Difference between revisions of "Hydrostatic Balance"

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{{edit|--[[User:Doos|Doos]] 12:34, 6 October 2006 (PDT)}}
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The hydrostatic balance is used to determine the [[Specific Gravity|specific gravity]] of a gemstone. Although it is a fairly simple apparatus to operate, it can be time-consuming. One must put the results into a calculation, something not all gemologists enjoy doing.<br />
 +
A specific gravity reading is considered a "primary test" in gemology yet, due to the time this method can consume, it is often regarded as a method of last resort in a standard gemological setup.
  
The hydrostatic balance is used to determine the [[specific gravity]] of a gemstone. Although it is a fairly simple apparatus to operate it can be time consuming and one would need to put the results in a calculation, something not all gemmologists enjoy doing.
+
==Basic==
  
==Basic==
+
:''Main article: [[Specific Gravity]]''
  
Specific gravity (SG) is a constant ratio of an object's weight compared to an equal volume of water. When an object in totally immersed in water it will experience an upward trust that is equal to the volume of water which is displaced by the object. This will make the object appear lighter in water.<br />
+
Specific gravity (SG) is a constant ratio of an object's weight compared to an equal volume of water. When an object is totally immersed in water, it will experience an upward thrust that is equal to the volume of water which is displaced by the object. This will make the object appear lighter in water.<br />
The first to observe this was Archimedes while investigating a possible fraud with a crown for King Hiero of Syracuse (present day Sicili).
+
The first to observe this was Archimedes while investigating a possible fraud concerning a crown made for King Hiero of Syracuse (present day Sicily).
  
===Archimedes and Archimedes' Law===
+
===Archimedes and Archimedes' Principle===
  
When King Hiero commisioned a new gold crown a certain volume of gold was provided to a goldsmith, who manufactured the crown to the liking of the king. Soon after rumours started that the goldsmith may have been frauduleus and added silver to the gold, making a nice profit for himself on the side.<br />
+
When King Hiero commissioned a new gold crown, a certain volume of gold was provided to a goldsmith who manufactured the crown to the liking of the king. Soon after, rumors circulated  that the goldsmith may have been fraudulent, that he may have added silver to the gold, making a nice profit for himself on the side.<br />
King Hiero asked Archimedes to investigate and one day while taking a bath, Archimedes noticed that his body mass caused the water in the bath to overflow. In his excitement over finding a possible answer to the problem he ran home naked, shouting "eureka" (I've found it out).
+
King Hiero asked Archimedes to investigate. One day while at the public baths, Archimedes noticed how his body mass caused the water in the bath to overflow. In his excitement over finding a possible answer to the problem, he ran home naked, shouting "Eureka!" (“I've found it!”).
  
He immersed equal weights of gold and silver in water and by observing the amount of water that was displaced, he noticed that the mass of gold displaced less water than the silver. He then immersed the crown and through a series of calculations he was able to determine how much gold and silver was used to create the crown. The fraud was detected and this was the first time in history SG was used.
+
Archimedes then immersed equal weights of gold and silver in water and, by observing the amount of water that was displaced, he realized that the mass of gold displaced less water than the silver. He then immersed the crown. Through a series of calculations he was able to determine how much gold and silver was used to create the crown. The fraud was detected. That was the first time in history the concept of SG was used.
  
Archimedes' Law (or the Law of Buoyancy) states that: the upward force on an immersed object is equal to the weight of the displaced fluid.
+
Archimedes' Principle (or the Law of Buoyancy) states that: the upward force on an immersed object is equal to the weight of the displaced fluid.
  
 
===Types of hydrostatic balances===
 
===Types of hydrostatic balances===
  
In gemmology 4 different types of hydrostatic balances are used.
+
In gemology, 4 different types of hydrostatic balances are used.
  
