Performance of katharometer for air infiltration measurement

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Performance of katharometer for air infiltration measurement

Tamura, G. T.

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

w h e r e K = infiltration r a t e V = volume of a n e n c l o s u r e C

=

t r a c e r gas concentration t = t i m e

K

-

-

v

- 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 the t r a c e r gas 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 the calibration

r e q u i r e d . With t h e u s e of the k a t h a r o m e t e r f o r i n f i l t r a t i o n 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 t h e 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 h e l i u m a s t h e 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 t h e National B u r e a u of S t a n d a r d s ( l ) , a s e r i e s of c a l i b r a t i o n 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 d e s i g n modification w a s 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 c a l i b r a t e d 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 u s i n g t h e t r a c e r g a s 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 .

P R I N C I P L E 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 m a i n t a i n e q u a l t e m p e r a t u r e s in t h e two c e l l s .

One e l e m e n t i s exposed t o t h e g a s m i x t u r e t o be m e a s u r e d and t h e o t h e r is 'enclosed in a s t a n d a r d gas. With t h e 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 d i f f e r e n c e

a c r o s s t h e 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 a n y changes in t h e conductivity of t h e g a s s u r r o u n d i n g t h e s e n s i n g e l e m e n t . If t h e h e a t e d e l e m e n t s a r e p e r f e c t l y m a t c h e d 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 d i f f e r e n c e a c r os s the b r i d g e r e m a i n s constant with changes i n t h e 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 h e n c e t h e r e s i s t a n c e of t h e h e a t e d e l e m e n t is dependent on t h e heat t r a n s f e r t h a t 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 t h a t t h e h e a t t r a n s f e r f r o m t h e e l e m e n t 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 h e a t e d e l e m e n t v e r t i c a l l y and c o - a x i a l l y in a t u b e with t h e t u b e 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 e x i s t e n c e of a s t a t i o n a r y l a y e r of a i r s u r r o u n d i n g t h e h e a t e d 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 h e a t e d e l e m e n t with a change in p r e s s u r e would i n d i c a t e t h a t t h e h e a t 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 h e a t 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 t h e t e m p e r a t u r e r i s e of t h e e l e m e n t 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 t h e conductivity when the r i s e i s s m a l l . F o r a h e a t e d e l e m e n t 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 , t h e r e s i s t a n c e change of t h e e l e m e n t would then b e d i r e c t l y p r o p o r t i o n a l 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 w a s m o n i t o r e d with t h e vacuum t u b e 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 t h e 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 w a s c o r r e c t e d by adjusting the d c voltage supply.

T o e l i m i n a t e p o s s i b l e e r r o r d u e t o t h e v a r i a t i o n i n humidity, t h e s a m p l e g a s w a s p a s s e d t h r o u g h a drying bottle containing calcium

c h l o r i d e 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 l e a k a g e m e a s u r e m e n t s w a s adapted t o s e r v e a s a n e n c l o s u r e in which t h e ventilation r a t e s w e r e d e t e r m i n e d . 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 o b s e r v a t i o n 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 l i n e t o obtain a p r e d e t e r m i n e d v e n t i - l a t i o n 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 e x f i l t r a t e through t h e i n t e r s t i c e s f o r m e d b y t h e m a t i n g s u r f a c e s of t h e box and t h e a c c e s s d o o r s , and through other e x t r a n e o u s c r a c k s .

PERFORMANCE T E S T S

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 d e t e r m i n i n g ventilation r a t e s , a s e r i e s of t e s t s w a s conducted t o

i n v e s t i g a t e (i) th e effect of t h e v a r i a t i o n i n t h e a m b i e n t 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 t h e v a r i a t i o n in t h e c u r r e n t on t h e s t a b i l i t y of z e r o , (ii) th e a c c u r a c y of t h e ventilation r a t e s indicated b y t h e 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 w a s o p e r a t e d with 300 d c v o l t s 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 f o r e a c h 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 w a s kept within

+

0.2 mv. A p p r o x i m a t e l y one h r w a s r e q u i r e d f o r t h e k a t h a r o m e t e r t o 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 t h a t t h e z e r o position could be e s t a b l i s h e d on t h e m i l l i v o l t 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 w a s 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 t h e r e c o r d e r w a s 1 t o 2 p e r c e n t 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 t h e 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 g a s 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 w a s 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. 1 O F with 1 O 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 a balance within

+

0. 2 m v between the circuit a r m 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 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 E F F E C T

