Measuring with a pipette

[Hannabrew]

So this may be a real dumb question but I can't seem to find any info online about how to measure with a type 3 graduated pipette.

Mine is the TC type so I understand there should be some liquid left after dispensing but my question is how to measure the liquid.

I would've expected the mL markings to start at 1 at the bottom and go up to 5mL towards the top but mine is the opposite, it starts at 5mL at the bottom and goes to 0mL.

Am I just supposed to do the math or is there a reason that it is backwards?

 

[flars]

The scale seems backwards because there should be liquid left after dispensing. The measurement is a subtraction. If you need two ml and the pipette is filled to 4.5 you would dispense to the 2.5 level. This is more accurate than trying to estimate how much volume will be left in the pipette after dispensing different specific gravity liquids.

 

[IslandLizard]

The scale seems backwards because there should be liquid left after dispensing. The measurement is a subtraction. If you need two ml and the pipette is filled to 4.5 you would dispense to the 2.5 level. This is more accurate than trying to estimate how much volume will be left in the pipette after dispensing different specific gravity liquids.

^ That!

 

[Hannabrew]

Interesting. That just seems like an inefficient way to do it as i could easily dispense to much or too little if I'm not careful and watching it closely while I'm dispensing right? If it was the way a normal cheap medicine-type pipette was, i could get my line exactly where i want it and then be assured that when i'm dispensing it it will only put in exactly what I want.

 

[flars]

Interesting. That just seems like an inefficient way to do it as i could easily dispense to much or too little if I'm not careful and watching it closely while I'm dispensing right? If it was the way a normal cheap medicine-type pipette was, i could get my line exactly where i want it and then be assured that when i'm dispensing it it will only put in exactly what I want.

I use a scale to dispense my phosphoric acid. The acid needed to reduce the pH of sparge water is a fair volume. I use 1 gram = 1 ml since the weight of 10% phosphoric acid solution at ≈ 66°F is 1.056 grams per ml.
Scale would not be accurate for smaller volumes of greater concentration.

 

[mabrungard]

You should only dose acids in a pipette by using a rubber bulb to suck up the acid and dose it into your liquid. The graduations on the pipette make sense when you fill the pipette and dose your prescribed amount out.

By the way, dosing with a scale is more accurate, but since acid SG's differ with the type of acid and its concentration or molarity, that is an extra thing to know. Maybe a future Bru'n Water update will include the capability to report the mass of an acid dose instead of its volume.

 

[Hannabrew]

So basically fill it to the top graduation mark (which is marked 0), and then dispense until it's at your desired mL marker?

I just worry about having to stop dispensing at exactly the right time to get the precise amount but maybe when I do it it will make more sense

 

[ajdelange]

All pipetting should be done with a pipetter. The cheapest and simplest form is a rubber bulb with two valves: one between the bulb and the pipet and the other on a stem off to the side between the pipet and the first mentioned valve. Some have a third valve at the top of the bulb. To use the pipet insert it into the bulb stem. Squeeze the valve at the top or bottom of the bulb and then squeeze the bulb expelling air and then release the valve. Now insert the tip of the pipet into the liquid to be pipeted and squeeze the valve between the pipet and the bulb. The vacuum in the bulb will suck liquid up into the pipet. As the liquid level approaches the reference mark release the pressure on the valve thus slowing the rate at which the liquid rises in the pipet. Let it fill a wee bit above the mark. Then raise the tip out of the liquid and gently and carefully squeeze the side arm valve. This lets air back in and the liquid will start to run out slowly. Release the valve when the meniscus is at the reference mark. Now move the pipet to the vessel to which you wish to transfer the liquid. Squeeze the side arm valve until the meniscus has dropped the required number of divisions.

Blow out type pipets are much easier to use. You fill them, as above, to the mark corresponding to the number of mL you wish to dispense, move to the target vessel, press the side valve until all the liquid that is going to run out has run out and then squeeze the rubber bulb while simultaneously squeezing the valve between the bulb and pipet. The blast of air blows out the last bit of the liquid.