The accuracy specified by ASME under B40.1 standards fall under 8 different “grades” to help describe a pressure instrument’s true level of ability to read accurately. Below are the different grades and their corresponding accuracies.
Grade 4A = 0.1% full scale
Grade 3A = 0.25% full scale
Grade 2A = 0.5% full scale
Grade 1A = 1.0% full scale
Grade A = 2-1-2% – 2% accuracy on first 1/4 of the scale, 1% on the middle 1/2, and 2% on the last 1/4 of the scale
Grade B = 3-2-3% – 3% accuracy on first 1/4 of the scale, 2% on the middle 1/2, and 3% on the last 1/4 of the scale
Grade C = 4-3-4% – 4% accuracy on first 1/4 of the scale, 3% on the middle 1/2, and 4% on the last 1/4 of the scale
Grade D = 5% full scale
Accuracy of the pressure, temperature, flow, or level instrument you’re reading is obviously important. Otherwise, how would you know if the reading you’re seeing is reliable or not?
For simplicity, we’ll go over a couple common scenarios. The first and easiest one to calculate is Full Scale accuracy. So, for example, if you have a pressure transducer that reads 0-200 psi and the accuracy is 1.0% FS (or Full Scale), that means the reading your transducer is sending out is +/- 1% of the full scale of the sensor. So, in this case it would be +/- 2 psi. If your transducer was showing a reading of 56 psi, then the “true” pressure could be anywhere between 54-58 psi (or +/- 2 psi from 56 psi).
Another common accuracy you may see on a pressure gauge would be a 2-1-2% or 3-2-3% accuracy. So, in the case of 3-2-3%, what this means is that on the first 1/3 of the scale the accuracy is +/- 3%, the middle 1/3 is +/- 2%, and the final 1/3 is +/- 3% accuracy.
If you’re concerned about the reading of your sensor, please give us a call or email. We’d be happy to answer further questions.
As odd as it might seem to see a gauge filled with an odd thick liquid, it’s actually quite common. The liquid in the gauge (usually glycerin) acts as a dampener during high vibration. Because a pressure gauge has many mechanical parts as well as a sensitive pointer, vibration on a gauge would cause the needle to shake back and forth making it difficult to read the pressure as well as cause the gauge a lot of wear and tear. Read our Blog Post on gauge options for vibrating application for more information.
The most common fill fluid for a gauge is Glycerin. Some other fill fluids could include Silicone, Glycerin/Water Mix, & Halocarbon.
Not much. They’re often used interchangeably in the pressure sensing world. If you’re getting specific, a transmitter usually refers to a sensor with a current signal such as 4/20mA and a transducer usually refers to a sensor with a voltage signal such as 0-10VDC or 1-5VDC
Proof pressure is the pressure the unit can handle over its nominal range without affecting its performance or accuracy.
Burst pressure is the pressure the unit can handle over its nominal range before it leaks, ruptures, or explodes into teeny tiny little pieces all over your facility.
Please note that for proof pressure, while the instrument shouldn’t break, consistently subjecting an instrument to its proof pressure will eventually damage it or make it fall out of calibration much quicker. Proof pressure is meant as a guideline for a once-in-a-long-while scenario. If you find yourself getting up to the proof pressure often, then you should look into a higher range sensor.