How to read a log graph?
Let's spend a little more time on this log rhythmic 1.85 graph that we use to quantify water supplies.
First let's breakdown what we're actually looking at period on our horizontal X axis we have the flow. In the US this is calculated as gallons per minute or GPM. this axis ranges from zero GPM which is at a number flow condition, all the way up to whatever value we need to represent the water supply. This could go up to 400 GPM 800 GPM or 3000 GPM.
When dinosaurs roamed the earth and apps were a thing we got before the main course, we would hand draw these charts. The charts usually would have multiple scales along this bottom X axis. having different options for scales meant that we could better showcase what the actual results are. It didn't matter which scale we used, it didn't alter the result, but it altered the way in which the results were shown on the graph. Rather than having a very steep line all the way on the left of the graph, we could stretch this axis and represent a water supply curve across most of the graph.
When we interpolate points along this supply, finding our interpolated value becomes significantly easier when this X axis is stretched.
When we would hand draw these graphs and select a scale, we would just circle the entire scale that we were using for the graph. This multi-scale option meant we could have one or two generic blank graphs and essentially be able to chart any of our flow test results that we wanted, regardless of how much water we were actually able to flow.
On the far-left side of this axis is the zero-flow condition. for a flow test we would call this position a static condition. When we have a municipal water supply that is a gritted pipe network, there is pretty much water flowing in the system all the time. Water flows to supply domestic needs like toilets and showers and washing machines and dishwashers, even in the middle of the night some amount of water is running to feed those ice makers in the refrigerator that only seem to make noise whenever it's otherwise quiet. in our city supply, there is always water moving through the system
However, for our purposes when we're looking at a flow test, when we are not flowing water, we are calling this a static condition. This term is slightly misleading, not just because water is always flowing within typical water supply, but because even this value will fluctuate. As pump kicks on or law and arrogation systems run or commercial facilities need more or less water, this static pressure will bounce around. so even though for our purposes we are in a static or no flow condition, just be aware that a static pressure will fluctuate, and that water is flowing some amount in a typical water supply.
Now when we get into using these graphs to chart fire pump supplies, this no flow condition is considered to be pump churn. A chern condition means that a fire pump is not flowing any water on its discharge side, it is only churning the water and it's impeller but it's not actually flowing any volume of water into a system. The term churn pressure is the pressure that the pump is creating when it is not delivering any volume of water to a system. So important markers here at the zero GPM point is a static point from a flow test or a churn pressure from a fire pump.
now as we move along the X axis to the right, our flow rates are increasing. Because the X axis is to the log 1.85 power, you'll see that 200 GPM is not twice the distance of 100 GPM. If we look at the total width of this chart that goes up to 500 GPM on scale A, you'll notice that halfway down the chart we are at roughly 275 GPM. Half of 500 is not 300, it's 250, but again we've distorted this scale so that R hydraulic relationship between the flow rate and the pressure is revealed as a straight line. for more on that topic check out our last video on what is a log graph.
So, as we're increasing flow rate the next important point along this axis is the residual flow rate from a flow test. When we're flowing water through hydrants as part of a flow test, we will measure the amount of flow that were able to generate out of a hydrant and measure a pressure at that flow rate. This is our residual pressure and our residual flow. We'll talk about this here a little bit more shortly.
Now let's look at the vertical Y axis. At the bottom we start with a pressure of 0PSI. In the United States we use pound per square inch or psi to represent pressure. As we go up vertically along this graph we are increasing in pressure. We reach maximum here of 150 psi. in my personal experience I've never had a flow test with a static pressure beyond 145 PSI, so this scale would have always worked for me.
So now how do we read these log graphs? How do we chart flow test results and read them on these graphs?
For a typical flow test, we will receive three pieces of information, at a minimum. We will receive a static pressure, a residual pressure or sometimes a pitot pressure which needs to be converted over to a residual pressure, and a residual flow. Now there's a whole lot more that goes into a flow test and we'll get into that in plenty of detail in the future, but these are the three critical pieces of information that we get from somebody in the field running a flow test.
As an example, let's say we have a flow test with a static pressure of 90 psi with the residual pressure of 45PSI at 900 GPM.
to chart these points, we first start on the left at a number flow condition and go up the chart until we hit our static pressure. Our static pressure is 90 PS I, so we will chart a dot at zero GPM and 90PSI. This is our static condition.
Now, we start at zero flow along the X axis and move to the right until we reach our residual flow rate. For this test it was 900 GPM, So I am going to use scale C so that I can chart the entire water supply on this graph. We find that point on the X axis, and then move up vertically until we reach our residual pressure, which in this example is 45PSI. We chart a second dot at this residual supply point.
Now here's the beauty of the log graph. We now draw a straight line connecting these two points and extending down past our residual flow and pressure. If this were a normal chart this would be a difficult curve to draw because it is logarithmic on the X axis, we're able to represent that as a straight-line relationship.
This is how we quantify a water supply and read a log graph.
In the upcoming modules will cover interpolation and extrapolation and how we evaluate this available water supply against our system demand needs, finding fire flow, and how we can account for variation in the water supply.
I'm Joe Meyer, this is MeyerFire University.
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