Plastic reservoir material gives different PH reading?

[headfortrinity]

I've noticed that when I use my hanna ph meter to ph water, I get different readings depending on the type of material the container is made out of (pp,hdpe,pete)? I tried searching but found no reference.
I'm thinking it has something to do with the density of the plastic, because when you take the probe off the bottom of the container and hold it suspended in the water you get an accurate reading. Has anyone else noticed this? or am I over medicating

 

[Surrender]

I believe the Hanna meters attempt to compensate for the temperature of the solution. Actually I think most PH testers do this.

Can you put a thermal probe down at the bottom of the reservoir and see if it's significantly cooler down there than in the middle?

 

[headfortrinity]

I believe the Hanna meters attempt to compensate for the temperature of the solution. Actually I think most PH testers do this.

Can you put a thermal probe down at the bottom of the reservoir and see if it's significantly cooler down there than in the middle?

I experiment with different styles of growing and I'm using a perpetual system, so I use a lot of smaller containers for reservoirs or mixing vessels, mainly 5 gal buckets, rubbermaid totes and some smaller cups like a pete keg cup or a polypropylene tupperware style container depending on what I"m testing, clone bubblers, soil containers, seedlings.
The temp in the containers is the same top to bottom and I always wait for the temp compensation before I read, yet different containers will register different ph readings depending on the plastic material, water volume also changes the readings. I've tried pouring the same water in the different containers and even the same water in similar containers only different water levels and the ph reading changes.

 

[PharmaCan]

I used to have a TV on the counter where I tested pH. When the TV was on, the meter would jump all over the place. I think light affects the meter. You could be getting different readings because the containers are letting in different amounts of light.

PC

 

[headfortrinity]

I used to have a TV on the counter where I tested pH. When the TV was on, the meter would jump all over the place. I think light affects the meter. You could be getting different readings because the containers are letting in different amounts of light.

PC

That's a good point. I'll blacken out the containers and test them again.