To test the absorbencies of different fabrics, I took 10cm (4″) square pieces of the (prewashed) fabrics (I didn’t prewash the zorb as you don’t need to)
I compared the weight of the samples when dry, and then let them absorb as much hot water as they can to make sure all the fibres were saturated (left them soaking in a bowl of water for a minute, squeezed the water out then put it back in to absorb more, repeated then left that to soak for a few more minutes). I then lifted them out of the bowl by one corner and waited until they stopped dripping – and then weighed them to see how much liquid they had absorbed. Comparing the wet weight to the dry weight.
Results are here:
Zorb (original) – Dry 3g – Wet 25g – Absorbed^ 22g
Bamboo/Cotton fleece (400gsm) – Dry 4g – Wet 23g – Absorbed 19g
Cotton flannel (2 layers) – Dry 4g – Wet 19g – Absorbed 15g
Bamboo/Hemp fleece 320gsm – Dry 3.5g* – Wet 19g – Absorbed 15.5g
Hemp/Cotton fleece 350gsm – Dry 4g – Wet 19g – Absorbed 15g
^ The zorb initially held a lot of water, but (like a sponge), the water just poured out of it for a long time. So while it can hold onto a lot of liquid, unlike the natural fibres, it can’t keep it all (it’s not “absorbing” as such, more “holding”). It’s not actually trapping most of it inside the fibres, the water will stay there only if there is nowhere else for it to go. (Which is why it can “compression leak”).
* my scales are only accurate to full grams, this sample kept bouncing between 3 and 4 grams. So I’m putting it as 3.5 g
So what does this mean? – How absorbent is a cloth pad?
That test above is only how much the fabric will absorb without dripping. And was really just done to show the relative absorbency levels of certain fabrics. In actual pad usage it’s not that simple. Pads aren’t bled onto in the same way as soaking fabric in water. So calculating the actual absorbency of a pad is a lot more complicated, but knowing the absorbency level of fabrics can help you get a rough idea.
First, to put this into reference with menstrual flow…
The “average” blood loss is said to be about 10-80ml of blood over the entire period – although there is some debate as to whether that measure is only counting blood or other vaginal secretions, as many find their flow is well over 80mls with no cause for alarm. Given discussions in different menstrual cup communities have some users who need to empty a large cup more than once on their heavy days, I think it’s fair to assume that you could expect around 1-2 menstrual cups (of about 30mls each) worth of menstrual flow a day on average for “regular” or “moderate” flow (given some will be lighter than this and some will be heavier, and making a pad more absorbent than you think it needs to be is far better than having it not absorbent enough). Also you’re expected to change your pad every 3-4 hours for hygiene reasons.
So….. as a VERY rough guide just for ease of calculation – a regular day may only be about 30mls or less over the whole day (not counting overnight) and you’d likely be wanting 3 pad changes during the day. So theoretically each pad needs to hold about 10mls.
The Water test
The best way to check the absorbency of a cloth pad is to pour water onto it. You need to use warm water, as for some reason this soaks in more realistically, since blood is also warm and the pad being worn will have the compression of your body against it, which helps the blood soak into the fabric.
Using a small measuring device (like an eyedropper or medicine cup that does mls) – replicate a menstrual flow onto the pad by pouring a small amount of water into it. Start with no more than 5mls at a time. Remembering that it’s unlikely the blood will be poured evenly over the entire surface, for many people there is a small (5-10cm or 2-4″) area where the majority of the flow is centered. So it’s best to do any testing assuming the worst case scenario (smaller surface area).
If the pad has PUL or DWR treated windpro, then the core can become saturated before the pad will leak through, so it becomes more of a matter of the top feeling wet and how long the pad is worn. Rather than if the pad leaks through. However if the core isn’t absorbent enough for the flow then it can start to leak along the wing out towards the edges of the pad. So waterproofed pad still need to be absorbent enough.
If the pad has no PUL/Windpro, then you need to be able to measure how much liquid the pad holds before any dampness at all is detected on the underside of the pad. If you feel any dampness of water, remember that this will be a red blood stain when the pad is being used – so you don’t want ANY leaking at all. Remembering a pad is worn while people are sitting, so it needs to hold the blood without compression leaking – test for this by placing something heavy like a cup of water onto the pad.
Using my fabric weight calculations
I measured by weight (grams) and flow is measure in volume (mls). 1ml of water is equal to 1g, but blood is slightly heavier than water. For this though we’ll just stick with 1ml = 1g.
If you think about the core in a 25cm/10″ long pad, the core inside is generally a strip down the centre about 5cm wide by 20cm long, so 1 layer of core for that length pad would be about the equivalent to one of the 10×10 squares of fabric I tested.
For a “light” pad I’d recommend 1 layer of bamboo (or hemp) fleece core, so that would equate to about 19mls liquid total*
For a “Regular” pad I’d recommend 2 layers of bamboo (or hemp) fleece core, so that would equate to about 38mls liquid total*
For a “Heavy” pad pad I’d recommend 3 layers of bamboo (or hemp) fleece core, so that would equate to about 57mls liquid total*
*This is really not a practical amount, see below.
However it is worth noting that while the entire core could physically hold that much liquid – if there’s no PUL behind it, then that liquid would have leaked through well before it could get to that level of absorption. Even if you had PUL in the pad it is HIGHLY unlikely for the whole core to get the chance to absorb that much blood, as people don’t bleed evenly all over the pad. Most people bleed through the pad in only a small area, so in a 25cm/10″ pad, the actually used surface area of the core may only be about 10cm/4″ (it may be even less). Even if the person did bleed evenly all over the core, and if the core was to hold the full amount of liquid then the pad would feel extremely wet and sodden.
No pad will realistically hold anywhere near that much liquid in a practical sense.
So when considering the absorbency of a core fabric, given the likelihood of only half (or less) of the pad having the majority of the flow centered on it, if you allow the core to become saturated (so a PUL layer would be necessary), you’d have to halve that – making it more like:
“light” pad (bamboo core) – equates to about 9mls
“Regular” pad (bamboo core) – equates to about 19mls
“Heavy” pad (bamboo core) – equates to about 28.5mls
Now, that’s still allowing for the core fabric to become completely and evenly soaked through, which isn’t going to happen in most cases – for some people, they may only bleed heavily into the pad in a spot about 5cm/2″ long, and if the pad doesn’t have PUL in it then the core can’t become that full or it can leak. Using a synthetic fleece backing is “leak resistant” (the DWR treated windpro can be as effective as PUL) but can still leak through if the core is wet enough. So for a pad without PUL or DWR windpro, then you should probably halve that again, making something more like:
“light” pad (bamboo core) – equates to about 4 mls
“Regular” pad (bamboo core) – equates to about 9mls
“Heavy” pad (bamboo core) – equates to about 14 mls
If you have no synthetic fleece backing and just something like cotton, it could very well be even less.