Revision Date: 08 November 2024
Author: M. Daniels
The PM600 pressure measurement modules are a version of the Q-RPT product family. The majority of the PM600 modules have an intrinsic absolute measurement mode. They can operate in both absolute measurement mode as well as a simulated gauge mode. Gauge mode measurements are accomplished by applying a method known as Dynamic Gauge Mode Compensation.
When changing measurement modes from absolute to gauge, the Q-RPT based products employ a two-step process. The first step is a taring of the absolute mode output. Gauge measurement mode is achieved by subtracting the value of atmospheric pressure from the Q-RPT’s absolute reading resulting in an AutoZero offset. In gauge mode, the AutoZero routine is run automatically whenever the 6270A is in the vented condition updating the autozero offset.
However, atmospheric pressure continuously changes over time in the period between when the 6270A is initially tared and when later vented. If the AutoZero offset is not updated during this time then the resulting gauge mode pressure does not truly reflect the differential pressure relative to ambient conditions. This is a common situation for industrial level sensors that use only a simple tare method. Therefore, a second step is required to automatically update the AutoZero value to compensate for changes in ambient pressure.
The PM600 has an integrated barometric sensor. When vented, the AutoZero value is updated and at the same time the barometric sensor value is logged. The AutoZero value is identified as the "zOffset". When the 6270A is no longer vented then the on-board barometer is queried at regular intervals of every few seconds. A change in barometric pressure is calculated and this value is then applied to original AutoZero offset value, thus dynamically updating the original zOffset value. This dynamic compensation process is continuously applied providing a near real time update. This method is known as Dynamic Gauge Mode Compensation.
The beauty of using this method is that the full uncertainty of the barometer is not applied to the measurement. The barometer is only used to measure the change in pressure over a short period of time on the order of a few seconds. Therefore, only the resolution and short-term stability of the onboard barometer become additional contributions to the PM600 gauge mode measurement.
The uncertainties associated with PM600 and the Q-RPT products are documented in Uncertainty Analysis publications. The publication “Guide to determining pressure measurement uncertainty for PM200, PM500,and PM600 Pressure Measurement Modules” cites references to the document titled “Guide to determining pressure measurement uncertainty for Q-RPT based products”. Both documents are available online.
Per the 6270A Product Specifications document, Table 3, Footnote 3 uses a barometer which contributes ±14 Pascal. The PM600 uses a different barometer than the other type of Q-RPT products that are discussed in the Q-RPT Uncertainty Analysis.
When changing measurement modes from absolute to gauge, the Q-RPT based products employ a two-step process. The first step is a taring of the absolute mode output. Gauge measurement mode is achieved by subtracting the value of atmospheric pressure from the Q-RPT’s absolute reading resulting in an AutoZero offset. In gauge mode, the AutoZero routine is run automatically whenever the 6270A is in the vented condition updating the autozero offset.
However, atmospheric pressure continuously changes over time in the period between when the 6270A is initially tared and when later vented. If the AutoZero offset is not updated during this time then the resulting gauge mode pressure does not truly reflect the differential pressure relative to ambient conditions. This is a common situation for industrial level sensors that use only a simple tare method. Therefore, a second step is required to automatically update the AutoZero value to compensate for changes in ambient pressure.
The PM600 has an integrated barometric sensor. When vented, the AutoZero value is updated and at the same time the barometric sensor value is logged. The AutoZero value is identified as the "zOffset". When the 6270A is no longer vented then the on-board barometer is queried at regular intervals of every few seconds. A change in barometric pressure is calculated and this value is then applied to original AutoZero offset value, thus dynamically updating the original zOffset value. This dynamic compensation process is continuously applied providing a near real time update. This method is known as Dynamic Gauge Mode Compensation.
The beauty of using this method is that the full uncertainty of the barometer is not applied to the measurement. The barometer is only used to measure the change in pressure over a short period of time on the order of a few seconds. Therefore, only the resolution and short-term stability of the onboard barometer become additional contributions to the PM600 gauge mode measurement.
The uncertainties associated with PM600 and the Q-RPT products are documented in Uncertainty Analysis publications. The publication “Guide to determining pressure measurement uncertainty for PM200, PM500,and PM600 Pressure Measurement Modules” cites references to the document titled “Guide to determining pressure measurement uncertainty for Q-RPT based products”. Both documents are available online.
Per the 6270A Product Specifications document, Table 3, Footnote 3 uses a barometer which contributes ±14 Pascal. The PM600 uses a different barometer than the other type of Q-RPT products that are discussed in the Q-RPT Uncertainty Analysis.