Objective Finish with: Heat Flux & Relative Humidity Measurement Pressure measurement Start tracer gas measurement

2 Heat flux sensor The flow of heat through the transducer creates a minute temperature difference between its surfaces. A multi-element, semi-conductor thermopile, consisting of hundreds of Bi/Te elements generates a D.C. voltage via the Seebeck effect. The resulting signal is directly proportional to the heat flux through the transducer. Semiconductor thermopile can be replaced of thermocouple thermopile

3 Heat flux sensor Thermocouple thermopile

4 Other methods for heat flux measurements Material / wall assembly testing Conductivity (R value) measurement: - Hot box - Hot plate

5 Relative Humidity: Why Measure? Comfort HVAC operation effectiveness Dust mites, mold Accumulation of water molecules on surfaces Drying of liquid water Effects on other physical processes in buildings

6 Measurement Techniques Psychrometer (± 3 – 7% accuracy) Mechanical sensors (± 5-10% accuracy) Thin-Film Polymer (± 2 – 5% accuracy) Chilled Mirror (± 0.2 – 2 o K accuracy on T dew ) Other Summary: ASHRAE Handbook Ch 14 – Table 3

7 Thin-Film Polymer Sensors See ASHRAE PDS IV – pg Most common RH sensor Water molecules adsorb/desorb w/ changes in RH Material props change (R or dielectric constant) Current converts change to V or capacitance Change related to RH Accuracy order 2-5% RH Fast response, low cost, good accuracy Contamination can be an issue

8 Polymer: Resistance Sensors Use change in resistance of hygroscopic polymer Works well for 20% - 90% RH Absorption/desorption time takes ~minutes Sensitive to contaminants –Need frequent calibration –Not a trivial process Source: Wiederhold Water Vapor Measurement

9 Polymer: Capacitance sensors Porous electrode absorbs water Faster response –Thin –Response time ~seconds Calibration issues –Need to be initially calibrated –Same contamination issues Source: Harriman ASHRAE Humidity Control Design Guide for Commercial Buildings

10 Chilled Mirror Directly measures dew point Calibration standard –Very accurate over large range –Slow, expensive, contamination issues Source: Wiederhold Water Vapor Measurement

11 Absolute vs. Pressure Difference Absolute = barometric pressure –Reference is a vacuum Differential ≈ gauge pressure –Reference is user defined (often atmospheric) Buildings: –We are almost always only interested in pressure difference. –Get in the habit of saying pressure with respect to reference i.e. House is +10 Pa w.r.t. outside, Garage is -5 Pa w.r.t. outside

12 Pressure Sensors Easiest and very accurate is a manometer –But, hard to measure low  p Make electronic recording Piezoelectric sensors Diaphragm with strain gages –Some issues Drift (auto-zeroing) Hysteresis Require power

13 Some Sensors Setra (pressure input → voltage output) – /_sensor.php5?snid=101http:// /_sensor.php5?snid=101 Energy Conservatory DG-700 – s4.htmhttp:// s4.htm

Tracer Gas Tests

IAQ Applications Quantification of outside air Air distribution system efficiency –Air change Efficiency –Contaminant removal effectiveness Leak detection House/chamber/duct/… Duct flow Re-entrainment of exhaust air into ventilation system Simulate toxic pollutant distribution Many other applications

A Good Tracer Gas? Non-toxic Environmental friendly Colorless and odorless Easily detectable Inert No other sources

Common Tracer Gases Used Carbon Dioxide Nitrous Oxide Freon Helium Sulfur Hexafluoride

Application 1:Quantification of outside air Volumetric Air Measurements Standard Test –ASTM E (2006) Test Method for Determining Air Change in a Single Zone by Means of a Tracer Gas Dilution

ASTM E741 Test Method Different methods: –Concentration Decay –Constant Injection

Concentration Decay Method Inject predetermined volume of gas into room Mix room air to get uniform concentration Monitor gas concentration decay Aim for 10 samples over measured time Use reactor model to predict concentrations

Theoretical Basis Space balance V º V º C in N Source C out ºº V space ·dC/d  = V·C in -V·C out +N 0 If C in = constant & V space /V = ACH º dC/(C in -C out ) = ACH· d  Integrate: ……. ACH =1/ Δ  ·{ln[C out (  = Δ  )- C in )]- ln[C out (  =0)- C in )]}

