The Bluelab Pulse has been carefully calibrated to accurately measure conductivity and moisture content in different media types. A variety of factors in the root zone however, can affect the transmission of the radio frequencies that Pulse uses to measures volumetric moisture content, including:
- Medium composition – for example coir absorbs radio wave more than perlite
- Medium density – for example if you have two identical media, but one is a year old and one is fresh in the pot, the older media will tend to be more compacted so will affect radio wave transmission more
- Quantity of roots – the Pulse's radio waves will go through (and get measurements from) the plant roots which are made up predominantly of water, and so the plant rootzone may be interpreted as mostly water
- Type of roots – harder older roots, like those of a tree, tend to absorb more radio frequencies than those of say a small flower
- Size and shape of the container – identical media in two different pots may retain different amounts of water based on the pot shape, size or number and position of drain holes. The same is true for bags, slabs or in the field
The above variations mean that, while Pulse gives results relatively accurate to each other, if you start looking at different media, plants of a different age, etc., you may start to see variances. For example;
- In one media type young plants may have a moisture content of 45-55% to be saturated.
- In the same media that is a year old (more compressed, larger and more roots) you may need 35%-45% moisture content for the same thing.
This is simply because there is less space for the water to go, as a lot of the air spaces and have been filled up by roots or through compression due to gravity over the year.
From a user’s point of view this means you need to understand how one medium behaves compared to another to have a good understanding of what the measurements mean.