Calibration of Heat Meters

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Technical Note (National Research Council of Canada. Division of Building Research), 1958-09-01

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Calibration of Heat Meters

Woodside, W.

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DIVISION OF BUILDING RESEARCH

'fECHNlICAIL

NOTJE

ｾ

262

NOT FOR PUBLICATION

PREPARED BY W. Woodside

PREPARED FOR Reoord Purposes

CHECKED BY AGW

FOR INTERNAL USE

APPROVED BY MBH

DATE September 1958

SUBJECT Calibration of Heat Meters

When a t mperature dift reno is applied aoros plan

• 11 (under steady-state oonditions) the temper ture d1trer nee

between the two pI nes in th w 11, parallel to the wall urf c ,

1 proportionsl to the rate of heat flo. Thu plane pI t

with thermooouple tt ched to it urt _{oes, press d ag inst the}

wall surfaoe, oonstitute a heat flow meter, inoe th ams mount

of heat pa'sses through both wall and plate.

The thermal resistanoe of a heat meter should be 11 in

oomper! on with the re i tanee

or

the all, 1n order that the he t

flow b ssent1ally unaffeoted by attaohment of the meter. Al 0

if heat flow under tran i nt or p riodlc oondition i to

me ured, he t met r should h v low th r I c pacity 0 t it

may follow oh nges in he t flow with little time 1 g. Bec of

th fir t requirement , the temp rature differenoe between t

urtaces of the heat t r plat is small. A large numb r of

thermocouples oonneoted differentially and In serie (i.e., th

rmo-pile) is therefore used to produo a larger and more easl1y asur d

voltage output.

The heat meters in use at the Division of Building Rese roh are of the Gier and Dunkle design and were purohased from Beckmann

and Whitley. Eaoh meter oontains a thin phenolio r in (bakelite)

plate around which are wound several hundred turns of fine oonstantan

wire'. Half of the wires re silver-pI ted in suoh a way that

silver-oonstantan thermopile i formed. This element is surrounded

by bakelite of the same thiokna and proteoted by bakelite cover

plates

1/64

in. thiok, re ulting in a meter with an over-all

thiokness of approximately

3/64

in. Two sizes of meter are

avail-able:

4

1/2 x 4 1/2 in. or 1 1/4 x 3

1/4

in. Meters with gre tar

ensitivity are made using several thermopil elements laid side

2

-two outer lugs are inter11811, output

ot

_{whioh i8 propotttJ1one4}

r. The two inner luge aft looated 1n the oentre ot _the

mperature.

Heat meters ot _{this design are current11 be1na used 1.}

the Division in oonnection with the following studies. (a) transient and periodic methodli for determlnins the

thermal properties of materials

(b) measurements of looal heat .flux. through vertical walls containing air spaces

(0) surface heat transfer measurements in oonneotion

with thermal radiation studies

(d) heat flux measurements in the ground and snow oover Ce) measurement of heat inflow and outflow in soil semple

undergoing frost heave. Calibration

The heat meter thermopile output is directly proportional to the rate of heat flow through the meter. For absolute deter-minations of heat flow the proportionality faotor must be

known.

This proportionality or oalibration faotor obviously depends

upoa

the thermal resistanoe between the two sets of thermopile junotlObI, the number of elements in the thermopile and the thermo-eleotrio power of a silver-constantan thermooouple. Since all of these factors are not known accurately, the calibration faotor for a heat meter oannot be oaloulated, and must therefore be obtained

experimental17. Also the calibration factor will vary with the mean temperature of the h&61t meter.

The manufaoturer furnishes with eaoh heat meter a calI-bration factor and a graph, common to all meters, showing the variation of this taotor with temperature. Because of the wide use ot heat meters in the Division, however, and some doubt in

the aoouraoy of the manufaoturer's calibration, an apparatus for calibrating heat meters has been construoted.

A heat meter may be oalibr t d by plaoing it n B ria

thermally with one of the test ｳｰ･ｯｾ･ｮ n guarded ot pI t

apparatus. However, if only one measurement i to be performed it must be assumed that the hot plate produo equal heat flo

with the t

on tb ot 1',

the •

A

on dir otio doe

Under t a d,.-8tat

b t flux U

or

whioh 1

Thus the pp ratus 1 oh of' which produo calibrating h t t r

thermal oonduot1viti Thi 1s 1n oontra t to in whioh only th vel' can b SU%' d.

