During the last 20 years, many DIMM instruments have been developed to measure astronomical seeing. The IAC has been involved in several projects to run different campaigns to characterize its observatories. However, the cost in manpower to maintain and operate these instruments has been too high and it is mandatory to minimize this effort by constructing a reliable, robust and available seeing monitor. A review of all sources of errors has been done in order to fit very reliable measurements: acquisition parameters, box size, signal threshold, SNR threshold, flux, deformations and vibrations for centroid calculations, best pixel scale, jitter in the images sampling, light bandwidth, CCD noise, as well as the centroid calculation algorithm. Experimental measurements about the influence of exposure time, number of images for computing the seeing or defocus have been carried out to identify the practical limits of the instrument. The IAC automatic DIMM design has been reviewed to improve its robustness and its availability to guarantee the minimum down-time and to maximize the time between failures. The new design will be shown as part of this work.