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Myth Busting: How important is TDS 000 for pure water window cleaning?


What have we been primed to believe and never challenge? Life is so busy and there are so many more important things to do than challenge everything that is presented.

This is the plan I will present a topic each column that is potentially a window cleaning myth. I will then present some ideas and arguments that support the topic (as I try to BUST it).

In many cases, I am no better informed than you … but I will take the time to think how YOU can be the true MYTHBUSTER. I will propose the sides of the story that I consider are relevant. WCM will run a blog for those that want to add another aspect, share their experiment, or declare their persuasion.

MYTH: You Need TDS 000 for a Spot-Free Results with Water Fed Window Cleaning.

Most people in the Water Fed Industry will demand that we all believe that TDS 000 is necessary for spot-free window cleaning and, if not, then it must at least be less than TDS010.

To bust this myth, we need to understand what is TDS and what is a spot.

TDS is a measurement of the total dissolved ‘solids’ in water. It is measured with a TDS meter that actually runs a small charge across two terminals. Because pure water does not conduct electricity, we know the measurement for Pure Water is TDS 000. The more solids in the water, the more the water becomes conductive, and therefore, the higher the reading on the meter.

A ‘spot’ is the total sum of solids that were once dissolved in water, but are left behind on the glass surface by the evaporation of that water. Because water usually beads before it dries, the shape of the ‘spot’ is usually a circle, and the density of the spot increases towards the middle as the concentration increases as the evaporation continues.

By the way, a ‘streak’ is a vertical rivulet of dirty water, that usually runs down on dry glass, causing it to dry more quickly, leaving a long line of solids rather than a set of circles.

Many customers do not want their windows cleaned when it rains because they have seen how dirty their car looks after rain.

What actually happens when it rains on a car is explained by understanding that there was a thin and very even layer of dirt particles on the car. This thin layer of dirt particles was agitated by the impact of the rain, and re-arranged into circle shapes by the evaporating rain. Pure Water and Rain do not (normally) leave spots.

So … when is water spot-free ( as compared to ‘pure’ )?

Well … you are the Myth Buster and there is not one answer. It depends on what is in your local water that might leave a visible spot. If your water is loaded with bright white calcium (from travelling through lime aquafers), then spot-free in your area may well be the infamous TDS 010. However, if you do not have loads of calcium in your water, you may find that the Spot-Free condition occurs as high as TDS 080. In Sydney in 2010, we were achieving Spot-Free at TDS 060 and below.

Why would I do the experiment?

You can potentially save hundreds of dollars ( or pounds) a year from ‘over-purifying’ water. In so many cases, RO units are delivering Spot-Free water with no real need for DI units and DI systems are being used to purify past the ‘Spot-Free’ threshold with no benefit.

How to Build the Kit to Test Spot-Free TDS.

If you have an RO system, the first thing to do is disconnect your DI Tank and then test the RO OUT TDS. What ever it is, use this water to perform the Test Below. If this leaves a spot-free result, or a commercially acceptable result, you can consider this information.

If you have a tank / housing of DI Resin, all you need is one Garden Hose Y adaptor with a ball valve on each branch of the Y (we will call this ‘Y (1)’ ), and another Garden Hose Y adaptor ( no need for ball valves ) ( we will call this one ‘Y(2)’ ).

Arrange your hoses like this :

§Tap to Branch A of Ball Valve Y (1)

§Branch B of Ball Valve Y (1) to the DI Tank

§Branch C of the Ball Valve Y (1) to BYPASS around the DI Tank Inlet direct to Branch C of Y (2)

§DI Tank Outlet to Branch B of Y(2)

§Branch A of Y (2) to Water Fed Brush

You now have the ability to ‘dilute’ your TDS 000 pure water from your DI Tank by ‘re-mixing’ it with Tap Water before you use it to clean windows.

Now: Open and Close the Ball Valves in Y1 to change the TDS at the Water Fed Brush.

If you close the BYPASS and open the DI Tank, you have TDS000

If you close the DI Tank and open the BYPASS, you have the TDS of your local tap water.

Adjusting the balance will change the TDS OUT, and give you the chance to ‘set’ the TDS OUT for the experiment.

How to Test Spot-Free TDS in Your Area.

The first setting should be TDS 050.

Now … go clean your windows the way you normally do, ensuring a thorough rinse method ( chase the water down the glass to the base ) and wait till it dries. Examine for spots.

Change the TDS OUT up and down accordingly to determine these two points:

§The TDS OUT reading at which Spot-Free occurs

§The SPOT that you can see at a TDS reading of 2 x Spot-Free.

This will give you two important numbers:

a) The point at which you get EXACTLY the same result as TDS 000 for your customers (and therefore the cost of purifying to TDS 000 is wasting money)

b) A reading that may be commercially viable for commercial cleaning (and therefore the cost of purifying your water beyond this may be a waste of money).

If you are performing this experiment, and you want to give feedback, please do so in the WCM Blog … it will be very interesting indeed. Remember to try and understand WHAT is in your local water (especially Calcium).

If you find you need TDS 010 or below, then your experiment has proven that you are not wasting any money and everything is perfect…