 
* The two-pan scales
 
* The two-pan scales
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====Two-pan scales====
 
====Two-pan scales====
  
The two-pan scales are the most time consuming scales to use. Some jewelry stores still use them but little of them are used for determining SG of stones. Mostly because the counterweights in general are not small enough to be of any help and they really need to be calibrated every few years.
+
The two-pan scales are the most time-consuming scales to use though some jewelry stores still have them.  But they are rarely used for determining SG of stones, mostly because the counterweights are generally too big to be of any help. Besides, they really need to be calibrated every few years.
 +
 
 +
Even so, the principle of the two-pan scale is very useful for explaining the hydrostatic balance.
 +
 
 +
[[image:hydrostatic2.png|250px|left]]
 +
 
 +
 
 +
A gemstone is placed on the left pan of the balance. Weights are placed on the right pan until the balance "zeros out". When the balance is equal, the total of the weights on the right pan is read and that represents the weight of the stone in air.
 +
<br clear="all" />
 +
[[image:hydrostatic.png|250px|left]]
 +
 
 +
 
 +
Then a freestanding stage with a beaker filled with water is introduced. After that, a thin coil wire basket is hung on the left side of the balance with the basket totally immersed in the water. At this point, the balance should be in complete balance by adding a small weight that is equal to the weight of the basket.<br />
 +
The stone is placed in the thin coil wire basket and then immersed in the water. Again, counterweights are placed on the right pan as before and the result of added weights will be the weight of the gemstone in water.<br />
 +
The wire basket should be of a material that is not water absorbent to avoid false readings. One should also take care that air bubbles around the basket and stone do not influence the readings, since air bubbles tend to rise upwards and can add to the upward thrust.<br />
 +
To avoid surface tension, a small droplet of detergent is added to the water.
 +
<br clear="all" />
 +
 
 +
After taking both the weight in air and the weight in water, the following calculation needs to be carried out.
 +
 
 +
 
 +
:<math> SG = \frac{weight\ of\ stone\ in\ air}{weight\ of\ stone\ in\ air\ -\ weight\ of\ stone\ in\ water}</math>
 +
 
 +
====Single-pan scales====
 +
 
 +
[[image:singlepan.png|250px|right]]
 +
 
 +
 
 +
Most gemological scales are of this type. They can be either analogue or digital and they come in various sizes and in every price range. For our purposes, it is best to get a carat scale as opposed to a gram scale. A gram scale may serve very well if it has a high precision that can read 0.01 grams, which is equivalent to 0.05 carat. These digital scales are very economical, under USD 100.
 +
 
 +
Various hydrostatic setups can be used with digital scales. One could use a bridge on which the glass beaker is placed, with a coil wire that is placed directly on the scale (see video below). Or the beaker can be placed directly on the scale with the stone hanging from a wire that is attached to a stage (as illustrated on the right), but note that in this last setup, SG= weigh of the stone divided by the weight that is read when the stone is hanged in water, and also the wire has to be as thin as possible.
 +
.
 +
<br clear="all" />
 +
 
 +
<!--=====Video presentation=====
 +
 
 +
In the following video (2:28 minutes), the use of a single-pan electronic scale is given. The hydrostatic addition uses a beaker in a bridge and the wire directly on the scale.
 +
 
 +
{| {{table}}
 +
|-
 +
|{{googlevideo|4628022311595565279}}
 +
|}
 +
 
 +
 
 +
{| {{table}}
 +
|-
 +
|[[image:video.png]] [http://video.google.com/videoplay?docid=4628022311595565279 Specific Gravity Video] (Hosted by Google Video)
 +
|-
 +
|Video showing the method of determining hydrostatic specific gravity - WMV/video format - 7.96MB
 +
|} -->
 +
 
 +
====Spring balances====
 +
 
 +
[[image:springbalance.png|100px|left]]
 +
 
 +
 
 +
This type of hydrostatic weighing can come in handy when handling large pieces of rough that are heavier than your usual balance can handle.<br />
 +
Spring balances are relatively inexpensive and can vary widely in accuracy. Before weighing the stone, one must of course weigh the part of the balance that is immersed into the water so that can be deducted. As usual, the stone is first weighed in air and then it is weighed in water.<br />
 +
The larger the rough and the higher the accuracy of the balance, the more precise your measurements will be.
 +
 
 +
A spring balance is usually used in the field just like a recreational fisherman would use it to weigh a fish.
 +
<br clear="all" />
 +
 
 +
====Direct reading scales====
 +
 
 +
These are also known as "Hanneman direct reading specific gravity balances" in honor of the one who reinvented (or at least popularized) this old wheel – Dr. W. Wm. Hanneman. It cleverly makes use of both Archimedes' Principle and Archimedes' Lever. It is an adoption of "The little balance" by Galileo Galilei.<br />
 +
Their operation is like the weighing scales you see at a physician's or weight-watchers' office, where a weight is shuffled from one end to the other.
 +
 
