The best way to make sure an incorrect delay does not ruin your rocket is to build all your rockets with the proper techniques and materials to withstand high speed ejections.
Gary Rosenfield of Aerotech successfully petitioned to amend NFPA 1125 to allow delay performance to vary by the greater of 20% or 1.5 seconds because "It is very difficult to maintain a time delay tolerance of +/- 1 second on some model and high power motors for a number of reasons…." 1.5 seconds! So if you think your rocket will be going 20 mph at a timely ejection, it could be going 50 mph if the ejection occurs 1.5 seconds late. But if you built your rocket right, it just leads to a few skipped heartbeats while waiting for that parachute.
Delays are special propellant designed to ignite the ejection charge a number of seconds after the main propellant grain burns out (enough time for the rocket to reach apogee). Seems simple enough. However, the burn rate of propellants varies depending upon pressure and temperature. When a motor is first ignited, the delay element is ignited too. The main propellant burns, resulting in high pressures and a faster burn rate for the delay element. Then the main propellant grain burns out, the pressure drops and the delay burns at a slower rate.
At low pressure, delays burn at about 1/32” per second. Under typical pressures during motor burn, the same delay will burn at 4 to 6 times that rate. It doesn’t take a rocket scientist to see that a small change in how long a motor burns under pressure can lead to a big change in the delay.
The warmer the propellant the faster the burn rate, the colder the propellant the slower the burn rate. Leave your motor in the desert sun a few hours, launch it and you will get a delay that is longer than advertised. Motors are tested at about 70 degrees. The closer you can keep your motors to that temperature before launch, the more consistent and reliable their performance should be.
Don’t let a delay ruin your day!