One of the most common questions posed to a newbie is how can you tell if a graduated tube has a blowout tip or not? This question can get very complicated if you don't know how to read the measurements of a pipette. The answer lies in the double ring at the upper end, which indicates that the pipette has a "blowout" tip.
The answer is simple, but you need to know the differences between the two types of graduated pipettes. Typically, a TD or TC pipette has a double ring at the top, while a TC pipette does not have double rings. The difference is largely in the design of the graduation marks. Normally, a TD pipette does not require blowing out the tip, but it will not work if the tip is too far up.
Besides the double ring on the upper end, another important thing to look for is the graduation marks. The Mohr pipettes are graduated to a point and have a band at the top. These bands are designed to expand the pipette's capacity and deliver all of its contents. If you want to get the entire contents of a TC pipette, you must blow it out. However, there are some methods to achieve this.
The first way to tell if a graduated pipette tip is a blowout tip is to look at its graduation marks. The two most common types of blowout tip are TC and TD. Both have double rings at the upper end, and the first graduation mark on a TD is well past the tip.
The first difference between a volumetric pipette and a graduated pipette is the position of the first graduation mark. The Mohr pipettes have straight tubes and the first graduation mark is placed well past the tip. The Serological ones have a wide base and a flat bottom. The difference between a TD pipette and a TC pipette is the positioning of the top graduation mark. The former has two graduation marks, one at the top of the tube, while the latter has a flat base and an angled bottom. The second difference is the diameter of the pipette.
Unlike the Mohr pipette, the Serological pipette has a blowout tip. Its first graduation mark is placed above the zero point. Then, the liquid should be released to the desired partial volume. The second graduation mark on a type 1 or 3 graduated pipette can be too narrow. A capillary tip has a very narrow internal diameter and is usually used to measure small volumes.
Unlike the serological pipette, a TD pipette has two scales, one pointing up and one pointing down. If you don't see a TD scale on your TD pipette, you'll need to look elsewhere. A TC pipette is the same as a TD-type one.
While a serological pipette is designed to deliver the last milliliter of the liquid, a graduated pipette will have one remaining drop. For this reason, a TD pipette will have a scale of zero and one that has graduated values. When the TD tip is in a position to contain the liquid, a TC pipette will need to be blown out.
A graduated pipette can be a blowout tip. The former has a scale that shows how much of a sample it can hold. This means that the tip must be blown out to deliver the whole sample. If you don't use a blotting pipette, you should have a blotting tip instead.