yes totally agree, its the best to measure directly, but again how critical the application, that make big difference.
Even for non-critical applications it is better to use sound engineering practices. People get very frustrated when things don’t operate as expected. I.E. it is obvious the motor is rotating, why does this thing think it is not?
@araasch, you make an excellent point regarding critical or non-critical monitoring in the sense that if the resulting metric is wrong people will not trust the system violating the whole purpose of the system.
Also, a few days ago you make the case for reliable sensing via direct parameters vs. secondary parameters. Regarding @majj_11’s project, is it possible to combine the data from two secondary parameters to produce a synthetic parameter that has near equal robustness of a primary parameter?
For example, if you are measuring the status (On or Off) of a simple exhaust fan like this:
If acceelerometer is above the set tolerance and a pressure sensor is reading air pressure from the fan discharge, the fan motor is on.
1 Like
Agree, but accelerometer is not a primary, or secondary. It is tertiary at best. I.E. a perfect motor would not vibrate. If 5 % of the production motors were of high quality, these motors would last the longest, yet would not be detected to be rotating by detecting accelerometer output. If the motor drove a fan and you wanted to use a secondary parameter sensor to detect this one could use a differential pressure sensor with one of its ports on the input plenum, the other on the output plenum. An alternate sensor would be to detect the position of a moving vane that is spring loaded in the output air stream. The position of the vane would be proportional to the amount of air that was being moved.
Agree with the simplest direct solution - e.g, - I have used vane sensors for airflow monitoring successfully and the side benefit is that you do not compromise the installed system in any way as you don’t need to attach, insert or otherwise interfere with someone else’s equipment ( and warranty 
)
1 Like
yes @shanevanj, that is the point, no need to interfere with someone else equipment, and in my case i have asked what access do i have, the answer is non, so it has to be a complete an independent system.
@shanevanj, you hit on one of my main concerns of monitoring someone else’s equipment. The second you modify or interface with something you become “the last person who touch it” if that equipment ever fails. For this reason, I think we should be more drawn to independent systems as @majj_11 states. Independent meaning self powered, non-invasive sensing.
2 Likes