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How to calculate PIR

First off let me state that all of this is way easier in metric, because in metric 1 L=1 kg. I would advice you to do all of this in metric.

What do you need?

  • Pressure Washer and Foam Cannon
  • Empty Bucket
  • Measuring beaker
  • Scale

First off let me note that you should mark the settings on your pressure washer you used when you calculated this as any deviation from them will mean you have to remeasure.

Start by filling your pressure washer bottle with exactly 1L of water and set your gun to your preferred setting. Now you weigh the Empty weight of your dry bucket(mdry).
Next you run your pressure washer with the Foam Lance on it into the bucket until the bottle is used up completely.
Now you weigh your Bucket (mfull) (If you don’t have a scale you can also use the beaker and measure how much water you have taken out or use a bucket with an internal scale) For the next step calculate the weight of the Bucket by substracting the weight of the empty bucket from the weight of the full bucket (mfull-mdry=mwater). Now as PIR is mostly given in percent you want to divide that number by 100 to get 1% (mwater/100=1%PIR)

If a product calls for x% of PIR you multiply x by your one percent number, then fill your bottle with that amount and fill it up to full (1L) get to the exact value.

Lets do an exemplary Calculation (with totally made up numbers, these are in no way meant to match your numbers, don’t worry if they deviate!)

Your empty bucket weighs mdry = 1500 g and the full bucket comes in at mfull = 14600 g.
Now first we want the weight of the water so we substract (mfull-mdry = mwater) 14600 g-1500 g = 13100 g Great! now we have our amount of water, which is 13100 g or 13.1 kg or 13.1 L of Water. Now we want to know what 1% is so we divide by 100 (mwater/100 = 1% PIR) 13100mL/100= 131 mL Great, lets say we want to make a 4% Solution, so you take that value by 4: 131*4 = 524 mL Which means that you put 524 mL of cleaner in your Foam Cannon, then you add up to the full Liter.

Either you fill up to the full Liter mark on your bottle or you add 1000-524 = 476 mL of Water.
Please remember that these numbers are completely exemplary.

Could you theoretically make less then a Liter of prediluted Product in your Foam gun ?

Yes you absolutely can, but you need to keep the ratio of water to concentrate.

But what if you have a product that states a PIR but you want to use it from a foam or pump sprayer?

In that case what you see is what you get, so fill in the correctly to percentage diluted chemical in the Sprayer

Updated on 11 July 2023

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