Mr. R. Tetu of the Division carried out the maior desie; and development woik. to them much credit and sincere thanks are due. This is'a confr-bution fronl the Division of Building Research, National Research Council of Canada and is published with the approval of thebirector of the Division.