HP T100 Specifications download pdf (Page 22)

SECTION IV CALIBRATION

Field Adjustments:

NOTE: Any change to the factory calibration will require the operator to re-check and

record the new diagnostic shunt value as defined below. Caution is advised before making

adjustments. Please read the entire section before making any adjustments.

The zero and span have been precisely set at the factory. It is recommended to check the zero

after first coupling to the inlet pressure face seal port (VCR®) or after welding to the tube stubs

for a butt weld model. This is especially true when using ranges of 100 psig or below. Normally

the span needs no adjustment.

Use of accurate and calibrated equipment is needed for this step. Wait up to 20 minutes warm

up for optimal accuracy after supplying power. Apply zero pressure for 0 psig- based units or

full vacuum for compound units and check for 4 mA output. Only a slight change to the zero pot

may be needed.

For psig units, full vacuum is defined as -14.7 psig or -1 bar for bar ranges and factory calibrated

as such. It is up to the user to define the required full vacuum level desired. If full vacuum cannot

be achieved due to equipment or barometric pressure limitations, the zero pot can be calibrated

at an alternate pressure such as -13.7 psig. In these cases, be sure to calculate the new offset

above 4 mA. This can be accomplished as follows:

Calculate the slope of output current vs. pressure:

Slope = mA span output current / pressure of full span for 4-20 mA circuits or

V span output voltage/pressure of full span for 0.1-5.1 Vdc circuits

The elevated zero at atmospheric pressure for a 4-20 mA. unit is:

Elevated Zero = 4 mA + (slope) x (abs. value of full vacuum. - abs. value of available vacuum)

As an example, assume that -13.7 psig is available and the desired zero level corresponds to -14.7

psig. Full scale is 30 psig.

The zero output at -14.7 psig is 4 mA. The full scale output at 30 psig is 20 mA.

Slope = (20 mA-4 mA)/(14.7 psig + 30 psig) = 16 mA/44.7 psig

The reading at -13.7 psig = 4 mA + ( 16/44.7) x (14.7-13.7) = 4.358 mA

To check span, first make sure no pressure leaks are present before making this calibration.

Next, apply full scale pressure from a traceable, certified and calibrated pressure source and

adjust to 20.00 mA output if required. Recheck zero and adjust again if necessary.

pg. 4-1

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HP T100 Specifications Page 22