Differential pressure transmitters are easily the most common transmitter used in industry, a range of process variables such as static gauge pressure, level and flow can easily be inferred from differential pressure.
There’s quite a lot of misconceptions as to the correct way to isolate and de-isolate these devices – get it wrong you could cause irreparable damage to the instrument and cause costly downtime.
First of all it’s essential to understand these devices measure a difference in pressure between the high (H) and low (L) connections. This difference in pressure is usually much lower than the difference between any one of these connections and atmosphere. For example, if 1000KPa is present on the High connection and 950KPa on the Low connection you have a differential pressure of 50KPa. This differential pressure is most likely within the measuring range of the instrument. However, not isolated correctly I risk exposing the instrument to exceeding high differential and potentially damaging pressures of any one of the connections to atmosphere (1000KPa or 950KPa).