Recently I had an enquiry from Frank, head of Downers Auckland laboratory needing clarification about the term “residual pressure” used in test D2041 (theoretical maximum specific gravity and density of bituminous mixtures).
As stated in the test, a vacuum is applied to the sample to reduce the residual pressure until it’s 30 mmHg or 4 kPa.
This is actually 30 mmHg absolute pressure, so a vacuum must be created so it is less than 30 mm of mercury absolute.
It’s fine to have a vacuum gauge connected to the pump, but a calibrated absolute pressure gauge is really the correct instrument that should be used to be able to confirm the residual pressure of the sample container.
As atmospheric pressure changes and vacuum gauges don’t account for this, an absolute gauge is the only way to tell exactly what pressure your sample is at. It’s all about your starting reference point. 30 mmHg (A) is referenced from 0 mmHg, not 760 mmHg which is standard atmospheric pressure. So a vacuum gauge will start from what ever your current atmospheric pressure is and go down to around 30, it may be 30 it may not be, it could actually be higher. So a standard vacuum gauge isn’t really suitable. An absolute digital pressure is the answer but until now there hasn’t been a reasonably priced absolute digital pressure gauge on the market and Frank was aware of this.
CPS (NZ) Limited is now able to supply BD Sensors “Baroli 05” range of digital pressure gauges. These are made in Germany, low cost, ± 0.25% accuracy and available in absolute ranges, and they can read in mmHG and kPa.
This great little gauge is $585 + GST plus $165 + GST for an IANZ calibration certificate.
Frank was very happy with this solution and ordered a 0-1 bar Absolute model.
When the gauge arrive three weeks later from Germany it was calibrated in our Auckland lab. So I decided to go and visit Frank to drop the gauge off and see it in action, and to understand more about this application which had caught my attention.
Frank started a test and all was going well until the pressure bottomed out at 75 mmHG (A). Hmmmm, maybe time to service the vacuum pump or get a new one! It was a fine, sunny day, so maybe doing the test on a rainy day would have helped. At least Frank knows the exact pressure of his tests from now on!
We will soon be stocking these gauges, if you would like to discuss replacing your mechanical gauges with this digital gauge, please do contact me.
Nick