differential pressure measurement

[Theboss]

Good day ladies and gentlemen,
Measuring dp is pretty straight forward. Install a dp cell transmitter and done. But what if you wanted to install two pressure transmitters one representing the HP side and the other the lp side. Then the difference between them is our dp. Is this method advisable???

 

[jstolaruk]

Good day ladies and gentlemen,
Measuring dp is pretty straight forward. Install a dp cell transmitter and done. But what if you wanted to install two pressure transmitters one representing the HP side and the other the lp side. Then the difference between them is our dp. Is this method advisable???

Sure, why not. Its common for measuring the pressure drop over filters or pumps in process water applications. I've used IFM Efector products for this in the past.

 

[Tom Jenkins]

Functional and advisable are two different criteria.

It would certainly function. There are a few negatives that may make it inadvisable:

- you have to make sure that you pick the right transmitter for both locations. If you are measuring flow through an orifice on the pressure side of a pump both need to be gauge or absolute. If you are measuring pressure drop through the filter on the suction side of a fan both need to be suction. If you are measuring pressure rise through a pump one may need to be suction an the other gauge pressure.

- you have to pay for two instruments instead of one (although DP are often slightly more expensive per instrument)

- you have to maintain and calibrate two instruments instead of one

- as calibration drifts you have the possibility of twice the error

I can imagine cases where distance between high and low points or other considerations make the two devices a better solution, but in my opinion if you can do it with a single DP transmitter that is the way to go.

 

[danw]

The use of a true DP transmitter for DP flow is such accepted practice that the use of two gage pressure transmitters would very likely be viewed as 'do they know what they're doing?" for good reason, most flow occurs at line pressures high enough that the low level DP that is to be measured is close to or in a zone of fairly high error, which gets compounded when subtracting one gage reading from another.

At extremely low DP's, like 0-0.5" DP from HVAC positive or negative room pressure or 0-5.7" from an averaging pitot tube, it makes no sense at all to use multiple gage pressure transmitters which will produce huge error. The error would be in the 10's of percent error range by the time two gage or absolute pressure readings were made and then subtracted.

Even at a typical 100"w.c. full scale DP for differential flow (orifice plate, venturi), you'd see far more error than with a DP transmitter, with significantly increasing error as the flow decreases from max. A gage pressure transmitter needs to be ranged for the static line pressure, not for the DP pressure.

For example, a flow app whose static line pressure is 100psi needs two gage pressure transmitters ranged for 100psi or slightly higher, say 125psi. A 0.2% error at 125 psi is 0.25psi or almost 100 inches water column. A DP transmitter would be ranged 0-100"wc for a typical orifice plate and its accuracy (% FS) is based on the DP range, not static gage pressure.

Example: The DP calc is high side measurement minus low side measurement.

Suppose the true measurement were 78.15psi (high side) minus 76.85psi (low side) = 1.30 DP = 36.01"w.c.

A 0.15psi 'low' error on the high side is produces 78.00psi reading
A 0.15psi 'high' error on the low side produces a 77.00psi reading

78.0 minus 77.0 = 1.0 = 27.7" w.c 36" vs 27.7", a huge error in a DP flow measurement, with a typical 100" max DP, totally unuseable with an averaging pitot tube with a 0-5.8" full scale DP range.

In an extreme case (at low flow or high drift error) the error could be a high error on the low side and low error on the high side, and the subtraction could produce a negative value, indicative of reverse flow, even though such is not the case (actually quite prevalent where low accuracy 2-3-2% bourdon tube pressure gauges are installed on etiher side of filter.)

Pressure drops across filters can be in the low PSI ranges (100"w.c. is just over 3 PSI) so low filter drops can have the same issues as DP flow.

Very low pressure gauge transmitters typically have to use the large sensor diaphragm of a DP, so the cost is typically a substantial fraction of a DP transmitter.

For high 10's of psi DP or above, two gauge pressure transmitters with a math subtraction is probably doable, but the error will be more than that of a single DP transmitter. But if you don't care whether the DP is ±2psi at 200 psi, then go for it.

 

[DwSoFt]

Use the right tool for the job. I may be able to cut wire with scissors but it will be a pain in the *** to do. and it wouldnt be as good as wire cutters.

 

[Theboss]

thanks for the input and advice gentleman.been having an argument with my chief with this. we have a venturi tube and our dp tx is working fine.we decided to remove dp tx and install two ifm pressure tx's instead. and his argument is that it wouldnt work because we will be measuring line pressure instead resulting in 0 dp.

 

[Tom Jenkins]

It could work, but all the cautions danw mentioned should be kept in mind. The way a venturi works is that the higher velocity at the throat increases the velocity pressure and reduces the static pressure by the same amount. (See Bernoulli's Law http://www.engineeringtoolbox.com/orifice-nozzle-venturi-d_590.html ).

So you are both right. The transmitters will measure line pressure (static pressure) but that pressure will be different at the two taps.

