Level Measurement Formulas Cheat Sheet
Level measurement on one page: hydrostatic level, DP transmitter range with dry and wet legs, zero suppression and elevation, radar and ultrasonic time of flight, and tank volumes.
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A one page A4 sheet to print, pin up in the workshop or keep on your phone. Everything on it is also shown below.
Download PDF A4 · 72 KBHydrostatic level
| Pressure | P = ρ × g × h |
| Level | h = P ÷ (ρ × g) |
| Water | 1 m = 98.07 mbar |
| Density change | Hydrostatic level reads wrong if density changes; correct or use radar |
DP transmitter: open tank or dry leg
| LRV | ρp × g × hmin (+ head below the HP tap) |
| URV | ρp × g × hmax (+ same head) |
| Transmitter below the tap | positive LRV = zero suppression |
DP transmitter: wet leg (closed tank)
| LRV | ρp g hmin − ρs g Hs |
| URV | ρp g hmax − ρs g Hs |
| Result | negative LRV = zero elevation |
| Symbols | ρs seal fluid density, Hs wet leg height between taps |
Remote seals (capillary)
| Both seals | LRV and URV include ρfill g × (seal height difference) |
| Tip | Mount the transmitter at or below the lower seal |
Radar and ultrasonic
| Distance | d = c × t ÷ 2 (time of flight) |
| Level | L = reference height − d |
| Speed of sound in air | c ≈ 331.3 + 0.606 × T (°C) m/s, 343 m/s at 20 °C |
| Radar | unaffected by temperature, pressure and vapour in most cases |
Interface level (DP)
| Total DP | ΔP = g × (ρ1 h1 + ρ2 h2) |
| Requirement | both taps always covered by liquid |
Tank volumes
| Vertical cylinder | V = π r² h |
| Horizontal cylinder (flat ends) | V = L × [r² acos((r − h) ÷ r) − (r − h) √(2 r h − h²)] |
| Sphere | V = π h² (3 r − h) ÷ 3 |
| Cone (point down) | V = π r² h ÷ 3 with r at height h |
Technology quick guide
| Method | Best for |
|---|---|
| DP / hydrostatic | Liquids with known density |
| Guided wave radar | Interfaces, foam, small tanks |
| Non contact radar | Most liquids and solids, vapour |
| Ultrasonic | Water and simple open tanks |
| Displacer | Interface, high pressure |
| Vibrating fork | Point level, high and low alarms |
Related Calculators and Tools
Frequently Asked Questions
What is zero suppression in level measurement?
When the transmitter is below the lower tap, the liquid head in the line adds a positive offset, so the LRV is above zero.
What is zero elevation?
With a wet leg on the LP side, the DP is negative at minimum level, so the LRV is below zero.
How does radar measure level?
It measures the time for a microwave pulse to travel to the surface and back: distance = c × t ÷ 2.
Learn more on the blog
DP transmitter level measurement: open tank, closed tank, wet and dry leg
A practical guide on instrumentationblog.in that explains the theory behind this cheat sheet.
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