Flow sensor impact on flow

Does anyone have a sense of how much an inexpensive in-line flow meter (like this http://jaycar.com.au/products_uploaded/ZD1200.pdf) would actually impede flow? I have an application where I need to get the highest flow possible and ideally would use some kind of sensor outside the hose but I believe those are very expensive. If something like the above is only going to reduce flow by 1% I could live with it but not 10%.

I realize I could just buy one and do a quick experiment but thought maybe someone had already been through this.

This depends a lot on factors like the pressure you're putting at it and what the flow is actually (without the sensor). If the sensor is oversized it wouldn't put much resistance into the flow and so not much influencing it. But if you already use the full capacity of your tubes and perhaps install a flow meter with a smaller diameter it surely will influence your flow.

If the pipe is long or turns lots of corners, you can use a differential pressure sensor to measure the existing drop in pressure due to friction. The flow rate will not be affected.

you can also split your pipe in a big and a small pipe and measure the speed in the small pipe. it would affect the main flow at max to the ratio of the diameters of the pipes. If the small pipe is 1/100 of the diameter of the big one a complete stop would have 1% impact. If it still flows it would affect less.

Problems you need to solve with this solution: 1) you must callibrate the readings with the real flow (l;ookup table) 2) the small flow can be too small to be readable (but then you can use a smaller flowmeter (e.g. centiliter iso liter) 3) the spilt and merge of the flow can/will cause turbulence which might influence the system 4) depending on the viscosity, and the purity of the liquid (is it water, oil, slurry etc) side effect may happen..

The sensor you quote looks like a non-contact turbine using a hall device to count rotations. It is one of the lowest loss sensors you can get so I'd suggest ideal for your job - whatever that may be. Similar (but larger) are used for accurate measurement of various fluids and gases in industrial commercial metering

How does the 'non-contact' part work if it is a turbine? Wouldn't you have to have a least a bit of the turbine in the flow (and therefore incurring losses because there has to be energy transfer for it to work)? Perhaps the point is that it is pretty small.

The actual application is flooding a backyard ice skating rink...you essentially fashion a shallow swimming pool, fill it up and let it freeze. It takes me about 20 hours to get 30,000 litres from my household supply. I was interested to see how flow changed during that time as other activities (showers, toilets etc.) put demands on the system.