Why is only mercury used to measure blood pressure,temperature?

Question

answar

Answer 1

It's not. Many other materials and instruments are being used. Use of mercury is discouraged because of environmental concerns.

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Answer 2

The other respondants are correct in that mercury is not used to much any more for safely reasons, so but as to why it's the best liquid in traditional gauges, mercury is the highest density room temperature liquid, does not wet (stick) to glass, and (for thermometry) has a decent thermal expansion coefficient. High density is important for pressure measurement because the tube does not have to be very long, and for thermometry, so you don't have to shake it back down the (thin) tube to get it back in the bulb after it's cool. Being a metal, it is also highly reflective, making it easy to see.

Answer 3

Not only mercury used to measure blood pressure and temperature.

there is Automatic Non-Invasive Blood Pressure (NIBP) Monitor to measure blood pressure and it's completely mercury free.

and there is various kinds of thermometers which are mercury free like Alcohol thermometer ,Electrical resistance thermometer ,Liquid Crystal Thermometer ,Infrared thermometer and constant-volume gas thermometer.

Answer 4

they quit using mercury because they found out that it was poisonous.  

Answer 5

Most liquid pressure meters use alcohol that is dyed red for visibility.

However, mercury is still the gold standard for use in pressure meters.  The reason is a little esoteric.  Pressure meters can be mechanical or fluid, but fluid meters are considered better because they are simpler and incapable of giving you wrong numbers due to equipment failure--anyone who flies planes knows that your altimeter, which is a mechanical pressure meter, has to be recalibrated regularly to make sure it is accurate.  A fluid meter, on the other hand, never needs recalibration after it is initially set, which is why they are great for blood pressure machines in the doctor's office.

Fluid meters do have one big problem, though.  All fluid meters work by having fluid in a glass tube, with the top having an air space and closed at the very top, the other open directly or indirectly to air pressure.  More pressure shrinks the air space at the top, raising the fluid level. Less pressure lets the air space at top expand, lowering the fluid level.  The problem is the air space itself--air expands or contracts with temperature, so on a cold day the meter could read different from a warm day.  Not so big a deal in an air-conditioned office, but in the real world, it could be a major problem.

There is one way around this.  If you took all the air out at the top, and made the glass tube long enough, the weight of the fluid would actually pull it down from the top, and you would have a fluid level with nothing but an (almost) empty vacuum above it.  This type of pressure meter is almost infallible and solves the problem of temperature variance (the fluid itself will expand an contract a tiny bit, but this is minor compared to having air at the top).  Unfortunately, if you use water as a fluid, the glass tube would have to be almost 36 feet long!  Any shorter, and the water would not weigh enough to pull down from the top of the tube, making it useless.

Most people who want to measure pressure, of course,do not have room for a 36 foot glass tube, and if you wanted to measure pressures _higher_ than one atmosphere, it would need to be even larger.  This is where mercury comes in--it is the heaviest fluid known, and it weighs so much it only takes 30 inches to make a vacuum at one atmosphere--this is the 29.98 inches that is referred to in weather reports as standard atmospheric pressure.

Among other pressure devices, using mercury can help make a fluid instrument much more small and compact, as well as the increase in accuracy and reliability, but obviously this needs to be traded off with safety concerns.  In general, though, it is still the standard for pressure instruments.