Frequently Asked Questions

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Strain Gauges

The calibration of the strain gauge plethysmograph is in percent volume change. How does this affect measurements?
Why should I choose a strain gauge instead of another type of plethysmography?
Why do I need the plethysmograph? Can I plug the strain gauge directly into my chart recorder?
What is the electrical resistance of a strain gauge?
Is mercury dangerous?
Does the age of a strain gauge effect its accuracy?

The calibration of the strain gauge plethysmograph is in percent volume change. How does this affect measurements?
Blood flow is usually expressed in terms of cc’s of blood flow per 100 cc’s of tissue per minute. This is equivalent to percent per minute. The reason for this is to normalize results so individuals of different sizes can be compared. If a different type of plethysmograph is used which is calibrated in terms of absolute volume change, it would be necessary to calculate the size of the limb in order to normalize the results.  © 2008 D. E. Hokanson, Inc.
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Why should I choose a strain gauge instead of another type of plethysmography?
Strain gauge plethysmography is quantitative and easy to use. Other types of plethysmographs are difficult or impossible to calibrate, so they are typically used only for qualitative applications.  © 2008 D. E. Hokanson, Inc.
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Why do I need the plethysmograph? Can I plug the strain gauge directly into my chart recorder?
 Strain gauge plethysmography is quantitative and easy to use. Other types of plethysmographs are difficult or impossible to calibrate, so they are typically used only for qualitative applications. You cannot plug the strain gauge directly into the chart recorder and get anything the plethysmograph takes the changes in the strain gauge and creates the data sent to the chart recorder. © 2008 D. E. Hokanson, Inc.
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What is the electrical resistance of a strain gauge?
This depends on length of the gauge. Gauges for digits might be as low as 0.5 Ohm; limb gauges can be up to 5 ohms.  © 2008 D. E. Hokanson, Inc.
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Is mercury dangerous?

Mercury is not dangerous when handled properly. Breathing mercury vapor is extremely dangerous, however mercury is not very volatile at room temperature.  A 24 centimeter limb-style mercury strain gauge contains about 1.25 grams of mercury. Gauges very seldom break and old gauges may be returned to Hokanson for recycling. If a gauge should break, the mercury should be cleaned up following the directions that come with the gauges.  © 2008 D. E. Hokanson, Inc.

 

 
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Does the age of a strain gauge effect its accuracy?
No, gauges either work or not. They do not lose accuracy with age. As a gauge ages, the copper electrodes at the ends are dissolved into the mercury. This shows up as a darkening of the mercury which begins at the ends of the gauge and progresses toward to middle. This process causes the pressure in the gauge to go down and eventually the gauge will lose electrical continuity when it is stretched. Hokanson gauges are guaranteed for 1 year, and working gauges over five years old are not uncommon.  © 2008 D. E. Hokanson, Inc.
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