The<unk><unk>  or<unk>  gauge system is commonly used to measure the size of a<unk> . It is most often abbreviated as Fr, but can often be seen abbreviated as Fg, FR or F. It may also be abbreviated as CH or Ch (for Charrière, its inventor). However, simply gauge, G or GA generally refers to Birmingham gauge. The<unk>  size is three times the diameter in millimeters. Thus, the<unk>  size is roughly equivalent to the circumference of a circular<unk> ; the true circumference being slightly larger (). A round<unk>  of 1<unk>  has an external diameter of mm, and therefore the diameter of a round<unk>  in millimetres can be determined by dividing the<unk>  size by 3: : 1 mm = 3 Fr : 1 Fr = 1/3 mm For example, if the<unk>  size is 9, the diameter is 9/3 = 3.0; mm. It is seen that the<unk>  unit is fully redundant with the metric system but introduces potential for rounding errors. This metrication problem is further complicated by the mixed use of metric and imperial units in medical professions using<unk> s. An increasing<unk>  size corresponds to a larger external diameter. This is contrary to Birmingham gauge, where an increasing gauge corresponds to a smaller diameter needle. The<unk>  size is a measure of the outer diameter of a<unk>  (not internal drainage channel, or inner diameter). So, for example, if a two-way<unk>  of 20 Fr is compared to a 20 Fr three-way<unk> , they both have the same external diameter but the two-way<unk>  will have a larger drainage channel than the three-way. Three-way<unk> s accommodate an extra channel for irrigation within a similar external diameter. The<unk>  gauge was devised by Joseph-Frédéric-Benoît Charrière, a 19th-century Parisian maker of surgical instruments, who defined the "diameter times 3" relationship.
French catheter scale
Trocar
Tile classification
Drain 
Monitor unit
Wound assessment
Pediatric assessment triangle
Infusion pump