NRC Publications Archive
Archives des publications du CNRC
Performance of Katharometer for Air Infiltration Measurement
Tamura, G. T.
For the publisher’s version, please access the DOI link below./ Pour consulter la version de l’éditeur, utilisez le lien
DOI ci-dessous.
Publisher’s version / Version de l'éditeur:
http://doi.org/10.4224/20359123
Internal Report (National Research Council Canada. Division of Building
Research), 1962-05
NRC Publications Record / Notice d'Archives des publications de CNRC:
http://nparc.cisti-icist.nrc-cnrc.gc.ca/eng/view/object/?id=560e3dea-f856-4a39-8ff0-8131078d962b
http://nparc.cisti-icist.nrc-cnrc.gc.ca/fra/voir/objet/?id=560e3dea-f856-4a39-8ff0-8131078d962b
Access and use of this website and the material on it are subject to the Terms and Conditions set forth at
http://nparc.cisti-icist.nrc-cnrc.gc.ca/eng/copyright
READ THESE TERMS AND CONDITIONS CAREFULLY BEFORE USING THIS WEBSITE.
L’accès à ce site Web et l’utilisation de son contenu sont assujettis aux conditions présentées dans le site
http://nparc.cisti-icist.nrc-cnrc.gc.ca/fra/droits
LISEZ CES CONDITIONS ATTENTIVEMENT AVANT D’UTILISER CE SITE WEB.
Questions? Contact the NRC Publications Archive team at
[email protected]. If you wish to email the authors directly, please see the
first page of the publication for their contact information.
Vous avez des questions? Nous pouvons vous aider. Pour communiquer directement avec un auteur, consultez la
première page de la revue dans laquelle son article a été publié afin de trouver ses coordonnées. Si vous n’arrivez
pas à les repérer, communiquez avec nous à [email protected].
NATIONAL RESEARCH COUNCIL
CANADA
DIVISION O F BUILDING RESEARCH
PERFORMANCE O F KATHAROMETER FOR
A1 R INFILTRATION MEASUREMENT
by
G. T. T a m u r a
Internal R e p o r t No. 254
of the
Division of Building R e s e a r c h
OTTAWA
May 1962
PREFACE
It i s often important t o be able t o determine the a i r changes
in any occupied space in a building which r e s u l t from natural or
induced a i r leakage. The direct measurement of t h i s infiltration r a t e
i s difficult. The most acceptable method a t present involves the
u s e of helium a s a t r a c e r gas and measurement of helium concentrations
using a katharometer. Some experiments on the techniques and
instrumentation a r e now reported.
The author i s a mechanical engineer and a r e s e a r c h officer
with the Building Services Section of the Division, and is in charge
of infiltration studies being c a r r i e d out in houses.
Ottawa,
May 1962
N. B. Hutcheon,
Assistant Director.
PERFORMANCE O F KATHAROMETER FOR
A1 R INFILTRATION MEASUREMENT
by
G. T. T a m u r a
T h e t r a c e r gas technique i s a useful method for determining
the ventilation r a t e of a n e n c l o s u r e whose boundary contains a number
of openings that a r e not well defined. F o r t h i s r e a s o n s e v e r a l i n v e s t i g a t o r s h a v e used t h i s technique t o d e t e r m i n e t h e ventilation c h a r a c t e r i s t i c s of h o u s e s w h e r e a i r infiltration t a k e s p l a c e through c r a c k s and
i n t e r s t i c e s f o r m e d by the d o o r s and windows located i n t h e outside walls.
T h i s technique involves t h e u s e of s o m e t r a c e r gas s u c h a s helium,
hydrogen o r carbon dioxide whose t h e r m a l conductivity d i f f e r s substantially
f r o m t h a t of a i r . T r a c e r gas i s introduced inside a n e n c l o s u r e and
allowed t o decay with t h e n a t u r a l ventilation of t h e enclosure. The
decay i s m e a s u r e d with a k a t h a r o m e t e r which is s e n s i t i v e t o t h e
changes in t h e t h e r m a l conductivity of the t r a c e r g a s - a i r m i x t u r e . The
k a t h a r o m e t e r usually contains two heated elements, one exposed t o
t h e t r a c e r g a s s a m p l e and t h e other enclosed in a s t a n d a r d gas and
f o r m s a p a r t of a Wheatstone b r i d g e circuit.
