Formula for converting mmHg to kPa as the unit for measurement of pressure when using newer tourniquet machines that measure pressure using the SI (Systeme International) Author links open overlay panelManoharArumugamM.B.B.S M.S. Ortho(Associate Professor and Consultant Orthopaedic Surgeon)PersonEnvelope Show ShareShare Cited ByCite https://doi.org/10.1016/j.joon.2009.04.002Get rights and content SummaryTourniquets use is common in hand surgery. The correct tourniquet pressure is important to achieve a bloodless operating field and to prevent tourniquet syndrome and tourniquet paralysis syndrome. Newer tourniquet devices use kPa as the unit to measure pressure. This article highlights the existence of such machines and also describes the formula to convert mmHg to kPa which can be done quite easily to ensure that the correct pressure (in kPa) is used. There are three different units of pressure used in chemistry. This is an unfortunate situation, but we cannot change it. You must be able to use all three. Here they are:
I. Converting between atmospheres and millimeters of mercury. One atm. equals 760.0 mm Hg, so there will be a multiplication or division based on the direction of the change. Example #1: Convert 0.875 atm to mmHg. Solution: multiply the atm value by 760.0 mmHg / atm. 760.0 mmHg0.875 atm x ––––––––––1 atm Notice that the atm values (one in the numerator and one in the denominator) cancel, leaving mmHg. Example #2: Convert 745.0 mmHg to atm. Solution: divide the mmHg value by 760.0 mmHg / atm 745.0 mmHg––––––––––––––– = 0.980 atm760.0 mmHg / atm Notice that the mmHg values cancel and the atm, in the denominator of the denominator, moves to the numerator. Note also that the answer of 0.980 atm has been rounded off to three significant figures. II. Converting between atmospheres and kilopascals. One atm equals 101.325 kPa, so there will be a multiplication or division based on the direction of the change. Example #3: Convert 0.955 atm to kPa. Solution: multiply the atm value by 101.325 kPa / atm. 101.325 kPa0.955 atm x –––––––––– = 96.8 atm (to three sig figs)1 atm Notice that the atm values( one in the numerator and one in the denominator) cancel, leaving kPa. Sometimes the 1 in front of atm is eliminated. In that case, think of the 1 as being understood to be there. Example #4: Convert 98.35 kPa to atm. Solution: divide the kPa value by 101.325 kPa / atm. 98.35 kPa––––––––––––––– = 0.9706 atm101.325 kPa / atm Notice that the kPa values cancel and the atm, in the denominator of the denominator, moves to the numerator. The answer is rounded to four significant figures. III. Converting between millimeters of mercury and kilopascals. 760.0 mmHg equals 101.325 kPa, so both values will be involved. This situation is slighly unusual because most conversions involve a one, usually in the denominator. The conversion examples above are examples of a one being involved. For example, 760.0 mmHg / 1 atm in examples 1 and 2. The 1 was assumed to be present. By the way, the 1 (as in 1 atm) has no influence on significant figures. In this conversion, both 760.0 and 101.325 will be involved and the location of each (numerator or denominator) will depend on the conversion. Example #5: Convert 740.0 mmHg to kPa. 101.325 kPa740.0 mmHg x –––––––––––– = 98.66 atm (to four sig figs)760.0 mmHg Notice that the mmHg will cancel, since one is in the numerator and one is in the denominator, leaving kPa as the unit on the answer. Example #6: Convert 99.25 kPa to mmHg. 760.0 mmHg99.25 kPa x –––––––––––– = 744.4 mmHg (to four sig figs)101.325 kPa Notice that the kPa will cancel, since one is in the numerator and one is in the denominator, leaving mmHg as the unit on the answer. Example #7: Convert 1.69 atm to torr 760.0 torr1.69 atm x –––––––– = 1280 torr (to three sig figs)1 atm Example #8: Convert 0.3587 atm to Pa 101,325 Pa<--- note the comma, not decimal point0.3587 atm x ––––––––– = 36340 Pa (to four sig figs)1 atm Example #9: Convert 820.1 torr to kPa 101.325 kPa820.1 torr x –––––––––– = 109.3 kPa (to four sig figs)760.0 torr Example #10: Convert 689 mmHg to Pa 101,325 Pa689 mmHg x –––––––––– = 91800 Pa (to three sig figs)760.0 mmHg Example #11: Convert 725.0 torr to mmHg 760.0 mmHg725.0 torr x ––––––––––– = 725.0 mmHg760.0 torr Example #12: A student reads a barometer in the laboratory and finds the prevailing atmospheric pressure to be 720.0 mmHg. Express this pressure in atmospheres, kilopascals, torrs, pounds per square inch, and pascals. Hint: 1 atm = 101.325 kPa = 760.0 torr = 760 mmHg = 14.696 psi = 101,325 Pa Solution: (720.0 mmHg) (1 atm / 760 mmHg) = 0.9474 atm |