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- Mettler-Toledo Excellence XS204 dual-analytical balance systems: range = 1~150 g
- Cahn Automatic Electrobalances: range = <1 g
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- g
Mettler-Toledo Excellence XS204 Analytical Balance
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The Cahn electrobalances are located in the Chemistry Laboratory and the Microbiology Radiation Van on the ship.
Figure 2. Cahn C31 Electrobalance.
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Upon opening Balance Master, the initial screen (Figure 3) will be seen:
Figure 3. Balance Master initial screen.
Balance Master Configuration
The technician should ensure that Balance Master is configured properly. Click on the “Configuration” window (upper left in Fig. 3, above) to open the configuration screen as shown in Figure 4.
Figure 4. Balance Master configuration screen.
The two COM fields should be set to the appropriate COM ports of the two Metter-Toledo balances, taking care to ensure that the reference and unknown balances are selected appropriately. The communication settings for the XS-204 balances are: 9600 baud, 8 bits, 1 stop, no parity, as shown in Fig. 4.
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The Scale Shift value should remain 1000 to keep the scale appropriate to the usual mass range being measured.
Figure 5. Measurement option window
Figure 6. Precision-based problem. Note that although only 31 counts were captured, the weight uncertainty reached the defined 0.0002 grams simply by random happenstance. This is the reason the precision method is not recommended.
Taring the Balance
All of the paired balance systems on the ship must be tared via their control software, not by the tare buttons on the balances themselves. The software tare measures the difference between the averaged tare value of the reference and unknown balances. As seen in Figure 7, the tare value is -0.0012823 grams; this is the difference in the average tare value of the reference balance and the average tare value of the unknown balance.
Figure 7. Completed tare step.
Making a Measurement
Once the tare is completed, the user can place the material to be weighed on the unknown (right-hand) balance, and a mass of standard reference masses approximately equal to the unknown to be weighed (refer to Precision Weighing at Sea for an explanation of this requirement). Enter the mass of the reference masses in the “Counter Weight” field, circled in red in Figure 8, below.
Figure 8. Counter Weight field (red circle) and Weigh command button (black circle).
Once the balances are loaded and the Counter Weight value is entered, press the “Weigh” button (circled in black in Figure 6, above) and the measurement will begin. It will stop automatically upon the completion of the measurement (after a set number of counts or a set precision value, depending on the options selected by the user).
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Use of the Java Balance software requires authentication to the LIMS, so once the software is started, a login screen will appear as shown in Figure 9. The link to this manual functions, but the instructions to email the shore-based developer account is out of date (any help requests should be routed to jr_developer@ship.iodp.tamu.edu instead). Although the login page states that it is for the dual Mettler-Toledo balances, this software is intended for use only on the Cahn electrobalances.
Figure 9. Java Balance login screen.
Once the user has logged in, the software will acknowledge login; click “OK” to proceed. The software will then which balance is being used, and then query whether to use the existing configuration profile or not as shown in Figure 10, below. Select Cahn (only) and use the previous configuration file if one exists.
Figure 10. Instrument selection and configuration choice screens.
Java Balance Configuration
If this is the first time that the Java balance software is being used on this computer, the proper configuration values may need to be set for COM port, baud rate, etc. The proper settings for the Cahn balances are shown in Table 1, below.
Property | Cahn Balances |
|---|---|
Baud rate | 4800 |
Parity | None |
Data bits | 8 |
Stop bits | 2 |
Flow control | None |
Table 1. Communication settings for the Cahn electrobalance
These values can also be accessed from the instrument’s main panel by clicking on the Configuration button on the upper left corner of the panel. This will invoke the configuration screen as shown in Figure 11, below. Be sure that you understand what each parameter should be before changing any of the fields!
Figure 11. Java Balance configuration editor.
The user can choose between milligrams and grams for the data output; milligrams are recommended for the Cahn electrobalances. Once the configuration settings are correct, click “Save” in the lower left corner, followed by “Exit.” The Exit button will invoke the second configuration screen as shown in Figure 12. You will have to get a developer to help set the parameters on this screen.
Figure 12. Java Balance second configuration editor.
Cahn Model 29 and Model 31 Electrobalance Calibration
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Once the configuration is correct, the user may now select the test options by clicking the “Test Options” button. This will invoke the screen shown in Figure 13, below. The user can select between counts (strongly suggested) or standard deviation and enter the value for the selected parameter. The Cahn balance provides a mass at 1 Hz, so a count value of at least 180 is recommended to cover three minutes of the ship’s heave.
Figure 13. Java Balance test option screen. It is not recommended to use Standard Deviation mode.
Taring the Balance
Before the balance can be used, it is necessary to tare the balance via the software, not by the tare buttons on the balances themselves. The software tare measures the difference between the averaged tare value of the reference and unknown balance pans. Click “Tare” on the left hand side of the panel, followed by “Start” in the lower center panel. These buttons are circled in Figure 14, below.
Figure 14. Taring the balance. Note that the “Start” button will be grayed out until the “Tare” button is pressed.
The taring action will take as long as a regular measurement (~3 minutes). Once the selected counts have been reached, click “Stop” followed by the “Get Mass” button to the right of the “Start” button. This will save the tare value to the Tare field. See the notes in Counterweighting the Cahn, below.
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Before taring the Cahn electrobalance, determine whether to tare with weighing paper or crucible:
Reference Material | Analysis |
|---|---|
Weighing paper | CaCO3 by Coulometer TOC by SRA ICP sample Reagents (<1g) |
Crucible | CHNS |
The Cahn returns the mass difference between the sample and reference/tare pans, so the best practice is to load the tare pan (right side) with a counterweight that is close to the mass of the container being used on the unknown pan. For example, if a folded piece of weighing paper is being used to collect the sample powder, place folding paper of the same size on the reference pan. You must tare to these papers and/or crucibles, not to empty pans.
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Once the Cahn has been tared, add the unknown to the sample/unknown pan and click on the “Weigh” button as shown in Figure 15. The “Reference Mass” field is not used in the Java Balance software for the Cahn, so that field can be ignored. Click “Start” and wait for the measurement to complete. Then click “Stop” and “Get Mass;” the final mass value should now be visible in the Final Mass field on the left side of the software panel.
Figure 15. Making a measurement.
Assigning a Mass to a Sample and Saving to LIMS
Once the mass has been obtained, use the controls on the lower left corner of the software to select an analysis as shown in Figure 16, below. Note that the balance software will show “CHNS” at startup. If you are measuring CHNS samples, you must select away from this analysis, then back to it, in order to proceed. If you are measuring any other type of sample, simply select that analysis.
Figure 16. Selecting the analysis.
Put the cursor in the “Text ID” field and scan the sample’s barcode, then click “Assign.” This will invoke a new window with a list of samples that match that Text ID (normally only one sample unless it is a QA/QC standard) and you select it by double-clicking.
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