The Thermo Fisher Scientific Technical Support team has received a handful of questions from customers regarding the usage of Thermo Scientific Model 17i Ammonia Analyzer and we would like to highlight some key solutions.
100% N2 or zero air is acceptable to perform zero calibrations.
For span calibrations, typically NO balance N2 is used for NO/NOx/Nt. For NH3 calibrations, NH3 balance N2 is typically used. For NH3, however, you should not run the calibration gas for long periods of time as it can strip the iron oxide layer from the NH3 hi temp converter. If NO2 is to be calibrated, a GPT test should be run same as for NO/NOx analyzer to determine converter efficiency.
You cannot obtain a NO2 calibration gas bottle at a low enough concertation for this test. Using a multi-gas calibrator to dilute NO2 gas is also NOT recommended as when diluting the NO2 with air the NO2 can react and turn into NO skewing the results low. A GPT test is how to perform calibrations on the NO2 channel.
For the Model 17i, O2 is not necessary for calibrations but is for reading sample gas. Calibrations are typically run for only a short amount of time and will not deplete the iron oxide layer in the high temp NH3 converter. The sample must contain the O2 level to keep the self-regenerative iron oxide layer intact over longer periods of time. If O2 is used in the calibration zero or span gas, that is also acceptable.
2. RS485 Communication
For either RS232 or RS485 serial communications, either serial connection port can be used. They are joined in parallel inside the analyzer. You can follow the wiring diagram from the manual. But to make it simple a standard DB9 serial cable may be used. For direct connection to a laptop a NULL (crossover) type cable from computer direct to the analyzer must be used. For connection to some other device such as a hub or something similar a straight type cable must be used.
The instrument uses a four wire RS-485 configuration with automatic flow control (SD). Refer to Table 8-2 for the DB9 connector pin configuration.
Table 8-2. RS-485 DB Connector Pin Configuration