DP Level Transmitter Range Calculator
Work out the calibration range of a differential pressure level transmitter. Covers open tanks, closed tanks with a dry leg and closed tanks with a wet leg or remote seal, with the transmitter mounted below the lower tapping.
- HP side = SG × (level + tap height) = 1 × (0 + 0.3) m at 0 %, 1 × (2.5 + 0.3) m at 100 % of liquid column
- LP side is vented or a dry leg, so it adds nothing
- DP = HP − LP, giving LRV = 29.42 and URV = 274.59 mbar
How DP Level Ranging Works
A DP transmitter measures the hydrostatic head of liquid above the HP tapping. Any liquid filled impulse line below the tap, and any wet leg or capillary on the LP side, also adds head that must be included in the range.
LP = SGwet leg × g × hwet leg (0 for open tank or dry leg)
DP = HP − LP
Using 1 mmH₂O = 9.80665 Pa, pressure in mmH₂O is simply SG × height in mm, which makes hand checks easy.
Worked example
Open tank, water (SG 1.0), level 0 to 2.5 m, transmitter 0.3 m below the tap. LRV = 1.0 × 300 = 300 mmH₂O (29.4 mbar) and URV = 1.0 × 2800 = 2800 mmH₂O (274.6 mbar). The positive LRV is a zero suppression.
Now close the tank with a water filled wet leg 3.3 m above the transmitter: LP = 3300 mmH₂O, so LRV = 300 − 3300 = −3000 mmH₂O and URV = −500 mmH₂O. This is zero elevation, and the transmitter output still rises with level.
Frequently Asked Questions
What is zero suppression in a level transmitter?
When the transmitter is mounted below the HP tap, liquid in the impulse line adds pressure even at zero level. The LRV becomes positive, which is called zero suppression.
What is zero elevation?
With a wet leg or remote seal capillary on the LP side, the LP pressure is higher than HP at low level, so the LRV is negative. That shift is called zero elevation.
Does the transmitter height matter with remote seals?
For a DP transmitter with two equal capillaries, the transmitter height cancels out. Only the vertical distance between the seals and the fill fluid SG matter.
Learn more on the blog
Basics of level measurement
A practical, field focused guide on instrumentationblog.in that explains the theory behind this calculator and how engineers apply it on real plants.
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