  Accurate conversion of neutron time-of-flight (TOF) to wavelength, and its
uncertainty, is of fundamental importance to neutron scattering measurements.
Especially in cases where instruments are highly configurable, the
determination of the absolute wavelength after any change must always be
performed. Inspired by the manner with which neutron spectrometers determine
the absolute wavelength, we evaluate for the first time, in the author's
knowledge, a commonly used method for converting TOF to neutron wavelength, the
distance of a monitor from the source of neutrons and we analytically calculate
the uncertainty contributions that limit the precision of the conversion. The
method was evaluated at the V20 test beamline at the Helmholtz Zentrum Berlin
(HZB), emulating the ESS source with a pulse of 2.86 ms length and 14 Hz
repetition rate, by using a mini-chopper operated at 140 Hz, beam monitors
(BMs) and data acquisition infrastructure. The mini-chopper created
well-defined neutron pulses and the BM was placed at two positions, enabling
the average wavelength of each of the pulses created to be determined. The used
experimental setup resulted in absolute wavelength determination at the monitor
positions with a $\delta \lambda_{mean} / \lambda_{mean}$ of $\sim$1.8% for
$\lambda >4$ \r{A}. With a modest increase of the distance between the
reference monitor positions a $\delta \lambda_{mean} / \lambda_{mean}$ of below
0.5% can be achieved. Further improvements are possible by using a thinner
monitor, smaller chopper disc openings and a higher rotational speed chopper.
The method requires only two neutron measurements and doesn't necessitate the
use of crystals or complex fitting, and could constitute a suitable addition to
imaging, diffraction, reflectometers and small angle neutron scattering
instruments, at spallation sources, that do not normally utilise fast choppers.
