HUMIDITY CONTROL INSTRUMENTATION FOR SAMPLE CHAMBERS USED IN X-RAY SCATTERING EXPERIMENTS: HUMIDITY SENSING AND GENERATION

Obafemi Otelaja, Department of Electrical and Computer Engineering, Howard University, Washington DC 20059Lin Yang (Mentor), National Synchrotron Light Source, Brookhaven National Laboratory, Upton NY 11973

ABSTRACT

The hydration level of open samples used in x-ray scattering experiments is changed by varying the relative humidity (RH) in a temperature controlled, aluminum sample chamber. The traditional relative humidity sensors used in these sample chambers usually have a slow response, and they are susceptible to damage at very high RH. The aim of this work is to realize a precise and durable method for humidity control using chilled mirror dew point hygrometer for RH sensing, divided-flow volumetric mix ratio technique for RH generation, and a newly designed temperature controlled and monitored sample chamber. By flowing helium gas, of predetermined flow rate, through a fritted gas dispersion tube into a flask containing temperature controlled water, the wet gas leaving the flask would be saturated at the water temperature. The wet gas is mixed with dry helium, also of predetermined flow rate, and the mixture is maintained at the saturation temperature and flown through the chilled mirror sensor into the sample chamber. From the chilled mirror sensor, we could obtain the dew point temperature, and with a four-wire surface platinum resistance temperature detector (PRTD) placed inside the sample chamber, we could obtain the sample temperature. Hence, we could determine the relative humidity in the sample chamber as a ratio of actual vapor pressure to saturated vapor pressure, since vapor pressure and saturated vapor pressure could be obtained from dew point and sample temperature respectively.

CHILLED MIRROR SENSING

The chilled mirror hygrometer (CMH) has been used a s a NIST-referenced standard in metrology laboratories for the past four decades. CMH provide a direct measurement of the dew point temperature of a sample of gas of unknown water vapor content. GE Eastern Optica chilled mirror analyzer and a D2 chilled mirror sensor were obtained for this purpose.

The mirror consists of a small polished rhodium mirror attached to a two-stage thermoelectric cooling (TEC) module. The Optica analyzer’s servo controller applies current to the TEC, whichcauses the mirror to cool. The mirror is illuminated by a Gas emitter, and the light reflected by the mirror is received by a photodetector. As gas flows over the mirror, dew droplets form on the mirror surface and the reflected light is scattered. As reflected light decreases, photodetector output decreases. This in turn controls the TEC via a control system that maintains the mirror at dew point. A PRTD embedded in the mirror monitors the dew point temperature. The dew point measurement range is -31 F to 77F (equivalent to 0.7% to 100%) to an accuracy of +/- 0.36 F dew/frost point.

Optica’s Digiloop Control vs. Traditional PID Control

HUMIDITY GENERATION

Divided-flow humidity generation operate under flow rate control whereby a continuous stream of saturated helium (wet) is mixed in selected proportion with a continuous stream of dry helium. The flow rates of the gas is to be predetermined using OMEGA FMA5400 mass flow controllers.

REFERENCES

Lin Yang. Cooperative Phenomena of Antimicrobial Peptides in Membranes: A study by Neutron and X-ray Diffraction. Ph.D. Thesis, April 2001.

General Eastern Dew Point Analyzer, Operator’s Manual

Mark E. Brownawell, United States Patent 5,056,547 “Relative Humidity Generation Techniques,” Oct. 15, 1991.

FUTURE WORK

Future work on this project is to complete the design of the divided-flow humidity generation technique. Also, final integration of sample chamber, chilled mirror sensor, Optica analyzer, Humidity generator, and flow controllers would be done.

SAMPLE CHAMBER DESIGN

• designed using AutoCAD LT

• water channel for temperature control.

• TEC module (4A max) placed between the sample block and the chamber. Prevents condensation on the sample

• 4-wire PRTD for sample temperature measurement

Sample Chamber

PRTD TEC Module

D2 Chilled Mirror Sensor and Sample chamber

0

V

R17

1

Q4

Q2N3906 W

0

ANALOG_Output

5

U10

OPAMP

+

-

OUT

V7

2.5

R16

100

TEC_Module

1

0

R5

100

V

0

R2

100

V6

2.5

Q3

Q2N3904

V

0

R4

100

Power Booster Design for TEC Module

wetpressurevaporwaterpartialsatVpdrypressurevaporwaterpartialdryVp

airsaturatedrateflowfwairdryrateflowfd

wheresatVpfwfd

satVpfwdryVpfd

, )( , )(

:

100)()(

)()( RH%

ACKNOWLEDGEMENT

I would like to acknowledge my supervisor, Dr Lin Yang for rendering his technical expertise throughout the summer, and as the work continues.I also express my gratitude to Dr. Ronald Pindak and Mr. Terrence Buck for providing this opportunity of great learning.