When was the last time you asked about digital counts? OK, how about the last time you asked about the accuracy of a digital pressure gauge? Ah! Now we’re talking!
Digital counts are an essential part of understanding the accuracy of your digital pressure gauge. Digital counts are also a key part of the advantage that digital gauges have over analog, or dial gauges.
To look at analog gauges for a moment, there are three factors of uncertainty involved in reading an analog gauge: the possibility of error at the transducer measuring the pressure; the error in the dial representing that measurement; and the human error in reading the gauge.
By contrast, digital gauges have two internal sources of error, but the third error in reading is removed. The first internal source of error is the same, the transducer. The second, however, is the process of converting the analog signal to a digital display.
Analog measurements have, in theory, infinite resolution across a given scale. By definition, digital displays have a finite number of possible readings. And that’s where digital counts come in.
Digital counts are the number of discrete increments your gauge is capable of displaying across its full scale. If your gauge goes from 0 to 10.0 PSI, you have 100 digital counts; but if your gauge goes from 0 to 10.00 PSI, you have 1,000 digital counts.
Back to analog-to-digital conversion: analog readings that fall between digital increments must be rounded, thus creating a small margin of error. Packing more increments—more digital counts—into the range of a sensor decreases the potential error. So, more digital counts means a more precise gauge.
But precision isn’t the entirety of accuracy. A gauge can be capable of displaying 1,000 increments, but if the transducer and conversion introduce too much error, you are left with a very precise, but wrong, measurement.
At APG, in calculating the accuracy of our gauges, we allow one digital count of error for the transducer, and one for the analog-to-digital conversion process. So, for a gauge with 1,000 digital counts, two counts of error equals a 0.2% minimum linear accuracy. Pretty good, right?
Let’s look at two examples.
A digital gauge with a range of 0 to 2,000 PSI has 2,000 digital counts. Two counts of error per 2,000 digital counts gives us a 0.1% minimum linear accuracy. In terms of PSI, every reading displayed on the gauge will be within 2 PSI of the actual pressure in the vessel or line being monitored.
For a smaller scale application, a digital gauge with a range of 0.00 to 80.00 PSI has 8,000 digital counts. Two counts of error per 8,000 digital counts equates to 0.025% minimum linear accuracy, which we would list at 0.1%. To put that back in PSI, every reading on the gauge will be within 0.08 PSI of actual pressure, but the gauge is capable of readings within 0.02 PSI across its full 80.00 PSI range.
So, digital counts? More is better, but more digital counts only means a more precise measurement. Truly accurate digital pressure measurements require transducers and analog-to-digital conversion that work with a high degree of accuracy in order to take full advantage of all those digital counts.
Contact us if you have any questions about the accuracy of your pressure gauge and how you can improve it.
top photo credit: Larry Jacobsen via flickr cc