 +
A direct reading scale can be constructed with little effort from household materials and has an accuracy of about 0.002 carat. Instead of having to go through the pain of calculations, this scale will give you an immediate read on the SG of the gemstone.<br />
 +
The full description on its construction is given in Dr. Hanneman's "Guide to Affordable Gemology".
 +
 
 +
==Legal requirements==
 +
 
 +
Some countries impose strict regulations on the use of weighing scales when used in a commercial environment. This means that the scale needs to be labeled with a document (sticker) proving the scale is calibrated to legal requirements.<br />
 +
These scales are in general much more expensive.
 +
 
 +
In the European Union this is regulated under the [http://europa.eu.int/smartapi/cgi/sga_doc?smartapi!celexapi!prod!CELEXnumdoc&lg=en&numdoc=31990L0384&model=guichett Non Automatic Weighing Instruments Directive] (NAWI).<br />
 +
For Canada, this is regulated through the [http://strategis.ic.gc.ca/epic/internet/inmc-mc.nsf/en/h_lm03862e.html Canadian Measurement Act].
 +
 
 +
==Related topics==
 +
 
 +
* [[Specific Gravity]]
 +
* [[Heavy Liquids]]
  
The principle of the two-pan scale is very useful for explaining the hydrostatic balance though.
+
==Sources==
  
[[image:hydrostatic2.png|250px]] <br clear="all" />
+
* [http://www.amazon.com/gp/product/B000WDQF18?ie=UTF8&tag=gemsandwhywelove&link_code=as3&camp=211189&creative=373489&creativeASIN=B000WDQF18 Gemmology 3rd edition (2005) - Peter Read ]
[[image:hydrostatic.png|250px]] <br clear="all" />
+
* ''Guide to Affordable Gemology'' (2001) - Dr. W. Wm. Hanneman
  
 
==External links==
 
==External links==
 
* [http://penelope.uchicago.edu/Thayer/E/Roman/Texts/Vitruvius/9*.html Archimedes' discovery]
 
* [http://penelope.uchicago.edu/Thayer/E/Roman/Texts/Vitruvius/9*.html Archimedes' discovery]
 +
* [http://math.nyu.edu/~crorres/Archimedes/Crown/CrownIntro.html Archimedes' discovery investigated]
 +
* [http://math.nyu.edu/~crorres/Archimedes/Crown/bilancetta.html Galileo Galilei's Little Balance described]
 +
<br />
 +
<br />
 +
'''Next:[[Heavy_Liquids| Heavy Liquids]]'''
 +
<br /><br />
 +
'''[[Table_Of_Contents| Return to the Table of Contents]]'''

Latest revision as of 13:55, 13 December 2016

The hydrostatic balance is used to determine the specific gravity of a gemstone. Although it is a fairly simple apparatus to operate, it can be time-consuming. One must put the results into a calculation, something not all gemologists enjoy doing.
A specific gravity reading is considered a "primary test" in gemology yet, due to the time this method can consume, it is often regarded as a method of last resort in a standard gemological setup.

Basic

Main article: Specific Gravity

Specific gravity (SG) is a constant ratio of an object's weight compared to an equal volume of water. When an object is totally immersed in water, it will experience an upward thrust that is equal to the volume of water which is displaced by the object. This will make the object appear lighter in water.
The first to observe this was Archimedes while investigating a possible fraud concerning a crown made for King Hiero of Syracuse (present day Sicily).

Archimedes and Archimedes' Principle

When King Hiero commissioned a new gold crown, a certain volume of gold was provided to a goldsmith who manufactured the crown to the liking of the king. Soon after, rumors circulated that the goldsmith may have been fraudulent, that he may have added silver to the gold, making a nice profit for himself on the side.
King Hiero asked Archimedes to investigate. One day while at the public baths, Archimedes noticed how his body mass caused the water in the bath to overflow. In his excitement over finding a possible answer to the problem, he ran home naked, shouting "Eureka!" (“I've found it!”).

Archimedes then immersed equal weights of gold and silver in water and, by observing the amount of water that was displaced, he realized that the mass of gold displaced less water than the silver. He then immersed the crown. Through a series of calculations he was able to determine how much gold and silver was used to create the crown. The fraud was detected. That was the first time in history the concept of SG was used.

