Instrument Support Level 3
Thermo 49C Ozone Monitor
Thermo Scientific 49C Ozone Monitor
The Model 49 is a powerful, easy-to-use, UV photometric based ozone analyzer which offers increased specificity via its balanced optical system. User programmable software capabilities allows selection of the frequency at which internal zero/span activation and instrument calibration checks will occur. Additionally, field programmable measurement range settings can be stored in memory for subsequent recall. Extended troubleshooting diagnostics now provide instantaneous indication of instrument operating parameter, status including: Pressure, Flow, DC Supply Voltages, Optical Bench Temperature, Ozonator Power Supply Voltage and Lamp Voltage.
The instrument has the capability to measure the amount of ozone in the air from 0.05 ppb concentrations up to 200 ppm. The instrument precision is 1 ppbV.
Due to both sample and reference flowing at the same time, the instrument has a fast response time of 20 seconds (0.2Hz). The gas concentration is displayed in real time and is logged via a standard RS-232 interface.
This instrument is run as part of the COZI Laboratory set up. It is run almost continuously to enable information about its long-term stability to be obtained. The instrument is calibrated regularly and is traceable to the Global Atmospheric Watch scale. Users are invited to run their new and existing ozone instruments/sensors alongside this instrument with the aim of evaluating their performance.
A common sampling manifold is available in addition to instrumentation for humidity testing. Two calibrators traceable to GAW are available to investigate instrument linearity.
The 49 Series Ozone Analysers are based on the principle that ozone molecules absorb UV light at a wavelength of 254 nm. The degree to which the UV light is absorbed is directly related to the ozone concentration as described by the Beer-Lambert Law:
K = molecular absorption coefficient, 308 cm-1 (at 0°C and 1 atmosphere)
L = length of cell, 38 cm
C = ozone concentration in parts per million (ppm)
I = UV light intensity of sample with ozone (sample gas)
I0 = UV light intensity of sample without ozone (reference gas)
The sample is drawn into the instrument through the sample bulkhead and is split into two gas streams. One gas stream flows through an ozone scrubber to become the reference gas (I0). The reference gas then flows to the reference solenoid valve. The sample gas (I) flows directly to the sample solenoid valve. The solenoid valves alternate the reference and sample gas streams between cells A and B every 10 seconds. When cell A contains reference gas, cell B contains sample gas and vice versa.
The UV light intensities of each cell are measured by detectors A and B. When the solenoid valves switch the reference and sample gas streams to opposite cells, the light intensities are ignored for several seconds to allow the cells to be flushed. The instrument calculates the ozone concentration for each cell and outputs the average concentration to both the front panel display and the analog outputs.
UV photometry is the recommended primary calibration procedure.
The instrument is run continuously in the COZI Laboratory. It is regularly run alongside the other AMF ozone instruments.
It is calibrated (at least every 6 months) using the AMF ozone calibrator (Transfer Standard – TS Model 49I-PS S/N: 0703820627) which is itself calibrated annually using the primary standard reference photometer #2 (SRP#2) from the National Physical Laboratory (NPL) (traceable through the National Institute of Standards and Technology (NIST). This allows the bias on a full range of ozone values to be evaluated.
The instrument zero is checked every month using an external scrubber (activated charcoal) on the inlet (20 minutes with the last 10 minutes of data being used) and as part of the calibration procedure. Results from this are recorded but the instrument is not rezeroed unless it is clear that there is a problem with the instrument, i.e. if the standard deviation of the zero data is large.
The standard difference between two SRPs measuring zero air should not exceed (root 2 x 0.25 ppbV) = 0.49 ppbV (1 sigma), 1 minute averaging time.
Since it is an absolute measurement technique, the accuracy of the instrument is purely determined by the random error (or precision) and there is no assumed systematic error. The routine zero data (1 minute) is used to calculate the error on the data (√2σ) and if this is > 3 ppbV for minute data then the data is flagged.
To give the precision for hourly averaged data (√2σ)/ (√60).
Instrument parameters (e.g. cell intensities, temperatures, flows) are logged alongside those of the concentrations and used to diagnose problems.
The ozone measurements are fully traceable to the international Global Atmospheric Watch (GAW) scale (NIST Standard reference photometer SRP#2). This means that the ozone photometer used to generate ozone for the calibrations is referenced to a photometer held at NIST.
Data from instruments that are part of a laboratory do not archive their data to CEDA and so do not have to conform to any AMOF standard.
Data can be provided for the period of time that the user has accessed the facility.