Are you using a deadweight tester or piston gauge? Be sure to account for gravity

To establish pressures using a deadweight tester (DWT) or piston gauge, you need to account for the acceleration of gravity where the instrument is operated. Gravity varies around the world. With a range of approximately 0.5 %, the local gravity correction is typically the largest environmental influence acting on the deadweight tester.

The most accurate method to determine the acceleration of gravity at the site where the deadweight tester will be used is to perform a site survey. Various companies provide this service. Although this is the most accurate method, it can also be expensive. To determine the acceleration of local gravity value when a survey or site measurement is unavailable (or unaffordable), the resources listed below are available on the Internet.

1. Use one of the following options to determine the latitude, longitude, and elevation at the site the deadweight tester will be used:

Obtain latitude and longitude coordinates using a GPS receiver 
or
Use an Internet-based geo-coding program

• https://www.google.com/maps. (To learn how to find latitude/longitude, go to Google here.)
• https://mapper.acme.com/

2. Enter latitude and longitude coordinates into one of the following Internet-based programs:

• NOAA/NGS website (limited to the contiguous 48 United States) – https://www.ngs.noaa.gov/cgi-bin/grav_pdx.prl
• PTB (National Metrology Laboratory of Germany) website (global) – http://www.ptb.de/cartoweb3/SISproject.php

For the P3XXX Series Deadweight Testers, you have two choices. We can provide the deadweight tester matched to standard gravity (9.80665 m/s2) and you can use software (such as Fluke Calibration PressCal) to account for the acceleration of local gravity. Or, we can provide the deadweight tester matched to the local gravity value that you supply. Fluke Calibration offers this service at no additional charge. Just provide your local gravity on the purchase order.

For piston gauges, such as the 2400 Series and PG7000 Series, the local gravity must be entered into either the terminal (PG7000 series) or software that will be used for the piston gauge calculation (WinPrompt or COMPASS® for Pressure software).

If you do not have a site survey, the methods available on the Internet provide a possible resource for determining the gravity at your location. The uncertainties estimated using this method range from about 0.0002 % up to 0.002 %. Use these at your discretion to determine if the local gravity provided is suitable for the intended use.