Calibration Prooed

Eaoh heat t r to b lite guard ring

3/64

in. i

nd having 1tit r 1 d1 Dal0

me tel' In the a e of th three suoh

oh bot pl te

4

-The guarded heat ters replaoed in the uniflow apparatus, u u l1y adj oent to one of the h atar plates. In the initial oa1i-bration s v 1" 1 meters war toed one above the other and

o I1brated slmultan ously. In the later oalibrations however, on1,. one met r is plaoed dj oent to e oh hater plete (or in tbs 0 8

of tbe small ｭ･ｴ･ｾｳＬ tbr e re pleo d aide ｢ｾ side). It i important that t th 1" 1 e 1 t no between the hot n oold

plat surt oes b uniform 0 l' th whol are in order t t t

he t flow be unidireotional. heet

ot

_{8 by 8 by /2 in. ins a i}

tibr board ar therefore int rpo d betw en the he t t r an the cold plate.. In the more reoent 0 libration

1/16-1n.

l' 1 ha

been used on both faoes of the heat tel" to impro e th I oontao • After installation of the heat meters the whole pp ratu 1s olamped at the tour oorners to further ens\.1r good th 1 oontaot.

output and tber.mooouple lead wires from aoh meter ar oonneoted to a seleotor switoh whioh is in turn oonneoted to high preoisio potentiometer.

When a thermal steady state has been attained, measurements of the power inputs to the heater plate test areas, and the outputs of th'e heat meter thermopiles and thermoooupies are taken over a period of approximately 3 hours. The power input measurement₁ whioh represents the heat flux through the heat meters, involves ｏｾＱ

the measurement or the voltage applied aorosa the test area ot _the

heater plate and the measurement of the Yoltage drop across a standard resistanoe in the beater oircuito It is estimated that

the heat input per unit area (BtU/hr. rt2 ) may be measured with an aoouraoy of the order ot

0.5

_{per cent. 2Tbe calibration faotor for}

eaoh heat meter, in wiits of Btu/hr. tt mv, is obtained by dividing the heat input b" the average value of the heat meter output masU!"ed in millivolts.

It Is not a1wa". possible to balanoe perfeotl t te per -turas of the two heater plat s. Howa ar the heat inputs to the two plates are easil" oorreoted for n" uoh 1mb lane providing t th rmal resistance or the insulation b 11 en th plate is known. This prooedure W8S followed for earlier heat meter

oali-brations by the Division. In the more recent oalibration , a

a librated heat meter. inst lIed between the two beater plates. The temperatures of th two plates were varied until the output of this ｭ･ｴ･ｾ was approxtmatel z rOe The beat flow egistered

by thl me,ter, ususlly Ie than 1 per oent of the heat input to

the two plates, is then u ed direotly to make oorr ctions due to imperfeot balancing.

- 5 ..

Disou sion

It is estimated that the tot the heat f'lux through a heat metex- 1 output of' the meter is in the order at ot _{six of the sma 11 heat meters in t}

d1fterent beat meter temperatures sho taotor. Taried linear17 with temper t this variation was almost the Baine for

were pertormed suoce8sively, without r meters atter eaoh oalibration. In OD

meter. were being calibrated, the me apparatus and then re-installed, and under the same oonditione. The repro oalibration faotors was les8 than

0.5

7

per oent for the other.

Also two meters which were calibrated in the uniflow apparatus were reoalibrated several times in situ 1n a

12-in.

test cell, the apparatus for ｷｨｩ｣ｨｾ heat meters were intended. The in situ oalibrations in the test cell were performed b7

establisbing a steady-state temperature gradient across a slab of rubber, the -thermal oonduotivity of' which had previously been determined in 8 guarded hot plate apparatus. Calibrations performed

in this manner have deviated from those obtained in the uni:rlow apparatus by as muoh as 10 per oent.

It would appear theref'ore that beat met r oalibration 1s Yery sensitive to the boundary condition imposed upon it during calibration and possibly that all meters should be oalibr ted

under conditions which simulate as closely possible the oonditio to be expeoted in use.

Cono1usion

A rough oalculation shows that the thermal conductance ot

the thermopile • sensitive' area

ot

_{the heat meter is of the order}

of two times the conduotanoes of the surrounding area, due to the presenoe of' the highly conducting silver-constantan thermo-pile wires. This nonuniformity of conductanoe over the area ot

the heat meter causes a distortion of the heat flow pattern. The

investigation of this and other possible reasons f'or the discre-pancies described above is being oontinued. The proportionality of' the heat meter output and the heat flow will be ohecked. This w1ll be done by m intain1ng tbe hot plate temperature oonstant-and performing oalibrations at different cold plate temperatures. Also the effeot of the relative positions of' the metal hot plate and the heat meter might be investigated, by performing oalibrations with the heat meter adjaoent to the hot plate and then with the

meter sandwiched between the slabs of rigid insulation. Finally, further determinations of reproducibility of oalibration should be made, both onoe installed and af'ter re-installation of' the meter. '