Type edc and select the experiment H90CALIB. This is a single-scan, single-pulse 1H experiment. Check the getprosol box. Acquire a spectrum. This only takes 10 seconds. Correct phase using apk.
Create another file using edc, and this time select the “Use Current Parameter” option. This will copy all the acquisition and processing parameters from the current file to the new one, including receiver gain, phase correction amount etc. Type p1 to view the 90deg pulse length. It should be somewhere around 12 us (microseconds). Double this value and enter it into the box. E.g. if the current value is 12.5, change it to 25 (the default unit is us). Acquire a spectrum. Don’t correct phase or do rga because we want to keep all the parameters the same for an exact comparison of this one against the prior one.
Use Multiple Display to compare the above two spectra. If the second spectrum has much smaller intensity than the first but is positive, the flip angle is less than 180deg, and you need to slightly increase the p1 (e.g. from 25 to 25.6) and observe again. If the second spectrum is negative, the flip angle is more than 180deg, and you need to slightly cut p1. Repeat until the final spectrum has a pretty much zero intensity or half-positive, half-negative (which is quite common). You have found a good 180deg pulse length. Let’s say it is 23.2 us. This means that the 90deg length should be 11.6 us.
In the picture above, the red spectrum uses a p1 that doubles that of the blue one. The sharp solvent peaks become essentially zero on the red spectrum, which means that its pulse length is a pretty good 180 degrees. There is some broad hump on the red spectrum, which might be background signal which you can ignore.