Flowline LD32‐S4_1 DeltaSpan - Manual (Page 8)

8 of 16 MN301040 Rev B
GETTING STARTED Step Seven
Pressure transmitters are designed to be submersed within the application fluid. The
transmitters can either rest along the bottom of the tank or be suspended at any
desired level within the tank. Please note that the physical location of the level
transmitter will indicate the lowest level of measurement within the tank. For example:
mounting the transmitter 1 foot from the bottom of the tank, then the lowest reading
of liquid will be 1 foot from the bottom.
How to convert Pressure into Liquid Height?
Pressure transmitters are all describe by the pressure range and not by Liquid Height. To convert pressure to
Liquid Height, use the following ratio:
1 psi = 2.31 feet of water or 1 psi = 0.704 meters of water
Therefore, a 15 psi transmitter will have a Liquid Height = 34.65 feet (10.56 m):
15 psi x 2.31’/psi = 34.65’ or 15 psi x 0.704 m/psi = 10.56 m
With the above ratio, you can always find the Liquid Height or water column
(wc) of any pressure transmitter.
How to select the correct pressure transmitter?
The objective is to use a pressure transmitter that will cover the entire range of the application. If the liquid
height of the tank is above the transmitter’s Maximum Liquid Height, then the sensor will not be able to read
the level of a full tank. Compare the tank’s Pressure @ Full against the transmitter’s pressure to select a sensor.
To calculate the tank’s Pressure @ Full, use the following formula:
Required Maximum Pressure = [Full Tank Height (feet) x SG] / 2.31 (feet/psi)
Required Maximum Pressure = [Full Tank Height (meters) x SG] / 0.704 (m/psi)
To select the correct pressure transmitter, follow the steps below:
 Measure the Height @ Full Tank (in feet or meters).
 Determine the SG for the liquid (if the customer does not know, check the MSDS sheet).
 Use the formula above to calculate the Pressure @ Full.
a. Ex: A 40’ tank with a liquid media SG of 0.9 has a Pressure @ Full of 15.58 psi.
b. [(40’ x 0.9) / 2.31’/psi] = 15.58 psi
 Select a pressure transmitter with a pressure greater than or equal to the Pressure @ Full.
a. In the example above for a 40’ tall tank with a SG=0.9, you need a pressure range greater than
or equal to 15.58 psi.
b. Using a sensor with a pressure range of 20 psi provides a Maximum Liquid Height of 51.33’,
which will read the entire range of the 40’ tall tank.
c. Using a sensor with a pressure range of 15 psi provides a Maximum Liquid Height of 38.50’,
which is short by 1.5’.
i. This means that the sensor will output 20 mA at 38.5’ of liquid and the top 1.5’ of liquid
will not be measured.