Device makes use of microwave spectral absorption in low-Q resonant Stark cell, and indications are that ultimate sensitivity of instrument is within 100 parts per billion of formaldehyde. Microwave source is very small and requires only six-volt dc bias for operation. Coarse tuning is accomplished mechanically and fine tuning by adjusting dc-bias voltage

Easley, W. C.

English

NASA Technical Reports Server

NASA TECH BRIEF 
Langley Research Center E1 . August 1973	 B73-10228 
NASA Tech Briefs announce new technology derived from the U.S. space program. They are issued to encourage 
commercial application. Tech Briefs are available on a subscription basis from the National Technical Information 
Service, Springfield, Virginia 22151. Requests for individual copies or questions relating to the Tech Brief program may 
be directed to the Technology Utilization Office, NASA, Code KT, Washington, D.C. 20546. 
Formaldehyde Monitor for Automobile Exhausts 
An instrument for detection of relatively low con-
centrations of formaldehyde has been constructed and 
tested in a Langley Research Center laboratory. This 
unique device makes use of microwave spectral ab-
sorption in a low-Q resonant Stark cell, and indications 
are that the ultimate sensitivity of the instrument is 
within 100 parts per billion of formaldehyde. 
The Stark cell is made from a short length of Ku-
band (12,000 to 18,500 MHz) waveguide with re-
Preamp
flectors placed at each end. Stark modulation is then 
used, in conjunction with a lock-in amplifier, for de-
tection. A Gunn oscillator is used for the microwave 
source and is tuned to 28,974.81 MHz, the .frequency 
of a strong formaldehyde-absorption line in Ka-band 
(26,500 to 40,000 MHz). 
The microwave source is very small and requires 
only a six-volt dc bias for operation. Coarse tuning is 
accomplished mechanically and fine tuning by ad-
justing the dc-bias voltage.
Recorder 
Lock-in 
Amplifier 
I	 Reference 
Detector	 Sample	 Output (33 kHz) In	 Exhaust 
I Isolator	 [1 Membrane	 To Pump 
 
II	 If  
I Assembly	 t 
Phase 
Shifter	
Attenuator H T-Junction H Tuner	 I	 I I _________ Tuning Resonant	 i 
Cavity 
I	 Stark  I	 Isolator	 I	 Cell 
Microwave Source 
.
Formaldehyde Monitor
(continued overleaf) 
This document was prepared under the sponsorship of the National
	 Government assumes any liability resulting from the use of the 
Aeronautics and Space Administration. Neither the United States
	 information contained in this document, or warrants that such use 
Government nor any person acting on behalf of the United States
	 will be free from privately owned rights.
https://ntrs.nasa.gov/search.jsp?R=19730000228 2020-03-17T07:11:43+00:00Z
Stark modulation and a lock-in amplifier are used 
for signal detection (see block diagram). The modu-
lation system consists of a small solid-state, square-
wave generator and associated power supply. Triggering 
of the square-wave generator is accomplished with a 
sine wave from the reference output of the lock-in 
amplifier. The sine wave is converted to a square wave 
and amplified. 
Microwave power, coupled out of the resonant 
cavity through the iris, enters the 1-junction and is 
split between the detector arm and the source arm. 
Power from the cavity which enters the detector arm 
of the T-junction contains the usable 33 kHz signal. 
This signal is detected and fed into the amplifier. 
An important component of the system is the 
sample concentrator. It consists of a small section of 
dimethyl silicone membrane supported by a thin 
section of perforated stainless steel. The membrane and 
support are placed between two halves of a teflon 
holder. The concentrator assembly is connected to the 
cavity. Nitrogen or air, at atmospheric pressure, con-
taining the formaldehyde to be detected flows past the 
membrane on the sample-in side. On the other side 
of the membrane the pressure is much lower due to 
vacuum in the absorption cell. This pressure drop 
across the membrane causes gases to pass through it.
The membrane material is much more permeable to 
formaldehyde than to nitrogen; thus, a significant in-
crease in the formaldehyde concentration relative to 
nitrogen results in the absorption cell. 
Notes: 
Requests for further information may be directed 
to:
Technology Utilization Officer 
Langley Research Center 
Mail Stop 139-A 
Hampton, Virginia 23665 
Reference: B73-10228 
Patent status: 
Inquiries concerning rights for the commercial use 
of this invention should be addressed to: 
Patent Counsel 
Langley Research Center 
Code 456 
Hampton, Virginia 23665 
Source: Wesley C. Easley
Langley Research Center
(LAR-1 1352)
. 
B73-10228	 Category 04, 02


Formaldehyde monitor for automobile exhausts

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