T h e r e l a t i o n s h i p between t h e concentration of t h e t r a c e r gas
and t h e ventilation r a t e m a y b e e x p r e s s e d by:
Integration of t h i s equation gives
where K
V
C
t
K
-
v
= infiltration r a t e
= volume of a n e n c l o s u r e
= t r a c e r gas concentration
= time
-
a i r change r a t e .
In t h e above equation i t i s a s s u m e d t h a t t h e t r a c e r g a s
concentration is u n i f o r m over the whole volume of t h e e n c l o s u r e at
a l l t i m e s , and that the a i r and the t r a c e r g a s l e a v e t h e boundary i n t h e
Same m a n n e r . Since the equation involves the r a t i o of t h e t r a c e r g a s
concentration, a n y quantities p r o p o r t i o n a l t o t h e t r a c e r g a s concentration
m a y be used t o d e t e r m i n e t h e ventilation r a t e . T h e r e f o r e t h e calibration
quantities with t h e absolute t r a c e r g a s concentration is not
r e q u i r e d . With the u s e of the k a t h a r o m e t e r for infiltration m e a s u r e m e n t ,
i t is e s s e n t i a l t h a t the output f r o m i t s b r i d g e c i r c u i t be p r o p o r t i o n a l t o
t h e t r a c e r gas concentration.
With helium a s the t r a c e r and with the k a t h a r o m e t e r designed
by Coblentz e t a1 of the National B u r e a u of Standards ( l ) , a s e r i e s of
calibration t e s t s w a s conducted i n t h e l a b o r a t o r y t o check t h e validity
of the above a s s u m p t i o n s and t o a s s e s s t h e p e r f o r m a n c e of t h e k a t h a r o m e t e r .
A design modification was m a d e on t h e k a t h a r o m e t e r which will b e
d e s c r i b e d l a t e r i n t h i s r e p o r t . T h e k a t h a r o m e t e r w a s calibrated by
introducing a known quantity of a i r into a l a r g e box and d e t e r m i n i n g t h e
ventilation r a t e using the t r a c e r gas technique. T h e r e s u l t of t h i s
investigation i s h e r e i n r e p o r t e d .
PRINCIPLE O F KATHAROMETER
A k a t h a r o m e t e r c o n s i s t s of two e l e m e n t s mounted in two s e p a r a t e
c e l l s in a m e t a l block t o maintain equal t e m p e r a t u r e s in t h e two cells.
One e l e m e n t i s exposed t o t h e gas m i x t u r e t o be m e a s u r e d and t h e other
is 'enclosed in a s t a n d a r d gas. With the addition of two fixed r e s i s t o r s ,
a Wheatstone b r i d g e c i r c u i t is f o r m e d and the potential difference
a c r o s s the b r i d g e i s m e a s u r e d t o d e t e r m i n e any changes in t h e conductivity
of t h e g a s surrounding t h e sensing element. If t h e heated e l e m e n t s a r e
p e r f e c t l y matched and the g a s e s s u r r o u n d i n g t h e e l e m e n t s a r e s i m i l a r ,
t h e potential difference a c r os s the b r i d g e r e m a i n s constant with changes
i n the input c u r r e n t o r t h e t e m p e r a t u r e of t h e block.
T h e t e m p e r a t u r e , and hence t h e r e s i s t a n c e of t h e heated e l e m e n t
is dependent on t h e heat t r a n s f e r that t a k e s p l a c e within a cell. A k a t h a r o m e t e r i s designed s o that t h e h e a t t r a n s f e r f r o m t h e element o c c u r s
m a i n l y by conduction. T h i s i s achieved by locating t h e heated element
v e r t i c a l l y and co-axially in a tube with the tube d i a m e t e r m a d e s m a l l
enough ( l e s s than 1 c m ) t o r e s t r i c t t h e convection c u r r e n t s within it.
