RTD Output

A bad connection in the resistance measuring circuit of an RTD circuit will increase the total resistance, so the RTD circuit would have more resistance in it that it should have resulting in a false higher temperature indicated reading at the receiver.

However, an increase in resistance in the 3rd wire of a 3 wire RTD circuit that does lead wire compensation drives the indicated temperature lower.

Hence, you are observing the effect of higher resistance from a loose connection in the RTD lead wire compensation circuit.
Lose connection results in air gap which causes the resistance to increase. That increase in resistance is responsible to show increased temperature accordingly.
Not enough information to help you. It does not read right because it is a loose connection. How does the screw get loose or is that your question? Do you torque the terminal to the manufacturer’s recommended torque?

William (Bill) L. Mostia, Jr. PE
ISA Fellow

Winner of the 2017 ISA Raymond D. Molloy Award

Sr. Functional Safety Consultant
SIS SILverstone, LLC
>RTD showing low reading when we touch the terminal lose and
>tight it's become normal what is the reason of it it's
>always like that?

I sincerely hope what you typed is not what you meant to say. Usually, when any connection of a sensor is loose and not making contact the feedback from the sensor goes low. (There are some special cases and some special monitors that cause the reading to go high on an open circuit, but those are not the normal situation.) What do you expect the input (feedback) to do when one leg of the sensor circuit is not making contact (is loose)?

Do you have two-wire, three-wire or four-wire RTDs?

Do you understand how an RTD works? If you don't have access to wikipedia, you can go to omega.com and there are some excellent descriptions of RTD circuits. You can probably find lots of videos on YouTube, also, about RTD circuit and how they work.

But, why even have wires connecting a sensor to some monitor or piece or equipment if, as you seem to believe, an open circuit (loose) should not result in a false reading? Why not just put the sensor in place, and program the monitor or equipment to read the device without wires?