1 INTRODUCTION
Large quantities of hypergolic propellants including hydrazine(HZ), monomethy hydrazine(MMH), and unsymmetrical-dimethylhydrazine (UDMH) are used extensively as rocket fuels in the national defence industry. These three hydrazine compounds are toxic, combustible and explosive. Some research indicated that the hydrazine compounds are even carcinogenic. The current recommended exposure level(TLV) are 0.1, 0.04, 0.5 ppm in air for HZ, MMH, and UDMH, respectively.
    Methods for determination of these three hydrazine(HZ,MMH,UDMH) include the earlier methods of colorimetry, titration with standard potassium iodate solution and sophisticated instrumental analyses involving gas chromatography, mass spectrometry, spectroscopy, photo-ionization, semi-conductors, chemiluminescence and electrochemistry. All of these methods except the electrochemistry suffer from the drawbacks such as complexity，insufficient sensitivity and lack of specificity, continuous measurement capability, or the cost-effectiveness necessary to meet present requirement. In addition, there is still no effective domestic instrument for determination of hypergolic vapor in real-time. Imported instrument for hypergolic vapor detection has the limitations of either expensive or inadequate to be adapted for actual applications.
    In this work, a new mutiple-channel monitoring instrument for propellant of hydrazine compounds has been developed. Electrochemical sensor has been employed which can provide fast, sensitive and real-time determination of HZ,MMH and UDMH vapors at 10^-6 levels when coupled with an instrument system.

2 INSTRUMENT DESIGN AND CHARACTERIZATIONS
There are four primary sections for the design of the instrument, including the design of electrochemical cell, the hydrazines dilution system, the intellectual electrochemical sensor and the mail processor system. The following report discusses the development of the four sections.
2.1 The development of electrochemical cell
The fundamental design of the instrument is the electrochemical cell with a three-electrode system.A schematic diagram of the electrochemical sensor design is shown in Figure 1.The three electrodes are all Teflon-bonded diffusion electrodes which are prepared by spraying the catalyst-Teflon dispersion onto the Teflon film.The catalyst materials are purchased as high purity powder of gold and platinum catalyst. The electrodes are sealed inside a polyethylene chamber which is filled with certain alkaline electrolyte.The alkaline electrolyte is prepared from reagent grade materials and distilled water.The chamber is assembled with special technology to ensure the cell impermeable.

Fig 1 The structure of the diffusion electrochemical cell

    The gold and platinum leads of the sensor electrodes are connected to a potentiostat which is capable of variable bias and this is required for optimum signal stability.
    During the operation of the sensor, mixtures of hydrazines in nitrogen are passed over the sensing electrode at constant flow rate. The electrochemical reaction occurs.The oxidation-reduction reactions of the three hydrazine compounds on the sensing electrode are as follows:
N₂H₄+4OH^- N₂+4H₂O+4e
CH₃NHNH₂+4OH^- N₂+CH₃OH+3H₂O+4e
(CH₃)₂NNH₂+2OH^- (CH₃)₂NN+2H₂O+2e
    The current produced by electrochemical reaction in the sensor between the sensing and counter electrode is measured and this is directly proportional to the concentration of hydrazine vapors. So the concentrations of the hydrazine vapor are determined.
    The main parameters of the hydrazine electrochemical cell are illustrated as follows:
    The concentration ranges:0-100ppm; minimum detectable sensitivity: 0.2ppm ; zero drift: <±5% of full scales per week; accuracy: 10% F.S. (when the concentration of hydrazines is 50ppm); response time: 90% of signal with 60s; fall time: 90s;operating temperature range: -10-40^oC; operating relative humidity range: 25%-85%; average life span: ≥one year.
    However, the present limitation of the electrochemical cell are its response time and average lifetime.These characteristics can be improved by further sensor developments and more field-testing .
2.2 The development of dynamic dilution system for hydrazines
Calibration, testing and evaluation of the instrument require some apparatus to generating a stable hydrazine stream in the desired concentration range. However, hydrazine compounds are prone to being decomposed and adsorbed on the container walls. It is difficult to keep such a system properly adjusted because a change at any point in the system will upset the flow at all the other points.
    A dynamic hydrazine dilution system is developed to make hydrazine vapor mixtures of 0-100ppm in a continuous manner .The dilution system is contained in a large fume hood,since the hydrazines are toxic and explosive.A syringe which is driven by a syringe pump is used to inject hydrazine compounds into a diluent gas stream flowing at a controlled rate(99.9998% N₂ was used as diluent) .The diluent gas passes through a pre-conditioning chamber which is used to warm the gas stream when relatively high concentration(>50ppm) of hydrazines are desired.The diluent passed the syringe needle, picking up the hydrazine vapor and into a container which the hydrazines and N₂ are mixed. The vapor mixture then may be divided by two Teflon valves.The vapor mixtures may either be collected in Teflon bags or be used directly from the sample exit stream.
    The theoretical concentration of the vapor can be found by knowing the delivery rate of the liquid and the flow rate of the diluent.The equation giving the ppm vapor concentration by volume is:

here : A=the delivery rate of hydrazine liquid(ml/min), D=the density of the liquid (g/ml), M=the molecular weight of the liquid(g/mol), F=the flowrate of the diluent(ml/min).
    During calibration procedure,spectrophotometric analyses are used to determine the concentration of hydrazine compounds in the generator output gas stream.The method in GJB2373-95 can be used in calibration procedure.According to the GJB2373-95,the concentration of the hydrazines were determined colorimetrically at a wavelength of 460nm for HZ, 470nm for MMH and 500nm for UDMH.

2.3 The development of intellectural electrochemical sensor
The principle of the intellectual sensor is illustrated in Figure 2.The concentration of hydrazine is converted to electrical signal at mA level after oxidation-reduction reaction in electrochemical cell.The mA signal is amplified to 0-200mV signal which is then converted to digital signal by the analog-digital conversion of the MCU.The digital signal which is calibrated as real-time concentration of hydrazines are shown with figures on the LED display .Audible and visual alarms can be given out when the concentration of hydrazines exceeds levels of the alarm.The audible alarm can come to 85dB at least.In the mean time,the output analogue signal at 1~5mA level is converted to the main processor directly by the hardware. The sensor is powered by intrinsically safe circuits from the main processor.

Fig 2 The intellecture sensor