There is insufficient information to completely answer the question.
I assume you are concerned about EMI from external sources affecting your thermometer -- not the other way around.
Questions to ask:
- What kind if EMI is your system going to be exposed to?
- What exactly are you worried about? Damage to the uC, damage to the
sensor, both?
- Corrupted Data?
- Consequences of actual EMI. E.g. danger to equipment or humans, down time, cost.
Taking these in turn:
Likely sources of EMI are from switching transient of high power local loads, especially inductive ones (motors). Nearby high power RF transmitters may also cause problems but cell phone tower are usually not an issue. Static electricity discharge can damage electronic devices too, but that is independent of the cable length.
Damage to the uC and sensor is unlikely if your sensor is connected to the uC by 3 wires in the same cable. Any noise will be mostly "common mode" (all wires are equally affected). You can put a few turn of the entire cable through a ferrite ring core if you are really paranoid.
A suitable RC filter on the uC pin(s) connected to the DQ signal may be appropriate. The details depend on your circuit. E.g. do you use 1 pin with direction control or separate read/write pins, etc?
If your concern is that you might get incorrect readings, then the CRC in the data stream will allow you to detect that and discard occasional bad readings.
Consequences of EMI: EMI protection can be expensive. If there are no dangerous consequences then replacing your thermometer may be the most economical option.
Summary: Your circuit may be fine as is, nothing further required.
The Maxim application note on 1-wire networks may help. See Appendices. The circuit in Appendix B looks appropriate for your application. Guidelines for Reliable Long Line 1-Wire® Networks
Tazer. Buy them from Amazon.com for as little as US$11.
The correct method for doing this is with expensive lab equipment that costs tens to hundreds of thousands of dollars. Anything less is a major compromise, to the point that you might as well have a Tazer.
We do our own ESD testing, which is similar to using a calibrated Tazer. And we will put our devices near known common RF emitters (Microwave oven, CB Radio, WiFi, Cell phones from various manufacturers). Of these tests, the ESD testing is the most informative.
Otherwise, we leave the EMI susceptibility testing to official labs, since we almost never fail this test.
Best Answer
The short answer is no! You cannot extrapolate from close proximity E field (or B field) measurements to a proper test set up in a lab. Near field is the region where em radiation need not be both E and B fields, but in the far field, after some propagation distance proportional to the wave length, the E and B fields merge (as in they are both present). You can have a radiator emmiting B fields which are invisible to an E field probe up close, but would be picked up by an E field probe in the far field measurement. The reverse is also possible with B field probes.
The long answer is it depends on the source, the fix, and the testing standard.
You need power and antennas to radiate at any given frequency, which means in practice you can either have high currents or long current paths causing your failure. Given the high frequency I'm going to guess a good radiator rather than a high current. If your fix involved snubbing or terminating (ie, stopping the noise at source) then you've got a good chance of seing some improvement, but if your fix involved tin foil or ferrites (ie, blocking the noise from getting to your probe) i would say its 50/50.
Be aware that the test houses I've worked with have a 6dB error tolerance, and things can easily change by that much just by a different setup of the same equipment. (Different cable routing etc). 3dB isn't all that much in the grand scheme of things. It could easily be just the probe on your bench in a slightly different position.