Contamination Monitors

Various contamination sensors are available for incorporation into a contamination monitor such as ICA (Mir). This uses QCMs, AO detectors, and impact detectors. All instruments are flight proven and fabricated to customer specifications in terms of class A, B, C protocols depending on budget and environment constraints. Five types of sensors can be used to monitor the environment in general:

  • Temperature-controlled Quartz Crystal Microbalance (TQCM)
  • Quartz Crystal Particle Microbalance (QCPM)
  • Ionization Pressure Gauge
  • Thermal Coating Calorimeter
  • Optical Scattering Sensor

Temperature-controlled Quartz Crystal Microbalance.

Temperature-controlled Quartz Crystal Microbalances (TQCM) are used to measure the deposition of molecular contamination. The TQCMs measure contamination by means of a frequency shift of a quartz crystal oscillator. This occurs when the crystal mass increases as a result of contamination accretion. The device is extremely sensitive, 1 Hz corresponding to 1.56 E-9 g/cm2. This sensitivity is achieved by using a specially cut crystal that produces an extremely small temperature dependence and by using a reference quartz crystal. The signal from the reference quartz crystal, when mixed with the signal from the sensing crystal, gives a beat frequency totally independent of temperature and power supply fluctuations. The TQCMs should be held at temperatures that are representative of typical instrument surfaces (optical and thermal) that are to be monitored. In addition, the TQCM can periodically volatilize collected contaminants by heating.

Quartz Crystal Particle Microbalance.

The Quartz Crystal Particle Microbalance (QCPM) is essentially the same as a TQCM. The only differences being the use of lower frequency quartz crystals (each resonates at approximately 10 MHz to aid in the accommodation of particulates) and the use of a vacuum deposited coating with an extremely low vapor pressure on the crystals. QCPMs have flown on 5 missions including STS, Mir (ICA).

Ionization Pressure Gauge.

An Ionization Pressure Gauge can be used to measure the total pressure. The vacuum measurement system consists of three assemblies; the cold cathode gauge, the signal conditioner, and the interconnecting cables. The cold cathode ionization gauge is used to convert the pressure input into an electrical signal. The gauge is based on the principle that the discharge current in a transverse magnetic field is dependent on the pressure of the gas. The cold cathode gauge and the high voltage supply are packaged into a common housing. Combining the cold cathode gauge and the high voltage supply in one housing eliminates high voltage connectors and cables and the possibility of arcing and corona. The output current of the cold cathode gauge is proportional to the input pressure. The signal amplifier changes the linear input pressure to current (voltage) relationship into an approximately logarithmic function.

Thermal Coating Calorimeters.

Thermal Coating Calorimeters (TCCs) have successfully flown on numerous missions including the IOCM, NOAA-C, ICA, APEX, OPM and NOVA contamination monitor projects. A sample disk rests on a 0.254mm thick Kapton ring within an inner cup. The thin ring support provides conduction isolation for the sample. The inner cup is flanged and has an exterior surface equal in area to the sample. This serves to maintain the inner cup surface close in temperature to the sample inner surface to minimize radiative and conductive heat transfer. The inner cup is supported by a Kapton ring 0.508mm thick, once again for conductive isolation. This resides within the outer cup or frame, which is mounted to the instrument. The sample mount and inner and outer cups are machined from aluminum and have very smooth surfaces to minimize their emittance. Several layers of Mylar aluminized on both sides and crimped and put between the sample and inner cup and between the inner and outer cup. This serves as a multilayer insulation (MLI) barrier to minimize radiation heat transfer within the calorimeter and thus minimize the heat leak out the back of the calorimeter. Various thermal coatings or surfaces are available. (E.g. replacing the sample disk with an arrangement of stacked razor blades (chemically blackened), can give an indication of the radiative heat flux input to the Contamination Monitor during on-orbit operations, and secondly, as a calibration surface for the contamination effects portion of the experiment). Another example would be to cover calorimeters with a second surface type mirror (SiO2/Ag). These mirrors are also known as Optical Solar Reflectors (OSR's). This surface will act as the primary contamination effects monitor for thermal control surfaces. This surface has been used for similar purposes to determine contamination effects on several shuttle missions.

Optical Scattering Sensor.

Contamination by spacecraft effluents can cause degradation to sensitive optical surfaces. An accepted figure of merit in the measurement of optical degradation is BRDF that quantifies the amount of incident radiation that is scattered due to non-uniform surface characteristics. Measurement of deposited mass is an indirect, non-linear measurement of the optical performance of degradation in terms of BRDF and not amenable to analytical solution without an extensive empirical database which considers the morphology of the contaminant deposit. For example, a given amount of mass deposited as a homogeneous film will have vastly different optical properties compared with the same amount of contaminant deposited as small droplets, the latter producing a high degree of scattering over a wide band of wavelengths. While TQCMs are useful in measuring a threshold amount of mass deposition, which has been determined by prior experiments to significantly degrade optical performance, the information provided has limited usefulness. A more direct measurement of real time changes in BRDF as a function of contaminant deposition would provide the user with greater flexibility in controlling and mitigating contaminant effects. The thickness of a layer of removable contaminant can be inferred from a measurement of changes in surface resistivity. The BRDF measurement includes degradation caused by a removable layer of contaminants and any permanent surface damage that may have occurred. Although direct measurement of the change in optical properties is important, the optical and resistivity measurements used in concert provide the amount of removable contaminant deposition with accuracy, which is not available to each instrument individually. The device consists of optical quality quartz glass on which several sub-sensors are placed. Surface resistivity is measured by several ring electrodes. The optical degradation measurement device consists of a laser diode and two (2) receiver diodes. The light beam emitted by the laser diode is partially reflected and partially scattered from the sensor surface. Light that is scattered into the receiver diodes is dependent upon the degradation of the surface and the thickness of deposits that accumulate on the surface.