Monthly Archives: April 2014

Absolute or Gage Sensor – Which is Best to measure Absolute Pressure?

If you want to make an absolute pressure measurement, you have two choices of sensor type.  The first and most obvious choice is to specify an absolute pressure transducer.  Another choice is to use a gage pressure sensor and offset the output signal to exclude the local atmospheric pressure.

An absolute pressure transducer is referenced to absolute zero and such transducers are manufactured to incorporate a vacuum chamber into the sensor body. This adds to the cost of an absolute transducer and so a more economical approach is to use a gage pressure sensor that is open to the atmosphere and then offset the output signal by 14.7 psi.

A gage pressure transducer with a fixed offset for atmospheric pressure, however, has the disadvantage of being affected by the variations in atmospheric pressure due to the weather. It is possible to quantify this error and determine if a gage-referenced sensor with an offset will result in a satisfactory measurement.  For historical records we can see the range of barometric pressure variations due to the weather.  These vary depending on geographic location.

The highest sea-level pressure on Earth occurs in Siberia, where the Siberian High often attains a sea-level pressure above 30.01 in Hg, with record highs close to 32.04 in Hg .  The lowest measurable sea-level pressure is found at the centers of tropical cyclones and tornadoes, with a record low of 25.69 in Hg).

Honolulu, Hawaii is the place in the US with the overall smallest range of changes in barometric pressure, ranging from 29.34 to 30.32 in Hg. San Diego is the city with the smallest range of changes in the continental US, with an average range of about 29.37 to 30.53 in Hg.

As for the places with the greatest range of pressure changes, St. Paul, Alaska ranges from 27.35 to 30.86 in Hg. In the contiguous US, Charleston, South Carolina has the largest range of changes, with a 27.64 to 30.85 in Hg.

Choosing between an absolute transducer with a full vacuum reference and a gage-referenced transducer with an offset is simply a matter of determining how much error will result from typical ambient atmospheric pressure changes due to the weather.

Here is a table to help guide the selection process.

absolute pressure

 

Here are two examples:

You want to make a 200 psia pressure measurement and you are in the southeast US. The weather will likely be similar to Charleston, SC and so looking at the chart you would expect an error of 0.788% to be the result of ambient atmospheric changes if you were to use a gage pressure sensor offset for atmospheric pressure. This error is excessive so the extra expense of a true absolute transducer would be warranted.

You want to make a 2000 psia measurement in Los Angeles. San Diego has a similar weather pattern so from the chart you can see that the error incurred by using an offset gage pressure sensor would be just 0.029% FS. This is very small compared to the 0.25% accuracy of the transducer and so would be the most economical choice.

Look at Validyne’s offerings of absolute and gage sensors and transducers

Test and Measurement Grade Pressure Transducers (P895,P896,P897V)

General Purpose Pressure Transducers

AP10 Variable Reluctance Absolute Pressure Sensor

 

Understanding Digital Displays

Digital display specifications can be confusing. Here is a short application note that describes the terminology and capabilities of a digital display.. 

Digital displays are specified as being either 3-1/2 digits or 4-1/2 digits. In a 3-1/2 digit display the three rightmost digits may read any value between 1 and 9. The left-most digit is limited to 1 or a blank. It is often easier to think in terms of counts: a 3-1/2 digit display can read up to 1999 counts, so it is suitable for displaying readings that have a maximum value of 20, 200 or 2000. Note that the decimal point may be placed within the right most three digits: 19.99, 199.9 or 1999.

Similarly a 4-1/2 digit display may read 19.999, 199.99, 1999.9 or 19999.

To specify the correct number of digits in a display, keep the following in mind: The resolution of the display should be five to twenty times better than the accuracy.

For example, if a 20 psi transducer with 0.25% FS accuracy is used, a 3-1/2 digit display configured to read 19.99 psi will provide a resolution of 0.01 psi when the accuracy of the transducer is 0.05 psi, and this is a good balance between resolution of the display and accuracy of the sensor.

A 4-1/2 digit display specified for the same transducer would read 19.999 psig at full scale, and the last digit would be unusable since it represents 50 times more resolution than the transducer accuracy.

However, if the transducer accuracy is 0.1% FS, then the 4-1/2 digit display would be appropriate because the accuracy of the transducer is now 0.02 psi and the 4-1/2 digit display would have a resolution 20 times greater than the accuracy. A 3-1/2 digit display, by contrast, would only read to 0.01 psi – just twice the accuracy of the sensor.

The diagram below shows how the 3-1/2 and 4-1/2 digital displays are used.

digital display

 

Check out Validyne products with a Digital Display

PS309 Portable Digital Manometer

PS409 Digital Panel Mount Manometer

P532 Ultra-Low Pressure Transmitter

DR800 Draft Range Transmitter

CD23/223 Carrier Demodulator with LED Display

CD379 Portable Carrier Demodulator with Digital Display