How DP level measurement works
A liquid column exerts a pressure equal to its height times its specific gravity (as water column). The HP side of the transmitter connects to the bottom tap and sees that head. The LP side either vents to atmosphere (open tank) or connects to the vapour space (closed tank), which cancels the vessel pressure.
kPa = metres of water column × 9.80665
Zero suppression
If the transmitter is mounted below the bottom tap, the HP impulse line holds a column of liquid that is always there, even at minimum level. The LRV is therefore above zero: the zero is suppressed.
Worked example
Open tank, SG 0.8, level range 0–2 m, transmitter 0.5 m below the tap.
- LRV = 0.8 × (0 + 0.5) = 0.4 m WC = 3.92 kPa
- URV = 0.8 × (2 + 0.5) = 2.0 m WC = 19.61 kPa
- Span = 15.69 kPa (the span depends only on SG × level range)
Zero elevation and wet legs
On a closed tank where vapour would condense in the LP impulse line, the LP leg is often filled deliberately (a wet leg) so that its head is constant. The wet leg pushes on the LP side, so at minimum level the DP is negative: the zero is elevated.
Worked example
Closed tank, water (SG 1.0), level range 0–100 in, transmitter level with the bottom tap, wet leg filled with water to 120 in above the transmitter.
- LRV = 1.0 × 0 − 1.0 × 120 = −120 inH₂O
- URV = 1.0 × 100 − 1.0 × 120 = −20 inH₂O
- Span = 100 inH₂O. The whole range is negative, and the transmitter output still rises with level.
Common errors in the field
- The span depends on the process SG. If the product density changes, the level reading changes even though the actual level didn't.
- A wet leg that slowly drains or evaporates shifts the zero. Check and refill it as part of routine maintenance.
- Temperature changes the SG of both the process and the fill fluid. For accurate work, use the SG at operating temperature, not at 15 °C.
- Remote diaphragm seals behave like a wet leg: the capillary fill fluid head between the seals must be included in the calculation.