Daynes ( 2 ) r e f e r s t o t h e work by L a n g m u i r , Lenher and T a y l o r which
d e m o n s t r a t e d t h e existence of a s t a t i o n a r y l a y e r of a i r surrounding t h e
heated w i r e . B e c a u s e t h e gas p r e s s u r e does not affect t h e r m a l
conductivity but a f f e c t s n a t u r a l convection, t h e r e s i s t a n c e change of
t h e heated element with a change in p r e s s u r e would indicate that the
heat l o s s by n a t u r a l convection m a y b e a significant p a r t of the t o t a l
heat l o s s . T r e a t i n g t h e c a s e of t h e heated w i r e i n a tube, i t i s shown b y
Daynes t h a t the t e m p e r a t u r e r i s e of t h e element i s i n v e r s e l y p r o p o r t i o n a l
t o the conductivity when the r i s e i s s m a l l . F o r a heated element with
a negative t e m p e r a t u r e coefficient of r e s i s t a n c e , the r e s i s t a n c e change
of t h e element would then b e d i r e c t l y proportional t o t h e conductivity.
It i s a l s o shown that for a s m a l l change in the r e s i s t a n c e of the sensing
element, the potential difference output from the bridge i s proportional
to this change. T h e r e f o r e , with a low t r a c e r gas concentration, the
change in the concentration i s proportional to the bridge output.
DESCRIPTION OF APPARATUS
The katharometer designed by the National Bureau of Standards (1)
consists of two matched t h e r m i s t o r s located in the two cylindrical cavities
of a b r a s s block. T h e s i z e of the cavity i s 1 3/4 by 5/8 in. in diameter.
One t h e r m i s t o r ( s t a n d a r d ) i s completely enclosed and the other t h e r m i s t o r
(sensing) i s located inside the cavity with four 1/32-in. diameter holes
in the b r a s s block, two a t the top and two a t the bottom of the cavity,
to p e r m i t a c c e s s of the sample gas by convection and diffusion. T h e two
t h e r m i s t o r s together with two 15 K ohm r e s i s t o r s a r e installed in p a r a l l e l
to f o r m a Wheatstone bridge circuit. The imbalance produced by the
sensing t h e r m i s t o r with changes i n the helium concentration i s r e c o r d e d
on a single -point millivolt r e c o r d e r .
Because i t was observed that the performance of the k a t h a r o m e t e r was affected by changes in the b a r o m e t r i c p r e s s u r e , i t s design
was modified to e n s u r e a constant difference between the p r e s s u r e s i n the
two cavities containing the t h e r m i s t o r s (Fig. 1). The standard t h e r m i s t o r
was enclosed in a g l a s s envelope with a s m a l l open tube at the bottom
end. T h i s tube was filled with light oil to obtain a movable seal. By
m e a n s of this s e a l , the p r e s s u r e difference r e p r e s e n t e d by the column
of oil i s kept a t a constant value with changes in the b a r o m e t r i c p r e s s u r e .
Although the two t h e r m i s t o r s a r e carefully matched, the deviation
in their c h a r a c t e r i s t i c s cannot be entirely avoided; it was found n e c e s s a r y
t o control the katharometer t e m p e r a t u r e and the c u r r e n t input. A simple
t e m p e r a t u r e control box was devised t o obtain a constant t e m p e r a t u r e
over a s h o r t period of time. T h i s box whose dimensions a r e 12 by 12 by
14 in. was fabricated f r o m copper sheet m e t a l and covered with 2-in.
foam insulation. T h e box was filled with water to act a s a constant t e m p e r a t u r e medium. The katharometer was secured to the side of the box
ensuring good t h e r m a l contact. By m e a n s of a s m a l l diaphragm pump,
the sample gas was passed through a copper tube inside the water -filled
container to a cavity f o r m e d by the katharometer and i t s p l a s t i c cover.
The cover was provided with a hole to p e r m i t the gas to escape into the
surrounding a i r . T o control the input c u r r e n t to the t h e r m i s t o r s , two
fixed r e s i s t o r s of 200 ohms and 15 K ohms w e r e added in p a r a l l e l t o the
Wheatstone bridge circuit ( s e e Fig. 2 for complete circuit). The balance
a c r o s s the c i r c u i t s containing the standard t h e r m i s t o r and the 200-ohm
r e s i s t o r was monitored with t h e vacuum tube v o l t m e t e r . Any
unbalance caused by t h e r e s i s t a n c e change in the s t a n d a r d t h e r m i s t o r
due t o t h e d r i f t in t h e c u r r e n t input was c o r r e c t e d by adjusting the
d c voltage supply.