Why would you remove a DP transmitter that is working and replace it? What is the perceived advantage?

 

[danw]

You're doing a flow measurement.

A DP transmitter will square root the DP to give you a 0-100% flow rate signal, just assign units to it and you're ready to go.

With two gage pressure transmitters, you need to subtract, normalize to 100% and do the square root to get to flow. Yes, the PLC is likely to handle the math, but there have been numerous threads over the years on this forum about how to take a square root (it's on normalized percentage, not a DP pressure value).

I echo Tom's questions, why would you not keep a working DP in service?

 

[Theboss]

they didnt trust the dp tx. ok let me get this correct. what ever dp u measure is that not square root off the dp and that represents your flow rate?

 

[Theboss]

@DANW: You say there would be a pressure difference between the pressure tx's, so we should see a dp right on the plc provided the plc does the math?

i think the main reason here that we are not getting the dp expected at maximum valve travel. 0-100kpa ~~ 0 300 tons/per hour

 

[rdrast]

We tried several times to use separate transmitters for the high and low side of a 0-100" water column. In every single case, the results were quite a bit worse than just horrible.

DP cells, even with almost 400 feet of reference legs attached however, work fine.

 

[danw]

@DANW: You say there would be a pressure difference between the pressure tx's, so we should see a dp right on the plc provided the plc does the math?

There will be a difference in pressure readings between the two gage pressure transmitters. The PLC has to subtract the values, then normalize to percentage, then square root the normalized percent value, then assign engineering units to the resulting value. Most PLCs probably have the math capabilities to do so.

The relationship of flow to DP is a square root relationship, but that does not mean that one takes the square root of some number that is a DP value in units of water column.

Take a full scale 100"w.c. DP. The square root of 100" is 10. 10 what? gpm? liters/sec? tons/hour.

What if that same 100"w.c. is in SI metric units?
100" = 24.88kPa
square root of 24.88 is 4.987 Same question, what units?

10.0 does not equal 4.987 for the same pressure drop. How can either be the same flow rate for the same DP?

What if you've got an integer 32768 from the A/D of 32678? Square root of a 4-20mA signal? square root of 32786 is 181.01

181.01 does not equal 10 or 4.987. They're all square root values of some representation of a DP drop, right?

To take a square root, the DP is normalized to percentage, and the square root is taken on the percentage.

100.0" = 100%
24.88kPa = 100%
32786 = 100%

100% = 1.00 square root of 1.00 = 1 = 100%

64.00" = 64% of max flow
15.93kPa = 64% of max flow

64% = 0.64
square root of 0.64 = 0.80
64% of max DP = 80.0% of max flow rate

So yes, the flow rate can be calculated from the DP, but I do not recommend doing so for flow when the pressures are measured with two gage pressure transmitters.

i think the main reason here that we are not getting the dp expected at maximum valve travel. 0-100kpa ~~ 0 300 tons/per hour

You are killing the messenger.

If you are are not getting the flow you expect, it is probably not the DP transmitter that is in error. The pump isn't up to the task or the system has pressure losses you haven't accounted for. The DP/venturi is just reporting what you don't want to hear/see/know.

If is quite simple to check the calibration of a DP transmitter, in fact you can do it with a water manometer - connect some tubing to the high side, leave the low side open to atmosphere and fill the tubing with water. Measure the height of the water column. Or send it to a local cal lab and let them cal against a standard.

A modern differential transmitter is better at measuring that water column than your ability to measure the height of the water column with a tape measure. But you can assure yourself that the DP is reading closely to what it should.

Because of the combined error of two gage pressure transmitters, you might actually get those two transmitters to give you the reading you want, but it won't be a true flow rate.

The method a DP uses to make a measurement is NOT the same as two gage pressure transmitter reading subtracted from one another.

The internals of a DP transmitter are designed so that the (relatively high) applied pressures mechanically subtract one from another and the sensor is scaled to read only the (relatively low) differential pressure, producing a far more accurate reading that the subtraction of two gage pressure transmitters ranged for line pressure measurement.

 

[danw]

If your flow rate is lower than expected, you need to check to make sure that you are using the square root function.

The percentage of DP is lower than its square root (square roots of numbers less than one are larger than the base number), so if your system is reading 0-100% DP and (incorrectly) assigning it flow rate values, those flow rate will be lower than expected.

 

[DwSoFt]

That is why typically a differential orfice meter uses a flow computer. it will compensate for these issues already and is certified for flow measurement b many different countries. there are many better designed now when going straight to a plc. Magnetic meters. coriolis meters. vortex meters.

 

[danw]

A Venturi might use annular rings for high and low pressure sensing. Annular rings are built into the meter (like an averaging pitot tube with drilled hole ports every couple inches bent into a circle that runs around the inner circumference of the tube) under the theory that an average pressure reading would produce a more accurate cross-sectional pressure indication than a single pressure reading.

If a sticky, gooey medium (like sludge) clogs the ports, the pressure readings will be low.