-

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 s a m p l i n g p o i n t s w e r e located n e a r t h e c o r n e r s 4 ft above t h e f l o o r l e v e l and the helium - a i r m i x t u r e was c i r c u l a t e d 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 t h a t due t o t h e high diffusion r a t e of h e l i u m , 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 t h e c a l i b r a t i o n box a s i t was a s s u m e d in t h e d e r i v a t i o n of t h e equation f o r computing t h e ventilation r a t e . T o d e t e r m i n e the h o u s e ventilation r a t e , i t i s p r o b a b l y 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 m e c h a n i c a l l y 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 t y p i c a l helium d e c a y c u r v e i s shown i n F i g . 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 t h i s 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 t h e k a t h a r o m e t e r i s m u c h lower t h a n t h e 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 p r o b a b l y c a u s e d by t h e l a g of t h e 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 c e l l and t h e r a t e of i n t e r c h a n g e of g a s i n s i d e and outside t h e c e l l through the four s m a l l h o l e s . T h i s i n t e r c h a n g e 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 d i f f e r e n c e between t h e 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 t h a t below 4 z i r changes p e r h r , t h e h e l i u m d e c a y r a t e i s low enough that t h e h e l i u m concentration of t h e

s a m p l e g a s i n s i d e 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 i n s i d e t h e c a l i b r a t i o n 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 , t h e s a m p l e gas i n s i d e t h e c e l l i s n o longer

r e p r e s e n t a t i v e of t h e g a s i n s i d e t h e box; t h i s condition i s a g g r a v a t e d a t a h i g h e r a i r change r a t e . It i s l i k e l y t h a t 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 h e l i u m concentration. Rapid i n c r e a s e in t h e h e l i u m c o n c e n t r a t i o n clccurs i m m e d i a t e l y a f t e r helium i s i n j e c t e d , r e a c h e s a p e a k and t h e n begins t o d e c a y due t o t h e ventilation in t h e e n c l o s u r e . B e c a u s e of t h e high r a t e of change, i t i s p r o b a b l e t h a t 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 t h a n t h e a c t u a l

concentration d u r i n g t h i s p e r i o d .

F I E L D VENTILATION TEST

T o d e t e r m i n e the h o u s e ventilation r a t e and t o i n v e s t i g a t e t h e r e l e v a n t f a c t o r s t h a t affect t h e ventilation r a t e , a s e r i e s of t e s t s was conducted in t h e field with the k a t h a r o m e t e r . By injecting the h e l i u m 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 , t h e helium w a s c i r c u l a t e d throughout the house. With t h e c i r c u l a t i o n fan o p e r a t i n g 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 d r a w n 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 t h e subsequent h e l i u m d e c a y c h a r a c - t e r i s t i c s w e r e r e c o r d e d .

The ventilation r a t e s of the two bungalows investigated during t h e winter and s u m m e r t r i a l s w e r e found t o 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 t h e helium t o decay

completely took s e v e r a l h o u r s s o t h a t the t e s t was t e r m i n a t e d before the final z e r o position was obtained. Although precautions a r e taken t o obtain a stable z e r o position p r i o r t o t h e t e s t , i t i s p o s s i b l e f o r the z e r o position t o 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 m e a s u r e d . By drawing outside a i r t o 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 l i n e a r

z e r o d r i f t during the t e s t m a y be a s s u m e d . C a r e m u s t be taken t o e n s u r e that the t e m p e r a t u r e and t h e background carbon dioxide content of the s a m p l e outside a i r a r e not substantially different f r o m t h a t 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 l e a v e s 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 t h e boundary i s due t o diffusion through the e x t e r i o r walls and ceiling. Although the amount of diffusion of helium t o outdoors i s r e l a t i v e l y s m a l l , i t m a y be significant a t a low ventilation r a t e . T h e decay of helium concent-ation may a l s o be p a r t l y due t o diffusion of helium into f u r n i t u r e , cupboards, c l o s e t s and walls of t h e 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 r o o m and enclosed s p a c e s , the effect of f u r n i t u r e , c l o s e t s , i n t e r i o r wall p a n e l s , etc. on the decay r a t e i s probably negligible.

CONCLUSION

B e c a u s e the k a t h a r o m e t ~ r i s sensitive t o changes in t h e 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 k a t h a r o m e t e r 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 p l a c e 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 c m , 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 t h e k a t h a r o m e t e r a s used in the t r a c e r gas technique gives r e l i a b l e 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 . T h i s would imply that the b r i d g e output i s proportional t o the helium concentration and, hence, proportional t o the t h e r m a l conductivity of the h e l i u m - a i r m i x t u r e . Above 4 a i r changes p e r h r , due t o the l a g of the k a t h a r o m e t e r , the k a t h a r o m e t e r r e c o r d s

ACKNOWLEDGMENTS

The author gratefully acknowledges the contribution made by Dr. D. G. Stephenson during the initial p a r t of the t e s t program, the a s s i s t a n c e given by M r . R. G. Evans in carrying out the t e s t s , and the guidance given t o this project by Mr. A. G. Wilson.

REFERENCES

1. Coblentz, C. W. and P. R. Achenbach. Design and performance of a portable infiltration m e t e r . Transactions, American Society of Heating and Air -Conditioning Engineers, Vol. 63,

1957, p. 477-482.

2. Daynes, H. A. Gas analysis by measurement of t h e r m a l 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.

F i g u r e 1

View of t h e K a t h a r o m e t e r Attached t o t h e T e m p e r a t u r e C o n t r o l Box

STANDARD THERMIS

NOTE

_:

KATHAROMETER LEADS SHIELDED

TOR

VOLTM ETER

FIGURE

2

FIGURE 3