Do you have more than one LI-7500 Open Path CO2/H2O Analyzer? Typically, the serial numbers of the sensor head and control box will match. For example, a sensor head with s/n 75H-1234 goes with control box s/n 75B-1234. However, sometimes you may want to replace the sensor head on one unit with the sensor head from another unit. This may be done for troubleshooting purposes or if you are returning a sensor head for calibration. There are some important points to consider when performing this swap.
When you return a sensor head for recalibration, we recommend returning the sensor head and control box together as we feel that the results of the factory calibration are better. The control boxes and cables have slight variations in components that might affect the relationship between electrical signals and gas concentrations. However, heads can be swapped from one box to another if the control box cannot be removed, of if it is required for another reason.
When an LI-7500 CO2/H2O Analyzer is factory calibrated, the calibration coefficients for the analyzer are specific to the sensor head, even though these values are stored in the control box rather than the sensor head. If you are swapping sensor heads, it is a good idea to save the original configuration files for each LI-7500.
Saving the Configuration File
1. Power up the LI-7500, open the LI-7500 Windows application, and connect to the instrument.
2. Click the “File” menu and choose the “Save Configuration As” option. Select a directory and save the LI-7500’s configuration to the computer.
Uploading a New Calibration File
When swapping sensor heads, the correct calibration coefficients for the replacement head need to be uploaded to the control box. This is accomplished by using the LI-7500 Windows application after the replacement head is connected.
1. Power up the LI-7500, open the LI-7500 Windows software application, and connect to the instrument.
2. Click on the “File” menu and select “Open Configuration.”
3. Then select the configuration file you’d like to open. This will open the saved configuration file in the LI-7500 Windows application.
4. Uncheck the box called “Update LI-7500” and click “Continue.”
5. View the values by clicking on the “Cal Coeffs” tab. Make sure all the values are correct for the new replacement head (Serial Number, CO2 coefficients, H2O coefficients). They should match those listed on the most recent calibration certificate that was shipped with the instrument. The values can now be uploaded into the control box with the steps below.
On the right-hand side of the window there is a “Values To Update” box. Make sure all those are checked except “Pressure Coefficients” (“Pressure Coefficients” will only be available on newer models) and then click the “Update Selected” button. This will save the polynomials to the control box.
Where’s the Calibration File?
If the configuration files for the replacement head were never electronically saved to a computer, or this file was deleted or saved on a different computer, there are two ways to load the calibration values:
1. LI-COR can provide the most recent configuration file electronically. This can be uploaded directly into the LI-7500. LI-COR can email the calibration certificate as a .pdf file as well.
2. If the printed calibration certificate is the only item available, attach the new head and then connect to the LI-7500 Windows application.
a. Click on the “Cal Coeffs” tab. The boxes should be filled with the previous sensor head’s configuration.
b. Go through all the boxes and change these values to those that are listed on the calibration certificate (Serial Number, CO2 coefficients, H2O coefficients, Pressure). On the right-hand side of the window there is a “Values To Update” box (see the screen capture above). Make sure all those are checked except “Pressure Coefficients” (it will only be available on newer models) and then click the “Update Selected” button. This will save the coefficients that were just entered to the control box.
Perform the zero and span calibrations after swapping sensor heads and uploading the correct configurations. This important step should help correct for minor differences between the control boxes and cables.
If one of the LI-7500s has a longer cable (6 meters rather than 3 meters), it is more important to perform zero and span calibrations after swapping heads and uploading the new coefficients. The cable length differences may influence the zero and span settings for the different LI-7500s.
If one of the LI-7500s is configured for normal operation, while the other has been verified for cold temperatures (usage verified down to -40 °C), be aware that swapping heads will not guarantee that the verification moves with the head. The cold temperature test verifies that both the sensor head and control box work adequately and within specifications at -40 °C. Again, make sure that the original configuration files are saved for each head prior to swapping heads. Also ensure that the zero and span calibrations are performed after swapping heads and uploading new coefficients.
Update: This post was updated on 01/25/2012 to correct errors in the original text.