Miller, T.A. Blaskiewicz, M. Boucher, R.B. Fedotov, A.V. Gassner, D.M. Germain, C. Germain, J-C. Kewisch, J. Minty, M.G. Seletskiy, S. Song, H. Thieberger, P. Wanderer, P. JACoW Geneva, Switzerland 978-3-95450-192-2 10.18429/JACoW-IBIC2017-WEPCF05 English 423-427 WEPCF05 ion electron dipole instrumentation operation Contribution to a conference proceedings 2018 2018-03 https://doi.org/10.18429/JACoW-IBIC2017-WEPCF05 http://jacow.org/ibic2017/papers/wepcf05.pdf Low Energy RHIC electron Cooling (LEReC) is planned during a 7.7 - 20 GeV/n run with Au+79 starting in 2019 (200 GeV/n center-of-mass typical), to explore the existence and location of the QCD critical point. An electron accelerator for LEReC is being constructed to provide a beam to cool both the blue & yellow RHIC ion beams by co-propagating a 10 - 50 mA electron beam of 1.6 - 2.7 MeV. For effective cooling of the ion beam, the electron and ion beam energies must be matched with 10⁻⁴ accuracy. As the energy of the RHIC ion beam can be known to <1×10-4, the absolute energy of the electron beam must also be measured to 10-4 accuracy. A 180° bend transport magnet will be used as an energy spectrometer for the electron beam providing fields in the range of 180 - 325 gauss. A Nuclear Magnetic Resonance (NMR) gaussmeter has been customized to measure the field in the magnet (to as low as 143 gauss) with an accuracy of 50 milligauss and a noise floor of < 20 milligauss. The concept of the magnetic spectrometer and details and commissioning performance of the NMR instrument are presented in this paper.