3.10 Inverse Ratio

Let's assume that you want the lights to go ON earlier. You could specify a 2.6 footcandles (27.9 lux) ON which gives about 3.5 minutes earlier ON than an ANSI standard control set at 1.5 footcandles (16 lux). This is desirable in some retail areas or places with heavy mixed pedestrian and vehicle traffic. If you specify a standard 3:1 positive ratio control, the turn-OFF will now be 9.8 footcandles (105.4 lux) (3X2.6 = 9.8). This is too late in the morning; it adds about 8 minutes of unwanted morning burning time.

The most common solution is to specify a control with a close positive ratio of 1.5:1. Now turn-OFF is about 4 footcandles (43.0 lux) (1.5X2.6 = 3.9). This is a good level. No one will complain about excessive morning burning time. Another solution is a control with an inverse ratio of 0.6:1 (ON = 2.6 footcandles (27.9 lux), OFF = 1.6 footcandles (17.2 lux)). This saves about 6 minutes/day over a close positive ratio electronic control and about 11 minutes/day over a standard AC relay control with 3:1 OFF/ON ratio.

There are some common problems when using inverse ratio controls:

- Inverse ratio circuitry is about considerably more complicated than close positive ratio electronic controls. Therefore it costs more.

- Inverse ratio controls won't invert in areas where it doesn't get completely dark at night. They should not be used in areas of high ambient light such as downtown urban sections, car sales lots, or on floodlights. Often these are the same areas that need the higher turn-ON.

- Don't specify inverse ratio with 1.0 footcandle (10.76 lux) turn-ON. You will have problems with control cycling. It will never get dark enough to properly operate the circuits.

Before specifying, do a simple test. Install some inverse ratio controls and some close positive ratio controls in adjacent lights. Do a dusk and dawn visit and see if you can see a difference. My experience says you won't. If an inverse ratio control is needed, DTL's preferred model is the DPR124-2.6-TMBK.

Alternative inverse ratio: If you really want inverse ratio but do not want the cost or other disadvantages, you can face a standard, close positive ratio, electronic control east and you will get the same result as an inverse control facing north. It will cost less and be more reliable.