T o eliminate p o s s i b l e e r r o r d u e t o the v a r i a t i o n i n humidity,
t h e s a m p l e gas was p a s s e d through a drying bottle containing calcium
chloride b e f o r e being pumped t o t h e k a t h a r o m e t e r
.
A l a r g e box designed f o r window a i r leakage m e a s u r e m e n t s
was adapted t o s e r v e a s a n e n c l o s u r e in which the ventilation r a t e s w e r e
determined. T h i s box, with a n i n t e r n a l dimension of 4 by 8 by 8 f t , i s
m a d e i n two h a l v e s supported on c a s t o r s with two a c c e s s d o o r s and
two observation windows. A m e a s u r e d quantity of a i r i s piped t o a n
opening i n t h e box f r o m a s e r v i c e line t o obtain a p r e d e t e r m i n e d v e n t i lation r a t e in t h e e n c l o s u r e . T h e p r e s s u r e developed by t h i s a i r input
c a u s e s the a i r t o exfiltrate through t h e i n t e r s t i c e s f o r m e d b y the mating
s u r f a c e s of t h e box and the a c c e s s d o o r s , and through other extraneous
cracks.
PERFORMANCE TESTS
In o r d e r t o develop a r e l i a b l e a p p a r a t u s and technique f o r
determining ventilation r a t e s , a s e r i e s of t e s t s was conducted t o
investigate (i)t h e effect of t h e v a r i a t i o n i n the ambient a i r t e m p e r a t u r e
and p r e s s u r e and, a l s o , of the v a r i a t i o n in the c u r r e n t on t h e stability
of z e r o , (ii) the a c c u r a c y of t h e ventilation r a t e s indicated b y the k a t h a r o m e t e r a s employed in the t r a c e r g a s method.
T h e k a t h a r o m e t e r was operated with 300 d c volts applied a c r o s s
t h e b r i d g e with a c u r r e n t of a p p r o x i m a t e l y 22 m a for each t h e r m i s t o r .
T h e unbalance between t h e c i r c u i t a r m s of t h e s t a n d a r d t h e r m i s t o r and
t h e 200-ohm r e s i s t o r was kept within + 0.2 mv. Approximately one h r
was r e q u i r e d for t h e k a t h a r o m e t e r to a p p r o a c h a t h e r m a l s t e a d y - s t a t e
condition s o that the z e r o position could be established on the millivolt
r e c o r d e r . T o e s t a b l i s h a s t e a d y z e r o position a p p r o x i m a t e l y 2 1/2 h r of
w a r m -up t i m e was r e q u i r e d .
T h e amount of helium r e q u i r e d t o obtain a m a x i m u m of 70 t o
80 p e r cent f u l l - s c a l e r e a d i n g (10 m v ) on the r e c o r d e r was 1 t o 2 p e r cent
b y volume depending on t h e r a t e of a i r change in t h e box. T h e quantity
of helium r e l e a s e d i n s i d e the e n c l o s u r e was m e a s u r e d with a wet t e s t
gas m e t e r . F o r equal m a x i m u m r e a d i n g s on the r e c o r d e r , a l a r g e r
volume of helium was r e q u i r e d a t higher ventilation r a t e .
AMBIENT AIR TEMF'ERATURE EFFECT
With the katharometer circuit a s shown in Fig. 2 and
without any t e m p e r a t u r e control, the z e r o position on the r e c o r d e r
fluctuated with changes in the ambient a i r temperature. F o r a 1 -deg
change in t h e ambient a i r t e m p e r a t u r e , the z e r o position drifted
approximately 3/4 of a millivolt. Even with ambient a i r t e m p e r a t u r e
fluctuations within a n a r r o w range, the z e r o position i s e r r a t i c and
difficult t o a s c e r t a i n . T o overcome this effect, the katharometer was
mounted on a t e m p e r a t u r e control box a s previously described. With
this arrangement the s u r f a c e t e m p e r a t u r e of the kathar om eter was
recorded with a thermocouple. Over a 2 1/2-hr period the katharometer
t e m p e r a t u r e varied 0. 1O F with 1O F variation in the ambient a i r temper
ature. This resulted in much improved stability of the z e r o position.
