Automatic chemical addition and makeup is an attractive on feature on today’s sophisticated cleaning machines. Who wouldn’t appreciate eliminating the drudgery of taking samples, titrating, refractometer readings, or whatever and adding chemistry to a cleaning or rinsing bath manually? The trouble is that automatic chemical makeup is just not possible or applicable in all cases.
Although there are a multitude of ways to apply automatic chemical makeup, most fall into one of two categories –
- Chemical is metered into the water supply in fixed concentration
- Chemical is added directly to the cleaning tank based on feedback from a sensor (pH, specific gravity, etc.) located in the tank itself.
Let’s tackle the fixed ratio metering first. These devices are, essentially, blind. They just keep metering in chemistry whenever there is a flow of water. In most cases, this method of adding chemistry works well for the initial filling of a system but may not be so great for ongoing makeup. Consider the case of makeup to replace evaporation losses. Evaporation usually consists primarily of just water (think distillation) leaving the chemistry behind. If the replacement liquid is water and chemistry, this will obviously result in an over-concentration of chemistry over time. Although a higher dose of chemistry may not be detrimental to the process, it is costly! On the other side, it may be appropriate to add chemistry to counteract depletion of the chemistry as a result of the cleaning process. Fixed metering seldom provides a mechanism to add more than the metered dose other than by manual means – and now you’re right back where you started.
I will concede that metered dosing makes a LOT of sense in rinsing situations where a small percentage of rust preventive, for example, needs to be added to one time use water that continuously exits the system to drain. In this case, there is probably no better choice.
Metered chemical addition based on feedback from some parameter that can be automatically monitored makes a lot of sense and intuitively is a better choice for chemical maintenance. This would be a hands down winner if it weren’t for the fact that finding the parameter to monitor and finding a way to monitor it weren’t so problematic. For more on this, check out this blog. Parameters including pH, refractive index (density), turbidity and others are potential “measuring sticks” but in most cases are not absolute. Chemistry suppliers usually provide a means or procedure to determine chemical concentration. There are automatic, on-line monitoring devices available to monitor nearly any property including such things as density and refractive index. The inherent problem here is that most tests are designed to measure the concentration of chemistry in water, not taking into account the effects of contaminants and other variables. Another shortfall is that this type of chemistry maintenance system seldom has the ability to automatically handle the reverse case where chemical concentration becomes too high. Finally, automatic chemical makeup can work to your advantage but the methodology must be given due consideration covering a wide range of scenarios. Remember, Murphy is alive and well!
– FJF –