# Instrument zero point checking and adjustment method

Instrument zero inspection and adjustment is a skill that instrument operator must master. This article introduces the method of checking and adjusting zero point of pressure transmitter, differential pressure transmitter, thermocouple display instrument and thermal resistance display instrument, It is of great help to the instrument operators for improving their technical level. When the operator has doubts about the indicator value of the instrument, it is necessary to check the instrument, the most commonly used method is to check whether the zero point of the instrument is correct. For pressure transmitter and differential pressure transmitter, check zero point after stopping process. The zero point of the instrument refers to the lower limit of the instrument's measuring range (that is, the minimum value that the instrument can measure under its specific precision).

The purpose of adjustment is to make the upper limit of the transmitter’s output signal correspond to the upper limit of the measurement range, Equivalent to changing the slope of the output characteristic of the transmitter. The purpose of zero adjustment and zero migration is to make the lower limit of the transmitter output correspond to the lower limit of the measuring range. Zero adjustment makes the starting point of transmitter’s measurement be zero, Zero-point migration is to transfer the starting point of measurement to a certain value(Positive value or negative value). Zero point transferring and adjustment can improve the measuring accuracy of the instrument. The adjustment amount of zero point adjusting is usually small; while the adjustment amount of zero point migration is large, which can reach one or several times of the range.

1. Pressure transmitter

When checking zero point, the pressure should be removed first, so that the transmitter is in a state of no pressure. As for the condition that pressure tapping pipe is longer and the transmitter’s installation position is below the sampling point of vapor pressure or liquid pressure, even if the pressure is removed, there will still be condensed water or liquid in the tapping pipe, Due to the static pressure of the liquid column, the current that the pressure transmitter outputs will be greater than the output current that zero point is corresponding to, this problem need to pay attention to, can drain the liquid column to solve. It is also necessary to confirm whether positive migration measures have been taken to offset the effect of static pressure, to avoid wrong adjusting the normal zero point.

1. Differential pressure transmitter

Before checking the differential pressure transmitter’s zero point, firstly close the negative pressure valve of the 3-way valve manifold, then open the balance valve; then close the positive pressure valve. The function of opening the balance valve is to make the pressure difference between the positive and negative measuring chambers of the differential pressure transmitter equal to zero. For the differential pressure transmitter without migration, observe whether its output is 4mA, otherwise need to do zero adjustment.

1. Thermocouple

On the site, it is impossible that the cold end temperature of thermocouple is equal to 0℃, most display meters have the function of automatic temperature compensation at the cold end. Therefore, the checking method usually used is to short-circuit the input end of the display instrument and observe whether the display value of the instrument is room temperature, the instrument is normal if the display value is room temperature. Of course, room temperature is just a popular term, strictly speaking, the display value of the instrument after shorting-circuit the input end is the ambient temperature near the input end of the instrument.

When the operator has doubts about the displayed value on the temperature instrument, DC potentiometer or other standard meter (such as process calibrator) can be used, first measure the thermoelectric potential Ux of the thermocouple, then according to the reference end, or the temperature value of room temperature, check the thermocouple type table, the thermoelectric potential U0 corresponding to the temperature is obtained, and then add Ux and U0 to get the total thermoelectric potential, then check the thermocouple type table to get the true temperature measured.

For example:

There is a S type thermocouple, the measured thermoelectric potential Ux of the thermocouple is 12.94mV, room temperature 28℃, look up thermocouple type table U0 = 0.161 mv, so Ux + U0 = 12.94 + 0.161 = 13.101mV, look up thermocouple type table, the actual temperature is 1295.2℃.

1. Thermal resistance

When the operator has doubts about the displayed value on the temperature instrument, a dc resistance bridge or other standard meter can be used to measure the resistance value of the thermal resistance, then look up thermal resistance type table, obtain the temperature corresponding to the resistance value, thus, can judge the error of the instrument. You also can use a dc resistance bridge to replace the thermal resistance to output the resistance value for checking the display instrument to judge whether the instrument is normal. When the thermal resistance or wire contact is poor, the display temperature on instrument will be higher than the actual temperature. If the display temperature is lower than the actual temperature, maybe it has short circuit phenomenon.

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