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Frequently， new machines are put into service with a ""startup screen"" in the compressor inlet piping to guard against the inevitable weld slag and construction debris that will remain in a new or rebuilt piping system afterconstruction. Regardless of the age of the installation， care must be exercised to ensure that measurements defining suction or discharge conditions are not influenced by such devices.
Inlet and discharge pressures are defined as the stagnation pressures at theinlet and discharge， which are the sum of static and velocity pressures at the corresponding points. Static pressures should be measured at four stations in the same plane of the pipe as shown in the piping arrangements. Velocitypressure， when less than % of the pressurerise， can be computed by the formula
旦 v)2
pv =匹旦 v)2P =匹P 匹20-1)
2gc x 144 9266.1
where旦 v is the ratio of measured volume flow rate to the cross-sectional area of the pipe.
When the velocity pressure is more than % of the pressurerise， it shouldbe determined by a pitot-tube traverse of two stations. For eachstation， the traverse consists of 10 readings at positions representing equal areas of thepipe cross section， as shown in Figure 20-2. The average velocity pressure pv is given by
P.旦3 pv = 288gcntp旦 v匹20-2)
where at each traverse point
...................
旦p = 9266P.1pv 匹20-3)
and nt equals the number of traverse points.
Barometric pressure should be measured at the test site at 30-minute intervals during the test.

Figure 20-2.Tra.erse .oints in .i.e. (Power Test Code10， Compressors and Exhausters， American Society of MechanicalEngineers， 1965.)
Temperature Measurement
Temperature may be measured by any of the following instruments:
1. Mercury-in-glass thermometers

2. Thermocouples

3. Resistance thermometers

4. Thermometer wells

Thermocouples are the preferred type of instruments because of the sim-plicity in basic design and operation. They can attain a high level of accuracy，are suitable for remote reading， and are robust and relatively inexpensive.
Regardless of the temperature-measuring device to be used， on-site cali-bration of the entire measurement system is desirable. Usually， a two-pointcheck can be made by employing frozen and boiling water. At the veryleast， all devices can be checked at a commontemperature， preferably in the midrange of expected temperatures so that any deviant devices can be discarded. This check is particularly desirable for low-head machines where the temperature rise will be slight.
Test plans frequently are prepared on the assumption that a laboratory thermometer can replace an operating instrument in an existing thermo-meter well. While this change may be satisfactory， the prudent tester needs to be aware that because of the propensity of thermowells to break off andperhaps enter the machine or cause a hazardousleak， their design is com-promised such that true gas temperature determination is impossible. The compromise may be to make the well short and/or to make it thick-walled. In either event the mass of metal exposed to ambient temperature may exceed that exposed tothegas， resulting in significant error if the gas temperature is much different from the ambient. High-pressure systemsrequiring thick-wall pipe are particularly susceptible to this fault. However， the use of a good heat-transfer fluid can minimize the error. The best gas temperature reading is attained by a calibrated fine-wire thermocouple with the junction directly exposed to the gas near the center of the flow. Asdeviations from this ideal aremade， the potential for error is increased.
Inlet and discharge temperatures the stagnation temperatures at therespectivepointsandshouldbemeaaresured within an accuracy of 1 0F
(0. 0C). When the velocity of the gas stream is more than 12 fps (36.6mps)， the velocity effect should be included in the temperature measurement with a total temperature probe. This probe is a thermocouple with its hot junction provided with a shielded cup. The cup opening points upstream. A trade-off has to be made in a field test situation where the gas is not clean.
　
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