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

LRL (Lower Range Limit) : is the lowest pressure at which the transmitter was designed to measure, respected the sensor lower range limit.

URV (Upper Range Value) : is the maximum pressure at which the transmitter calibrated.

LRV (Lower Range Value) : is the minimum pressure at which the transmitter calibrated.

Span (Range Calibrated) : 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 : 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|

Field Transmitter Range Calculation

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|

Transmitter Span Range Calculation

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 /|URV| = 3.33 : 1

set span:

URV – LRV = 100 mbar (1.5 psi)

This span is based on the zero point.

Also Read : Importance of Turndown Ratio

Reference : endress hauser

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3 comments

Prasenjeet Sen February 6, 2017 at 3:10 am

Hi All,

In the above, please elaborate the effect of Turn Down Ratio of higher or lower during operation or selection of transmitters.

Reply
S Bharadwaj Reddy February 6, 2017 at 4:17 am

Hi, Please search the website with word “Turndown” its already there. Thanks

Reply
Kumaresan September 2, 2017 at 5:51 pm

Hi , I have certain doubts in case 2
1. If TD = URL/URV , then it should be 1000/0 as per the formula mentioned

2. Span = URV – LRV, then it should be 0-(-300), which is 300.
Please correct me if am wrong

Reply

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