Archimedes' Principle (or the Law of Buoyancy) states that: the upward force on an immersed object is equal to the weight of the displaced fluid.

Types of hydrostatic balances

In gemology, 4 different types of hydrostatic balances are used.

  • The two-pan scales
  • The one-pan scales
  • Spring balances
  • Direct reading scales

Two-pan scales

The two-pan scales are the most time-consuming scales to use though some jewelry stores still have them. But they are rarely used for determining SG of stones, mostly because the counterweights are generally too big to be of any help. Besides, they really need to be calibrated every few years.

Even so, the principle of the two-pan scale is very useful for explaining the hydrostatic balance.

Hydrostatic2.png


A gemstone is placed on the left pan of the balance. Weights are placed on the right pan until the balance "zeros out". When the balance is equal, the total of the weights on the right pan is read and that represents the weight of the stone in air.

Hydrostatic.png


Then a freestanding stage with a beaker filled with water is introduced. After that, a thin coil wire basket is hung on the left side of the balance with the basket totally immersed in the water. At this point, the balance should be in complete balance by adding a small weight that is equal to the weight of the basket.
The stone is placed in the thin coil wire basket and then immersed in the water. Again, counterweights are placed on the right pan as before and the result of added weights will be the weight of the gemstone in water.
The wire basket should be of a material that is not water absorbent to avoid false readings. One should also take care that air bubbles around the basket and stone do not influence the readings, since air bubbles tend to rise upwards and can add to the upward thrust.
To avoid surface tension, a small droplet of detergent is added to the water.

After taking both the weight in air and the weight in water, the following calculation needs to be carried out.


<math> SG = \frac{weight\ of\ stone\ in\ air}{weight\ of\ stone\ in\ air\ -\ weight\ of\ stone\ in\ water}</math>

Single-pan scales

Singlepan.png


Most gemological scales are of this type. They can be either analogue or digital and they come in various sizes and in every price range. For our purposes, it is best to get a carat scale as opposed to a gram scale. A gram scale may serve very well if it has a high precision that can read 0.01 grams, which is equivalent to 0.05 carat. These digital scales are very economical, under USD 100.

Various hydrostatic setups can be used with digital scales. One could use a bridge on which the glass beaker is placed, with a coil wire that is placed directly on the scale (see video below). Or the beaker can be placed directly on the scale with the stone hanging from a wire that is attached to a stage (as illustrated on the right), but note that in this last setup, SG= weigh of the stone divided by the weight that is read when the stone is hanged in water, and also the wire has to be as thin as possible. .


Spring balances

Springbalance.png


This type of hydrostatic weighing can come in handy when handling large pieces of rough that are heavier than your usual balance can handle.
Spring balances are relatively inexpensive and can vary widely in accuracy. Before weighing the stone, one must of course weigh the part of the balance that is immersed into the water so that can be deducted. As usual, the stone is first weighed in air and then it is weighed in water.
The larger the rough and the higher the accuracy of the balance, the more precise your measurements will be.

A spring balance is usually used in the field just like a recreational fisherman would use it to weigh a fish.

Direct reading scales

These are also known as "Hanneman direct reading specific gravity balances" in honor of the one who reinvented (or at least popularized) this old wheel – Dr. W. Wm. Hanneman. It cleverly makes use of both Archimedes' Principle and Archimedes' Lever. It is an adoption of "The little balance" by Galileo Galilei.
Their operation is like the weighing scales you see at a physician's or weight-watchers' office, where a weight is shuffled from one end to the other.

A direct reading scale can be constructed with little effort from household materials and has an accuracy of about 0.002 carat. Instead of having to go through the pain of calculations, this scale will give you an immediate read on the SG of the gemstone.
The full description on its construction is given in Dr. Hanneman's "Guide to Affordable Gemology".

Legal requirements

Some countries impose strict regulations on the use of weighing scales when used in a commercial environment. This means that the scale needs to be labeled with a document (sticker) proving the scale is calibrated to legal requirements.
These scales are in general much more expensive.

In the European Union this is regulated under the Non Automatic Weighing Instruments Directive (NAWI).
For Canada, this is regulated through the Canadian Measurement Act.

Related topics

Sources

External links



Next: Heavy Liquids

Return to the Table of Contents