I should have checked this a long time ago...

Keep in mind IR thermometers are sensitive to the reflectivity/emissivity of the materials they are used on, and the shiny PEI surface can be a bit tricky. Try measuring the temperature of a mirror that your warmed up with your hand and you’ll see a similar problem.

If you want to test more accurately, put some matte tape on the bed and then measure the temperature then.

That said there is also a chance for the heat bed to be a bit lower than target temperature, since the temperature control and feedback occurs at the bottom of the bed where the thermistor is, not at the top, and is separated by a layer of magnetic adhesive and coated metal.

The actual number is mostly meaningless. If you have adhesion problems just bump the set temp up by a few degrees. Also the temp will vary across the plate but also vary over time. As the heat soaks into the plate it will be more consistent across it so if there are hot spots now, it's not a huge deal.

Well, you are supposed to adjust your slicer until you get a temperature where the plastic adheres consistently.

Knowing the real temperature doesn't actually help, as the performance will vary with coating, printer placement, z-level adjustment, etc.

I wouldn't worry about the value difference too much but more the consistency. If you need 75 degrees on the edge then keep dialing up the temp until you get it.

It's most likely only going to affect large prints at the end of the day.

Your printer will measure the bed from a single location so moving your IR sensor will vary across the bed. If your using a custom firmware I'm sure you could set this as an offset assuming your IR thermometer is accurate ;)

Heat of the bed will be the warmest point and the periphery cooler (just like your body temp)

A lot of printers also heat the bed unevenly, so the temperature could be significantly higher in the centre of the build plate. This video shows it fairly well. You could try adding a few minutes delay to your start gcode after the printer thinks it's reached the correct bed temp, to let it even out a little.

Adhesion I'd totally recommend cleaning your surface with soap and water (surface depending) and then messing with temps. With abs and petg I've found that even a bit of finger print residue is enough to cause a part to come away from the bed, next I'd check first level height. I've very rarely found the bed temp to be the reason I lost adhesion.

I think you might be using the IR gun incorrectly.

The area which is being measured may be partially off the edge of the bed.

Yeah tbh it didn't really matter what the number is tube by results not input. If you only get bed adhesion at 80°C there's probably an underlying issue but rock what you got. Haven't tested mine there's probably a variance off true value for a lot of people. No one can deny the results you observe. If people say say it to 75 and that's too low based on your observations either fix your printer if you trust them or crank her up.

I wonder if gluing down a thin layer of borosilicate glass would even out the hotspots as it comes to temperature? maybe place it between the magnetic base and the heater. it'd take longer to come up to temp, though.

that said, I expect my beds to be 10 degrees cooler than the thermister says- simply because the thermister location is usually closer to the heat source and covered from ambient air. it's all relative anyhow, and every brand/formulation of filament is going to be slightly different as to what works best.

An enclosure might help a bit, but not a lot. What are you using for a plate, just spring steel? You will lose a small amount of heat to that. If it’s glass and steel, then you will lose more. The temperature sensor is pretty close to the actual heating element, and the heating coil doesn’t cover the entire bed, so there will be hotter areas and cooler areas—like an electric griddle if you’ve used one of those in a kitchen before.

Glass helps because it’s a good thermal conductor, but because it’s another layer to heat (and lose heat into) you need to bump the temperatures a bit, and it also comes with the caveat of weight, which isn’t ideal on a bed slinger.

Overall though, if you’re printing PLA and having bed adhesion issues at 68 degrees the temperature probably isn’t the issue.

That is wicked smaht

Metal flex sheet is going to conduct heat differently than the Creality standard surface plus it's shiny metal meaning it's difficult to measure with IR If you are having issues, then up the bed heat

The edges are going to be cooler if everything is working the way it's designed (especially on a bed slinger), since the heater doesn't go all the way to the edges and because of increased airflow. One of the reasons I was thinking about getting a Prusa XL is because it has multiple independent bed heaters.

An enclosure will help some. When I'm printing near the edges I also up the bed temp 5 to 10 degrees depending on room temp.

What the heck are you printing that requires 75C of bed temp with no walls?