Actually the flow rate of the hour glasses with sand can be changed by some means! There are ways to change the speed at which the sand falls through the hour glass. For example, an external force can be used to shake the timer body, affecting how fast it falls. In addition, pressure  can be used to change the speed at which the sand of the glass falls. As the sand falls, the volume of the upper glass cavity increases and the pressure decreases, while the volume of the lower glass cavity decreases and the pressure increases. It is this small pressure difference that slows the sand down, and then the upper and lower gases reconnect, the pressure difference disappears, and the sand falls faster. If you want to change the falling speed of the sand, you can start from the outside and affect the pressure of the upper and lower glass body.