Concentration Decay Method Air Change Rate: In the case of zero inlet concentrate and perfect mixing in the space ACH = (ln C 2 - ln C 1 )/Δ  (in hours) C 1 = Tracer Gas Concentration at start of test C 2 = Tracer Gas Concentration at end of test

Tracer gas result [minutes]

Decay Test Advantages –Don’t need to release precise amount –Don’t need to measure volume (if C out = 0) Disadvantages –Need to keep building well-mixed –Recontamination from buffer spaces –House needs to stay in one condition for entire test

How do you estimate uncertainty? 1.Use standard error of slope 2.Follow ASTM E741 –ΔACH < 10%

Advanced Tracer Gas Testing Multi-zone flows –Easiest – Use several unique tracer gases –Harder – Use flow and mass balances

Consider Two-Zone Building V1V1 V2V2 V4V4 V3V3 V1V1 V2V2 E vdAvdA V3V3 V5V5 Tools Mass balance on tracer gas Mass (flow) balance on air Measured concentrations in each space º º º º º º or V 6

Equations How many unknowns? Equations? Flow direction for interzonal flow Air exchange rate for spaces Sums of flows ºººº ºººº ºººº

Comments Reduce mass balance to one equation by solving C 2 equation for C 1 and substituting into C 1 equation –2 nd order ODE –Same thing for C 1 equation 5 unknown flows –Overall flow balance can be used to get four unknown flows –Measured tracer gas concentrations can be used to eliminate two more flows –Additional data needed for solvable system Often use multiple tracer gasses

Single zone Example: Coffee Houses Lohaus and Waring (2006) ArE 381E Course Project

Example 2 Air distribution system efficiency How well is outside supply air distributed to breathing zones in occupied areas? Air exchange efficiency ASHRAE Standard 129 – Measuring Air Change Effectiveness Uses Tracer Gas Techniques Age-of-Air Measurements

Why Worry About Good Mixing? Poor Mixing Occupant complaints ASHRAE Standard, Ventilation for Acceptable Indoor Air Quality ASHRAE Standard is based on amounts of outside air getting to breathing zone not to supply air louvers Short – circuiting airflow patterns Where a significant portion of supply air flows directly to the exhaust, bypassing the occupied portion (breathing zone) of the ventilated space.

Air Exchange Effectiveness The definition is based on a comparison of the age of air in the occupied portions of the building to the age of air that would exist under conditions of perfect mixing of the ventilation air.

Age of Air The age of the air at a give location is the average amount of time that has elapsed since the air molecules at that location entered the building. Amount of time outside air has been in an area Two Methods of determination –Step-up constant tracer gas injection –Tracer gas concentration decay

How to measure Age of Air? Step down method: Injection and mixing –Air in the room is marked with tracer gas (injection and mixing) –Ventilation turned on Age of Air Measurements –Locations of interest –In the exhaust (C)

Constant Injection V = N / (C out - C in ) º V º V º C in N Source C out You need to get to steady state injection

Constant Injection Advantages –Can determine time-dependence of air exchange rates Disadvantages –Need to keep building well-mixed –Recontamination from buffer spaces –Need to have mass flow controller –Need to measure volume (for ACH(  ))

How to measure Age of Air and Air Exchange Effectiveness Age of air at a location = Average tracer gas level during test Tracer gas level at beginning of test Air change effectiveness (E) E = < 1.0 (less than perfect mixing) E = 1 (perfect mixing) avg age of air – Exhaust avg age of air – age of air in breathing zone E = avg age of air =

Significance of Air Exchange Effectiveness ASHRAE Standard Ventilation for Acceptable Indoor Air Quality - Outside air requirements = QA/E as E decreases, OA should increase US Green Building Council LEED Rating requires an E > 0.9 in all ventilated zones

Tracer Gas Instrumentation

Tracers which we use SF 6 Gas analyzer –ppm with IR absorption or photo-acoustic IR –ppb with GC/ECD CO 2 Tracers gas analyzer ( CO 2 sensor )