It can be expected that during a n o r m a l t e s t period the katharometer
t e m p e r a t u r e will r e m a i n constant within this range.
-
CURRENT E F F E C T
With the t e m p e r a t u r e -controlled katharometer , the katharometer
circuits with and without current contr 01 w e r e tested to determine t h e i r
effectiveness on maintaining the z e r o stability. Without any c u r r e n t
control the z e r o position drifted 0. 8 m v during 1 1/4 h r . By maintaining
0. 2 m v between the circuit a r m s containing the
a balance within
standard t h e r m i s t o r and the 200 -ohm r e s i s t o r t o obtain constant input
c u r r e n t , the drift in the z e r o position was reduced t o 0. 12 mv. The
katharometer circuit was a l s o checked with the standard t h e r m i s t o r
replaced with a 200 -ohm r e s i s t o r , and without any c u r r e n t control. During
a 1/2-hr period the z e r o position drifted by 1. 3 mv. It can be seen that
the change in the z e r o position due to the variation in the input c u r r e n t
i s reduced with matched t h e r m i s t o r s in the bridge circuit; this i s
f u r t h e r reduced with careful control of the input current.
+
PRESSURE
-
EFFECT
T o check the b a r o m e t r i c p r e s s u r e effect, the katharometer
was subjected to various p r e s s u r e s and a change in the z e r o position was
noted. With the cavity of the standard t h e r m i s t o r sealed with plasticine
in the open tube, the katharometer was placed inside the calibration box
and the p r e s s u r e inside the box was brought up t o 10 m m of water with
increments of 2 m m of water. At 10 m m of water, the z e r o position
drifted 0. 2 mv, and a f t e r the p r e s s u r e was r e l e a s e d , the original z e r o
position was r e s t o r e d . The t e s t was repeated with oil in the open tube
of the katharometer. No change in the z e r o position was indicated.
This demonstrated the effectiveness of the oil s e a l to counter the effect
of the change in the b a r o m e t r i c p r e s s u r e on the z e r o position. The
p r e s s u r e t e s t s have a l s o indicated that the r e s i s t a n c e and, hence, the
t e m p e r a t u r e of the thermistor is sensitive to changes in the gas p r e s s u r e
and, t h e r e f o r e , the heat t r a n s f e r in the cavities of the katharometer
is due in p a r t t o natural convection. With the standard thermistor
enclosed in a glass envelope, the heat t r a n s f e r coefficients in the two
cavities a r e somewhat different. This would have the same effect
a s a mismatch in the temperature sensitivity of the two t h e r m i s t o r s .
CALIBRATION TESTS
To determine the accuracy of the katharometer a s employed
in the t r a c e r gas technique, the ventilation t e s t s were conducted on the
l a r g e box previously described. By introducing a constant r a t e of a i r
flow measured with a variable a r e a flowmeter, a known ventilation r a t e
of the box was obtained. The helium gas at a p r e s s u r e of 10 p s i was
injected into the box through a plastic tube. With a small diaphragm
pump, a continuous sample of helium-air mixture was drawn through
a tube t o the drying bottle and then t o the katharometer located outside
the calibration box. The unbalance of the bridge circuit caused by the
changes in the helium concentration was recorded on the millivolt
r e c o r d e r . F r o m the helium decay curve, the ventilation r a t e was
computed and compared with the ventilation r a t e calculated from the
r a t e of a i r input.
Since the sample gas Ilow r a t e m a y affect the performance
of the katharometer, the flow r a t e was varied f r o m 460 t o 2300 ccm.
This did not a l t e r the z e r o position on the r e c o r d e r . T h e r e was no
marked difference in the r e s u l t s of ventilation t e s t s r u n with sample gas
flow r a t e s of 1050 and 2300 ccm. The majority of the ventilation t e s t s
were run with a sample gas flow r a t e of 2300 ccm which gave approximately
200 a i r changes p e r h r inside the cover of the katharometer. At
ventilation r a t e s of 8 and 10 a i r changes per h r in the calibration box,
readings were recorded with the katharometer inside the calibration
box and exposed directly t o the helium-air mixture; the ventilation r a t e s
obtained were the s a m e a s those obtained with the sample gas pumped
t o the katharometer. It can be concluded that with the sample gas flow
r a t e used for the calibration t e s t , the helium decay c h a r a c t e r i s t i c s in
the calibration box and in the vicinity of the katharometer a r e the same.
