The gauge shows less fuel depth than the tank contains because a bottom-mounted pressure transducer responds to hydrostatic pressure, not level directly. If the existing 0–40 ft scale treats the signal as water head, JP5 at a specific gravity of 0.8 produces only 80% of the pressure produced by the same depth of water. Correct the displayed height by dividing it by 0.8.
Where does the level signal travel?
Follow the signal from the process to the indication. The JP5 column applies hydrostatic pressure at the bottom connection. The pressure transducer converts that pressure into 4–20 mA. The analog gauge then converts current into an indicated range of 0–40 ft.
| Path element | Input | Output or setting | Commissioning check |
|---|---|---|---|
| Tank and pressure connection | JP5 depth | Hydrostatic pressure | Connection is below the measured liquid column and open to the process |
| Pressure transducer | Hydrostatic pressure | 4–20 mA | 4 mA represents the configured zero pressure |
| Analog gauge | 4–20 mA | 0–40 ft indicated | Applied current produces the expected uncorrected indication |
| SG correction | Water-equivalent indicated height | Actual JP5 height | Corrected value agrees with an independent level reference |
Layer one first: inspect the pressure connection, wiring polarity, loop supply, terminals, and current before changing scale factors. An electrical or impulse-line fault cannot be repaired with a specific-gravity calculation. Prove the current at the gauge input before proceeding.
Was the 0–40 ft scale based on water or JP5?
The correction depends on the reference fluid used when the transducer and gauge were ranged. Hydrostatic pressure follows
P = ρgh
where P is pressure, ρ is liquid density, g is gravitational acceleration, and h is liquid height. Specific gravity relates fuel density to the reference water density:
ρ_fuel = SG × ρ_water
For a water-referenced indication, the gauge interprets pressure as:
ρ_water × g × h_indicated = ρ_fuel × g × h_actual
Substituting ρ_fuel = 0.8 × ρ_water and cancelling common terms gives:
h_actual = h_indicated / 0.8 = 1.25 × h_indicated
| Existing range basis | Meaning of the displayed value | Required action |
|---|---|---|
| Water head | Water-equivalent height | Multiply the indication by 1.25
|
Already calibrated for JP5 at SG = 0.8
|
Actual JP5 height | Apply no additional SG correction |
| Unknown | Scaling cannot yet be classified | Read the transducer pressure range and gauge configuration, then compare them with the pressure expected at a known depth |
The decisive check is the configured pressure represented by 20 mA. If it equals 40 ft of water head, use the 1.25 multiplier. If it equals the pressure produced by 40 ft of JP5 at SG = 0.8, the correction is already built into the range.
How does loop current convert to corrected JP5 depth?
For the stated linear gauge range, first convert current to the uncorrected indication:
h_indicated = ((I_mA - 4 mA) / 16 mA) × 40 ft
If the range is water-referenced, apply the SG correction:
h_actual = ((I_mA - 4 mA) / 16 mA) × 40 ft / 0.8
For the supplied values, this reduces to:
h_actual = (I_mA - 4) × 3.125 ft
| Loop current | Gauge indication | Corrected JP5 depth at SG = 0.8
|
|---|---|---|
| 4.0 mA | 0 ft | 0 ft |
| 8.0 mA | 10 ft | 12.5 ft |
| 12.0 mA | 20 ft | 25 ft |
| 16.0 mA | 30 ft | 37.5 ft |
| 20.0 mA | 40 ft | 50 ft |
The last row exposes the range consequence: a water-referenced 0–40 ft indication corresponds to 0–50 ft of JP5 at SG = 0.8. Check the calculation at 12 mA; half-scale current must produce 20 ft uncorrected and 25 ft corrected.
How should the corrected range be configured?
Choose one correction point in the signal path. Re-ranging the indicator is preferable when it accepts engineering-unit scaling. A controller can perform the same linear conversion when the gauge cannot be rescaled. Do not correct both devices, because that multiplies the result by 1.25 twice.
- Record the transducer pressure range and identify the pressure represented by 4 mA and 20 mA.
- Confirm whether the existing 0–40 ft gauge scale is water-referenced or already fuel-compensated.
- If it is water-referenced, configure the receiving device for 0–50 ft across 4–20 mA, or apply
h_actual = h_indicated / 0.8in the controller. - If the required display must remain 0–40 ft for a 40 ft tank, recognize that 40 ft of JP5 produces the pressure represented by only 32 ft of water head. On the stated water-based scale, that operating point is 16.8 mA.
- Set alarms and bar-graph limits from corrected engineering units, not from the old scale markings.
The derived 40 ft operating current is 4 + 16 × (32 / 40) = 16.8 mA. Prove the selected implementation by applying 16.8 mA: it must resolve to 40 ft after correction, while the uncorrected gauge would show 32 ft.
Which installation effects can imitate an SG error?
| Observed symptom | Likely mechanism | Check |
|---|---|---|
| Constant offset at every level | Zero elevation, trapped pressure, gauge-reference error, or incorrect 4 mA zero | Check current and pressure at the defined zero-level condition |
| Error grows proportionally with level | Incorrect span or specific-gravity factor | Compare two known levels and calculate slope |
| Reading changes abruptly or sticks | Blocked pressure connection, trapped gas, wiring fault, or unstable loop current | Inspect the process connection and measure loop current directly |
| Correct at low level but wrong near full | Wrong span, nonlinear indication, or transducer range limitation | Inject multiple current points and compare indicated values |
| Corrected value exceeds tank height | Water-based full-scale range extends beyond the physical tank | Compare corrected full scale with tank geometry and usable level range |
A bottom-pressure measurement also includes any pressure acting above the liquid unless the measurement arrangement cancels it. Compare the transducer’s pressure reference with the tank pressure arrangement before assigning the entire error to SG. The proof is a zero and span test that separates a fixed offset from a proportional scaling error.
How is the complete measurement verified?
- Test the indicator independently with known currents at 4 mA, 12 mA, 16.8 mA, and 20 mA. For the corrected water-referenced case, expect 0 ft, 25 ft, 40 ft, and 50 ft.
- Reconnect the transducer and compare measured loop current with an independently established tank depth.
- Calculate the expected water-equivalent indication as
h_indicated = 0.8 × h_actual. - Confirm that the correction stage returns
h_actual = h_indicated / 0.8. - Check all displays, alarms, trends, and control calculations use the same corrected value and that no second SG multiplier remains elsewhere.
At a verified JP5 depth of 40 ft, the water-referenced path must produce a 32 ft uncorrected indication, approximately 16.8 mA on the stated linear range, and a final corrected display of 40 ft.
FAQ
Can I multiply the gauge reading by 0.8?
No. For a water-referenced gauge measuring JP5 at SG = 0.8, divide the indicated height by 0.8, which is equivalent to multiplying by 1.25.
Does 20 mA still mean 40 ft of JP5?
Only if the transducer or receiver was ranged for JP5. With a water-referenced 0–40 ft scale, 20 mA corresponds to 50 ft of JP5 at SG = 0.8.
Can I verify 40 ft of JP5 without filling to 20 mA?
Yes. Inject 16.8 mA into the receiving path; it should show 32 ft before SG correction and 40 ft after correction.