Transmitter Turndown Ratio, Set Span and Zero Span

Explanation of the terms: Transmitter’s Turn down (TD) ratio, set span and zero based span

Consider a Pressure Transmitter and we will discuss some important parameters related to it.

Range :

It is the measurement limit and covers from the minimum to the maximum pressure that the transmitter can measure, e.g., 0 to 5080 mmH2O. The maximum span is 5080 mmH2O.

URL (Upper Range Limit) :

It is the highest pressure at which the transmitter was designed to measure, respected the sensor upper range limit.

LRL (Lower Range Limit) :

It is the lowest pressure at which the transmitter was designed to measure, respected the sensor lower range limit.

URV (Upper Range Value) :

It is the maximum pressure at which the transmitter calibrated.

LRV (Lower Range Value) :

It is the minimum pressure at which the transmitter calibrated.

Span (Range Calibrated) :

It is the work range where the calibration is done is known as span, for example, from 500 to 3000 mmH2O, where the span is 3000-500 = 2500 mmH2O. The Span is equal to URV – LRV.

Turndown(TD) or Rangeability :

It is the relation between the maximum pressure (URL) and the minimum measured pressure (minimum calibrated span). For example, a transmitter range is 0-5080 mmH2O and will be used on 10:1, indicating which transmitter will measure 0 to 508 mmH2O. TD = URL/ Calibrated Span.

Case 1:  

|Lower range value| ≤ |Upper range value|

Figure : 1000 mbar (14.5 psi) sensor

Here,

1 : Set span
2 : Zero based span
3 : Nominal value ≅ Upper range limit (URL)
4 : Nominal measuring range
5 : Sensor measuring range

LRL : Lower range limit
URL : Upper range limit
LRV : Lower range value
URV : Upper range value

Example:

Lower range value (LRV) = 0 mbar

Upper range value (URV) = 100 mbar (1.5 psi)

Nominal value (URL) = 1000 mbar (14.5038 psi)

Turn down:

TD = URL /|URV| = 10 : 1

set span:

URV – LRV = 100 mbar (1.5 psi)

This span is based on the zero point.

Case 2:  

|Lower range value| ≥ |Upper range value|

Figure : 1000 mbar (14.5 psi) sensor

Example:

Lower range value (LRV) = -300 mbar (-4.5 psi)

Upper range value (URV) = 0 mbar (0 psi)

Nominal value (URL) = 1000 mbar (14.5038 psi)

Turn down:

TD = URL / |LRV| = 3.33 : 1

set span:

URV – LRV = 300 mbar (4.5 psi)

This span is based on the zero point.

Reference : endress hauser

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