Because helium i s much lighter than a i r , i t was thought that
due t o stratification, the helium-air mixture in the box might not be
homogeneous. F o u r sampling points w e r e located n e a r t h e c o r n e r s
4 ft above the f l o o r level and the helium - a i r m i x t u r e was circulated
with a t a b l e fan. L a t e r t e s t s conducted without the t a b l e fan and with
a single sampling point located n e a r t h e floor l e v e l gave s i m i l a r
ventilation r a t e s . T h i s indicated that due t o t h e high diffusion r a t e
of helium, t h e helium - a i r m i x t u r e was e s s e n t i a l l y homogeneous i n the
calibration box a s i t was a s s u m e d in the derivation of t h e equation for
computing the ventilation r a t e . T o d e t e r m i n e the house ventilation
r a t e , i t i s probably n e c e s s a r y t o c i r c u l a t e t h e a i r mechanically t o
e n s u r e a homogeneous m i x t u r e .
Ventilation t e s t s w e r e conducted with a i r flow r a t e s of 1/2
t o 10 a i r changes p e r h r . T h e r e s u l t s of t h e s e t e s t s a r e shown in Fig. 3;
a typical helium decay c u r v e i s shown i n Fig. 4. Good a g r e e m e n t i s
obtained with t h e a i r change r a t e up t o 4 a i r changes p e r h r . Above
this r a t e , t h e a i r change r a t e d e t e r m i n e d with the k a t h a r o m e t e r i s much
lower than the a c t u a l r a t e . T h i s d i s c r e p a n c y i s probably caused by
the l a g of the k a t h a r o m e t e r which i s dependent on t h e volume of t h e cell
and t h e r a t e of interchange of gas inside and outside t h e c e l l through the
four s m a l l holes. T h i s interchange of gas t a k e s p l a c e by diffusion and
by s t a c k effect due t o t h e difference between the c e l l a i r and the outside
a i r t e m p e r a t u r e . It would a p p e a r that below 4 z i r changes p e r h r , t h e
helium d e c a y r a t e i s low enough that t h e helium concentration of t h e
s a m p l e gas inside t h e c e l l i s e s s e n t i a l l y t h e s a m e a s t h a t inside t h e
calibration box. Above 4 a i r changes p e r h r , however, due t o t h e l a g
of t h e k a t h a r o m e t e r , the s a m p l e gas i n s i d e t h e cell i s n o longer
r e p r e s e n t a t i v e of the gas inside t h e box; t h i s condition i s aggravated
a t a higher a i r change r a t e . It i s likely that t h i s would a l s o b e t r u e f o r
t h e c a s e of s i m i l a r r a t e s of i n c r e a s e of helium concentration. Rapid
i n c r e a s e in the helium concentration clccurs i m m e d i a t e l y a f t e r helium
i s injected, r e a c h e s a peak and then begins t o decay due t o t h e ventilation
in t h e enclosure. B e c a u s e of the high r a t e of change, i t i s probable that
t h e k a t h a r o m e t e r d e t e c t s helium concentration lower than the a c t u a l
concentration during t h i s period.
F I E L D VENTILATION TEST
T o d e t e r m i n e the house ventilation r a t e and t o investigate t h e
r e l e v a n t f a c t o r s that affect the ventilation r a t e , a s e r i e s of t e s t s was
conducted in the field with the k a t h a r o m e t e r . By injecting the helium
in t h e supply a i r duct of the f o r c e d a i r heating s y s t e m , the helium was
circulated throughout the house. With the circulation fan operating
continuously, a s a m p l e h e l i u m - a i r m i x t u r e was drawn f r o m t h e r e t u r n
a i r duct t o t h e k a t h a r o m e t e r and the subsequent helium decay c h a r a c t e r i s t i c s were recorded.
