Converting Pressure Units
American Petroleum Institute (API) recommendations are to use a base temperature of 60.0° F when converting from inches to or from any other pressure unit. In CygNet, the following conversions fit this category:
- Inch of Water (inH2O)
- Inch of Water gauge (inH2O g)
- Foot of Water (ftH2O)
- Foot of Water gauge (ftH2O g)
- Inch of Mercury (inHg)
- Inch of Mercury gauge (inHg g)
The CygNet conversions were based on the National Institute of Standards and Technology (NIST) international standard base temperature of 39.2° F. These conversions were changed to follow the API standard, thus changing the base temperatures in our calculations to 60.0° F.
Additional units have been added so that if you want to continue to use a base temperature of 39.2° F you may elect to do so. The unit description and abbreviation are adorned with the temperature associated with the conversion. For example:
| Unit Abbreviation | Unit Description | Base Temperature | 1 kPa converted |
|---|---|---|---|
| inH2O | inch of water at 60° F | 60° F | 4.01864651985211 |
| inH2O39 | inch of water at 39.2° F | 39.2° F | 4.01474213311279 |
| inH2Oc | inch of water conventional | None specified | 4.01463092092823 |
SCADA Data
If you are using the Alternate Units in the Point Service, the pressure value will be different. Alternate Units are for display only, so this does not affect any data stored in the VHS.
If you are using the CxScript.Conversions object to convert the value from metric to imperial and storing the data in the VHS, you will see a difference between your converted values.
The difference in the calculation is as follows:
| kPa | inH2O (at 39.2° F) | inH2O (at 60° F) | Difference |
|---|---|---|---|
| 0.01 | 0.04014742133113 | 0.04018646519852 | 0.0000390438673899957 |
| 0.1 | 0.40147421331128 | 0.40186465198521 | 0.00039043867392996 |
| 1 | 4.01474213311279 | 4.01864651985211 | 0.00390438673932003 |
| 10 | 40.1474213311279 | 40.1864651985211 | 0.0390438673931968 |
| 100 | 401.474213311279 | 401.864651985211 | 0.3904386739320100 |
| 1000 | 4014.74213311279 | 4018.64651985211 | 3.9043867393202200 |
FMS Data
Some CygNet Measurement (FMS) unit conversions may be affected by unit conversions, depending on licensed mode and version. See FMS Data Items Units for more information.
