Pressure Looks Right, But the Signal is Wrong: Finding the Fault with the Fluke 717

 

A pressure transmitter may look fine at first glance, yet it may be sending the wrong 4 to 20 mA signal to the control system. And if the control system displays a value that doesn’t match what’s happening in the process, where should the check start?

 

When this happens, start by checking the pressure applied to the transmitter against the current it produces. This is where the Fluke 717 Pressure Calibrator can help, because it measures pressure and mA at the same time. It can also supply 24 V loop power and calculate the mA error percentage, so a separate meter or loop supply isn’t needed for the check.

 

 

Where the Fluke 717 Fits

 

The 717 is made for everyday pressure work in the plant, such as checking P/I transmitters, finding out if a transmitter has drifted, or testing a pressure switch. It can measure pressure using its internal sensor or with one of the Fluke 700Pxx pressure modules connected, and it works with non-corrosive gases and liquids through a 1/8 NPT female fitting.

 

One thing to keep in mind is that the 717 doesn’t generate pressure on its own, so a hand pump or another pressure source is needed to go with it. For work that needs automatic pressure generation, documented calibration records or HART communication, the Fluke 729 Automatic Pressure Calibrator or the Fluke 754 Documenting Process Calibrator-HART may be a better fit. To learn more about calibration routines in the plant, read our article on what automation and control engineers need to know about calibration.

 

 

Expected Transmitter Output

 

Before testing, confirm first the transmitter’s configured pressure range. For example, if a transmitter is set to read 0 to 100 psi across a 4 to 20 mA output, these are the expected readings at five points across the range:

 

% of range	Applied pressure	Expected output
0%	0 psi	4 mA
25%	25 psi	8 mA
50%	50 psi	12 mA
75%	75 psi	16 mA
100%	100 psi	20 mA

 

If the transmitter has a different configured range, the pressures will change, but the mA values stay the same at each percentage. And for any point in between, the expected output is 4 mA plus 16 mA multiplied by the percentage of range.

 

Now, if the mA reading doesn’t match the expected output, check the transmitter’s configuration and calibration. But if the output is correct at the transmitter and the control system still shows a different value, the problem is somewhere else in the loop, so check the wiring, the input card and the scaling in the control system.

 

 

 

Calibration Drift

 

A transmitter that has drifted can still respond when the pressure changes, so it’s easy to think nothing is wrong. The problem is its output may be reading a little high or a little low across the range.

 

That’s why it’s better to check several points across the range, like the five in the table above, instead of relying on just one reading. The 717 shows the mA error percentage at each point, so it’s easy to see right away if the output is still within the required tolerance. If the error is about the same at every point, it’s usually a zero error, and if the error gets bigger toward the top of the range, it’s usually a span error.

 

To learn more about why instruments drift and how often they should be checked, read our articles on the common causes of calibration problems and when to calibrate.

 

 

Pressure Switch Testing

 

Just because a pressure switch clicks doesn’t mean it switched at the right pressure. The 717’s switch test captures the set point, reset point and deadband, so those readings can be compared with the switch’s required settings. This is especially important when the switch is connected to an alarm or interlock.

 

 

Setting Up a Transmitter Check

Here’s a simple way to set up the check:

1. Confirm the transmitter’s configured range and the tolerance it needs to meet.
2. Connect the pressure source and the Fluke 717 to the transmitter’s pressure port using a tee, so both of them see the same pressure.
3. Connect the 717’s test leads to the transmitter’s mA loop. If the transmitter isn’t powered by the control system during the test, turn on the 717’s 24 V loop power.
4. Vent the 717 to atmosphere and zero the pressure reading.
5. Apply pressure at each test point, wait for it to settle, then record the pressure, the mA output and the error percentage.
6. Repeat the same points on the way back down, then compare the results with the tolerance.

 

 

Fluke 717 Pressure Ranges

The Fluke 717 comes in different models, and each one has its own internal sensor range. It’s best to choose the model that covers the instruments tested most often.

 

Model	Internal sensor range
Fluke 717 1G	-1 to 1 psi (-68.9 to 68.9 mbar)
Fluke 717 15G	-12 to 15 psi (-850 mbar to 1 bar)
Fluke 717 30G	-12 to 30 psi (-850 mbar to 2 bar)
Fluke 717 100G	-12 to 100 psi (-850 mbar to 7 bar)
Fluke 717 300G	-12 to 300 psi (-850 mbar to 20 bar)
Fluke 717 500G	0 to 500 psi (0 to 34.7 bar)
Fluke 717 1000G	0 to 1,000 psi (0 to 69 bar)
Fluke 717 1500G	0 to 1,500 psi (0 to 103.4 bar)
Fluke 717 3000G	0 to 3,000 psi (0 to 207 bar)
Fluke 717 5000G	0 to 5,000 psi (0 to 345 bar)
Fluke 717 10,000G	0 to 10,000 psi (0 to 690 bar)

 

For work across several ranges, the 717 can also be used with Fluke 700Pxx pressure modules. There are 29 modules to choose from, covering 0 to 1 inH₂O up to 10,000 psi, including differential and absolute pressure.

 

Don’t forget that a pressure source suited to the test range is also needed. We have the Fluke 700HPPK Pneumatic Test Pump Kit, which generates up to 3,000 psi, and our pressure calibration tools page has other pumps, gauges and calibrators to compare.

 

 

Fluke 717 Specifications

Pressure accuracy (internal sensor)	0.05% of full scale
Pressure modules	29 Fluke 700Pxx modules, 0 to 1 inH2O up to 10,000 psi
Engineering units	11, including psi, bar, mbar, kPa, inH2O, inHg, mmHg and kg/cm2
mA measurement	0 to 24 mA, 0.001 mA resolution, 0.015% + 2 counts accuracy
Loop power	24 V DC
Functions	Zero, min, max, hold, damp, pressure switch test, mA error %
Pressure fitting	1/8 NPT female
Media	Non-corrosive gases and liquids
Operating temperature	-10 °C to 55 °C
Size and weight	201 x 98 x 52 mm, about 600 g with holster
Power	Single 9 V alkaline battery
Warranty	3 years
Included	Protective holster with test lead storage, test leads and alligator clips, 9 V battery, instruction sheet

 

 

Choosing the Fluke 717

For technicians who regularly check pressure transmitters and switches in the field, the Fluke 717 is a practical tool to have. Since it measures pressure and mA and supplies loop power in one unit, most checks only need the calibrator and a pressure source.