The ventilation r a t e s of the two bungalows investigated
during the winter and s u m m e r t r i a l s w e r e found to be well under 1 a i r
change p e r h r . At t h e s e r a t e s , the t i m e for the helium to decay
completely took s e v e r a l hours s o that the t e s t was terminated before
the final z e r o position was obtained. Although precautions a r e taken
to obtain a stable z e r o position p r i o r t o the t e s t , it i s possible f o r
the z e r o position to d r i f t during the t e s t causing an e r r o r in the
ventilation r a t e measured. By drawing outside a i r to the k a t h a r o m e t e r ,
the z e r o position at the end of the t e s t m a y be obtained and a linear
z e r o d r i f t during the t e s t may be assumed. C a r e must be taken t o
e n s u r e that the t e m p e r a t u r e and the background carbon dioxide content
of the sample outside a i r a r e not substantially different f r o m that of
the room a i r .
In the calculation of the a i r change r a t e i t i s a s s u m e d that
the helium leaves the boundary through c r a c k s and i n t e r s t i c e s around
a house in the s a m e manner a s the exfiltration a i r . It i s quite probable
that p a r t of the helium leaving the boundary i s due to diffusion through
the exterior walls and ceiling. Although the amount of diffusion of helium
t o outdoors i s relatively s m a l l , i t may be significant a t a low ventilation
r a t e . The decay of helium concent-ation may a l s o be p a r t l y due to
diffusion of helium into furniture, cupboards, closets and walls of
the house. Ventilation t e s t s conducted by Bahnfleth et a1 ( 3 ) with
cupboards and closet doors opened and closed indicated no change in
the ventilation r a t e s . Because of the interchange of helium in a room
and enclosed s p a c e s , the effect of furniture, closets, interior wall
panels, etc. on the decay r a t e i s probably negligible.
CONCLUSION
Because the k a t h a r o m e t ~ ri s sensitive to changes in the
ambient a i r t e m p e r a t u r e , humidity, b a r o m e t r i c p r e s s u r e and c u r r e n t
input, provision should be made t o maintain constant operating condition.
As indicated by the sensitivity of the katharometer to changes in p r e s s u r e ,
heat t r a n s f e r by convection t a k e s place in the cavity enclosing the
t h e r m i s t o r . Because the diameter of the cylindrical cavity i s well over
1 cm, the heat t r a n s f e r in the cavity probably o c c u r s mainly by n a t u r a l
convection. The calibration t e s t s have shown that the katharometer
a s used in the t r a c e r gas technique gives reliable r e s u l t s for ventilation
r a t e s f r o m 1/2 t o 4 a i r changes p e r h r . This would imply that the bridge
output i s proportional to the helium concentration and, hence, proportional
to the t h e r m a l conductivity of the helium-air mixture. Above 4 a i r changes
p e r h r , due t o the lag of the k a t h a r o m e t e r , the katharometer r e c o r d s
ventilation r a t e s lower than the actual values.
ACKNOWLEDGMENTS
The author gratefully acknowledges the contribution made
by Dr. D. G. Stephenson during the initial part of the t e s t program,
the assistance given by Mr. R. G. Evans in carrying out the t e s t s ,
and the guidance given to this project by Mr. A. G. Wilson.
REFERENCES
1.
Coblentz, C. W. and P. R. Achenbach. Design and performance
of a portable infiltration meter.
Transactions, American
Society of Heating and Air -Conditioning Engineers, Vol. 63,
1957, p. 477-482.
2.
Daynes, H. A. Gas analysis by measurement of thermal conductivity.
Cambridge a t the University P r e s s , 1933.
3.
Bahnfleth, D.R., T. D. Moseley and W. S. H a r r i s . Measurement
of infiltration in two residences. Transactions, American
Society of Heating and Air -Conditioning Engineers, Vol. 63,
1957, p. 453-476.
Figure 1
View of t h e Katharometer Attached t o
t h e T e m p e r a t u r e Control Box
STANDARD THERMISTOR
VOLTM ETER
NOTE :
KATHAROMETER LEADS SHIELDED
FIGURE 2
KATHAROMETER CIRCUIT DIAGRAM
FIGURE 3
CALIBRATION T E S T S O N K A T H A R O M E T E R