Differential pressure transmitter range calculation for level


Thread Starter

John Flynn

I want to calculate the range of a dp transmitter used for level measurement which has glycerine on the LP leg and process (tank) fluid on the HP side.
Can anybody suggest the formula for this or give me an e-mail link to a site which has this type of info available?
I would greatly appreciate it.
What is the fluid on the HP side? Its S.G. will have a great effect on the outcome for use as a level measurement. Perhaps you should try an empirical method.
Your heading asks about level - but the description was a little confusing to me. But anyway - check out: "http:www.almegcontrols.com":http:www.almegcontrols.com go to products then continuous -then - go back to products and hit controls - 4-20. Another product is available, but not on line. It is "user set" control with 4 240 V relays. Works with 4-20 or 3 wire. thanks Bob Hogg
You can use this formula
dp = P(hp) - P(lp)
= den(process fluid)*H(hp)
- den(glycerine)*H(lp)
1.den(process fluid) is density of process fluid
2.den(glycerine) is density of glycerine
3.H(hp) is height of HP leg, in this case liquid height in tank to be measured.
4.H(lp) is height of glycerine filled LP leg.

this formula can be used for open tanks where P(lp) becomes zero. In case you use any fluid, other than glycerine in LP leg, use its density in place of den(glycerine)
More information on the message i posted yesterday, for those who asked. Thank you to those who replied.

The tank in question is closed and will contain a fluid of density 823kg/m3. The HP side of the txer is filled with glycerine of density 1260kg/m3 at h2=2.88m. The LP side is at h1=2.38m. (both measurements from centreline of the txer).

I calculated the dp based on HP-LP 4mA=((823*g*2.38)-(1260*g*2.88))/100=-163.8mbar

However other colleagues of mine use a different formulae : 4mA=[(h2-h1)*den.glyc]-den.tank
Their measurements were in inches so their results were in "H2O which they converted to mbar by multiplying by 2.4864.

Their result is 4ma= -59.04mbar
20mA= -23.23mbar

I'm wondering which is correct?
If both the levels are varying, then you should use 2 transmitters from the line that separates them. Thus you can have individual readings (calculations) for each. Or the difference of the two readings (calculations) if the transmitters are located one at center and the other from end to end.

Please note the glycerine volume will change and hence the density with temperature widely.

My calculation matches with what you have calculated. As far as your colleagues' calculations are concerned, they are calculating the span (20mA) correctly, but I think they have to use

h1*den.tank - h2*den.gly

to calculate zero (4mA), which will give same result as yours.