When running in Autopilot mode (see: Simple Spectrometer Field Setting) the compensating coil voltage is set automatically and the probe appropriate for the field desired is selected. The gaussmeter is placed in SEARCH Mode and the dipole power supply regulator is turned on. In this case the dipole current is adjusted to achieve the desired field. If the NMR gaussmeter is not "locked" a backup Hall Probe is used until the NMR "locks". The user should just stand back and let it work. What follows are instructions for using the NMR gaussmeter in situations where Autopilot doesn't work or some special supplemental measurements are required.
In principle it is possible to make the field measurements using the SEARCH mode in the Teslameter. In this mode you select a probe and the meter explores the whole field range of the probe until it finds and "locks" on the resonant signal indicating that it has a field measurement. A "lock" is indicated on the controls display by positive field values. This has the advantage of simplicity but in practice can be time consuming and doesn't always work. The problem being, in situations where there is a lot of noise mixed in with the signal, the circuitry has problems distinguishing the signal from the noise and gets lost before it ever finds a lock. The problem is exacerbated when the field being measured is at the high end of the probe's range. In this case the search starts at the low end and keeps getting hung up on the noise and never gets to the field range of interest. The solution to this problem is to tell the device approximately what field it's looking for and use the AUTO mode to find the lock. In the procedure below that is what we will be doing.
In any case, for "gap probes" (group 0) you must energize and adjust the gradient compensating coils for the field ranges to be measured before trying to make measurement.
For studies involving 10% changes in the field settings the compensating coil current can be set once and left alone.
Recommended Procedure:(turn the REGULATOR OFF for all
non-autopilot field measurements)
For group 0 probes set compensating coils appropriately (see figures).
Put meter in MANUAL mode with SEARCH OFF
Select a probe and polarity (Group 0:
Probes 0, 1, 2 negative; Probes 3, 4, 5 positive)
Type in DAC number for the field range being measured (see below)
Select AUTO and wait for a lock (positive field reading)
Verify that you have a good lock by checking the oscilloscope for a
clear resonant signal.
Go back to 2. for the next probe
If you have problems see the table listing problems and possible
solutions.
Selecting DAC #'s
In selecting the DAC # to use for the field of interest use either the graph in Figure 4.11 or the polynomial below that.
Problems and Solutions
| Symptom | Diagnosis and Cure |
| Weird numbers on displays, controls for | Need to reboot. |
| all magnets fouled up | See instructions below. |
| NMR Teslameter does not respond to | Meter's communications are |
| somehow hung up. | |
| commands and display shows all zeros. | Push RESET. |
| Will not lock | Very high noise level |
| makes resonance hard to find. | |
| Still will not lock | Very high noise level makes |
| resonance hard to find. Search | |
| for the resonance manually by | |
| adjusting the DAC in manual | |
| mode until you see the resonant | |
| signal. (It helps if you know | |
| what field you expect so you'll | |
| know where to look). | |
| You find resonance manually | Check probe polarity. |
| but still can't get a lock | Try decreasing and |
| increasing DAC number by 1. | |
| Optimize signal by adjusting | |
| compensating coils. | |
| Can't find resonance manually | Try a different probe. Use |
| readings from other probes to | |
| tell you where to look for | |
| the resonance with the probe | |
| that's giving you trouble. | |
| Make sure compensating coils are | |
| energized properly. | |
| Make sure magnet is on. |