I have an F1, I have a Combro and I have a Skan.
The F1 lies unused for the majority of the time. The only time it gets used is when taken out to do field testing of BC. It is very susceptible to changes in light but so long as it has good batteries it can be used, for this purpose I suggest a thick perspex plate in front as protection for the screen against fliers and I also use a 5 gallon white plastic container with top and bottom cut off as a sunshield tunnel over the whole chrono. I have calibrated it against my Skan and under optimum conditions it agrees within 1-2% of the Skan readings
The Combro gets used for the muzzle velocity readings when calculating BC, with careful use and good batteries, always remembering to set the calibration value every time the batteries are changed it gives readings +/- 1 or 2 FPS with my Skan.
The Skan is my bench chrono. Every rifle I own or work on gets run through the Skan as it is the same chrono that the police will use in event of an investigation. All results are logged and stored on the computer, every gun that's ever been shot, tuned or repaired by me is recorded for my piece of mind and to prove to the police that it was to the best of my knowledge legal when it left me. If ever my Skan produces a strange result I check the calibration figure and clean the sensor windows. If it won't show the correct calibration value it goes back to Skan for repair and re-calibration. So far it has been totally reliable apart from the sensor window covers coming un-glued from the plastic tunnel (probably due to getting 50+FtLb guns tested through it reasonably regularly, even with silencers these produce enough muzzle blast to rattle the windows! ) This "problem" has been cured with double sided sticky tape, I know another Skan user who has replaced with thicker perspex but so far I've not felt the need.
The Skan is plugged into the mains 24/7, this ensures the electronics are always at correct operating temperature (timing is done by quartz crystal oscillators, the accuracy is dependent on temp). The workshop is kept at 20-21 deg C, there is no direct sunlight or artificial illumination shining into or on the sensors.
Before taking my other chronos out for a BC test I will run a couple through them in conjunction with the Skan to check their calibration, while they will almost certainly be a lot cooler at the range they should both be at similar temps when recording velocities for BC calculations so I can assume they will be reasonably calibrated to each other at the range, but it's quite simple to put one or two through the F1 with the Combro on the muzzle and the F1 a couple of feet out rather than 50 yards away just to confirm they are within 1% of each other.
The instructions for the F1 Chrony state 99.5% accuracy, if we are talking about velocity readings in the 800 fps area this equates to +/-4 fps so don't get carried away with trying to get any chrono to read identically to another, they are acceptably accurate if used correctly, I use my Skan as my primary measuring tool as it is identical to the tool the police may use if ever there is a question over the legality of a gun I have worked on. I can honestly say that the gun was legal when I worked on it and if any tampering has been done I can disclose to the police how my guns are marked with "tamper evident" seals and marks, if there is any evidence of tampering then the gun is not guaranteed to be as it was when I measured it last. While this may or may not be admissible evidence for a court it goes a long way to prove that I am being honest and not attempting to disregard any legislation regards muzzle energy for sub12 and sub6 guns.