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『Siemens Mag Flow Meter Error F70 Coil Current: Fault Diagnostics Guide』Related information(flow rotameter|digital flow meter|balo meter|hydraulic flow meter|volumetric meter|positive displacement meter|pitot tube flow meter|displacement meter|cfm meter|variable area flow meter|inline flow meter|differential pressure flow meter|calibrated flow meter|velocity meter|paddle wheel flow meter|oval gear flow meter|anemometer hvac|multiphase flow meter|doppler flow meter|thorpe tube flowmeter|heat flow meter)
Siemens Mag Flow Meter Error F70 Coil Current: Fault Diagnostics Guide
Quick Answer: Error F70 on a Siemens electromagnetic flow meter indicates that the transmitter detects a problem with the coil current circuit. The root cause is usually a broken coil circuit, a shorted cable, moisture inside the sensor terminal box, or a failing transmitter circuit board. Most faults can be fixed on site without sending the meter back to the factory if you follow a structured check of resistance values, cable insulation, and supply voltage.
When a Siemens Mag 5000 or Mag 6000 transmitter flashes Error F70, the coil excitation current is outside the expected window. The transmitter tries to drive a known current through the field coils and measures the actual current flowing. If the measured value drifts above or below the limit, the converter locks out and shows F70. In practice, this error stops flow measurement completely. The analog output typically drops to 3.6 mA and the empty pipe detection may also trigger.
We have seen this fault many times on customer sites. A water treatment plant in the Philippines had a Mag 3100 sensor buried in a pit. After heavy rain, the terminal box flooded. The coil circuit shorted internally and the transmitter threw F70. Another case involved a brewery in Vietnam where the signal cable was run inside a cable tray that also carried a VFD motor cable. The induced noise corrupted the coil current measurement and caused intermittent F70 alarms.
Before you open the sensor housing, power off the transmitter and wait 2 minutes. The coil excitation on a Siemens meter can reach 40 V peak. If you measure resistance with a multimeter while the transmitter is energised, you risk damaging your meter or getting a false reading. Always work safely and follow your site electrical permit system.
Step by Step Coil Current Diagnostic
First, identify your sensor model. The most common sensors paired with Siemens transmitters in the field are the Mag 3100, Mag 3100 HT, Mag 5100 W, and Mag 1100 Food. The coil resistance changes with sensor size, from around 10 ohm on a DN15 to about 120 ohm on a DN300. If you do not have the factory test sheet, ask Silver Instruments for the expected coil resistance. Send us your sensor serial number and tag number to [email protected].
Open the sensor terminal box. Look for moisture, corrosion, or insect activity. In tropical countries like Indonesia or Nigeria, we see many sensors with water inside because the cable glands were not tightened after wiring. Wipe the terminals dry and inspect the O ring on the lid. If the terminal block is cracked, change it before proceeding. A cracked terminal block can let moisture creep into the coil circuit over weeks and give you a false resistance reading.
Next, disconnect the coil wires from terminals 85 and 86 (older sensors) or terminals C and D (newer Mag 3100). Mark the cable colours before you disconnect. Siemens uses three wire coil excitation: two outer wires drive the current and the centre wire is a shield or return. Measure the resistance between the two outer coil wires with a multimeter set to the 200 ohm range. The reading should match the sensor factory value within plus/minus 10 percent. If the reading is open circuit or above 10 megaohm, the coil circuit is broken inside the sensor body. This failure is not repairable in the field. You need a replacement sensor.
If the coil resistance is correct, measure insulation resistance between one coil wire and the stainless steel sensor body. Use a 500 V DC insulation tester. A healthy sensor shows more than 20 megaohm. Anything below 2 megaohm points to moisture ingress into the coil housing. In some cases, baking the sensor at 60 degrees Celsius for 24 hours can bring back the insulation resistance, but this is a temporary fix. A sensor with low coil insulation will fail again in humid weather.
Check the Transmitter and Wiring
If the sensor checks out, the problem moves to the transmitter or the cable. Disconnect the coil cable at the transmitter end too. Measure the loop resistance of the cable from transmitter to sensor and back. On a typical installation with 50 metres of 3 core shielded cable, you should read less than 5 ohm. High loop resistance can be caused by loose screw terminals, green copper corrosion inside the cable, or a damaged junction box along the way. Tighten every terminal and recheck.
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Most engineers skip this part: measure the coil cable capacitance. If the cable capacitance is above 10 nF, the transmitter coil driver cannot settle the current waveform correctly and F70 appears. This often happens when someone extended the cable with a different cable type in a junction box. A sugar refinery in Guatemala once used a spare telephone cable to extend the coil circuit. The capacitance was 47 nF and the transmitter locked out every 15 minutes. Replace the cable with the right type, Siemens part number FDK-083F5046 or equivalent, to fix this.
The transmitter coil drive circuit sits on the main board. If both the sensor and cable test fine, suspect the transmitter. A quick way to test is to connect a known resistive load across the transmitter coil output terminals. For a Mag 6000, a 47 ohm 10 W power resistor across terminals 85 and 86 works as a dummy coil. Power on and check if Error F70 clears. If the error disappears with the dummy load, the transmitter is fine and the fault is in the external circuit. If the error stays, the transmitter needs repair. Silver Instruments can supply a refurbished Mag 5000 or Mag 6000 transmitter with a six month warranty. Tel: +86-135-1213-1526, Whatsapp: 86-135-12131526.
Common Field Scenarios That Trigger F70
Scenario one: a chemical plant in Saudi Arabia reports F70 only when the process temperature rises above 80 degrees Celsius. The sensor coil resistance increases with temperature. If the coil resistance at ambient temperature is already at the high end of tolerance, a hot process can push it outside the trip window. The fix is to check the winding temperature coefficient with the sensor data sheet and adjust the transmitter reference value via the service menu.
Scenario two: a marine ballast water system on a vessel registered in Singapore shows F70 after dry dock. The sensor cable was pulled too hard during maintenance and the shield drain wire broke inside the cable gland. Without a solid shield connection, the coil current waveform picks up VFD noise from the ballast pump drive and the transmitter detects a distorted current. Re-terminating the shield at both ends solves it.
Scenario three: a dairy factory in New Zealand has intermittent F70 on a CIP return line. The sensor is installed in a pit that floods with cleaning chemicals. The coil terminals corrode slowly. The error appears only during the rinse cycle when the pit humidity is highest. Upgrading the cable gland to an IP68 rated type and filling the terminal box with silicone gel potting compound stops the corrosion permanently.
FAQ
Can I clear Error F70 by power cycling the transmitter?
You can try, but the error will return if the underlying fault is still present. The transmitter runs a coil current check every 30 seconds. If the condition remains, F70 comes back. Power cycling is a diagnostic step, not a fix.
What is the acceptable coil resistance for a Siemens Mag 3100 DN100?
Most Mag 3100 DN100 sensors have a coil resistance between 30 ohm and 55 ohm at 20 degrees Celsius. Always check the original test certificate for the exact value. If you lost the certificate, email us the serial number and we can look it up for you. Email: [email protected].
Can I measure coil current with a clamp meter while the meter is online?
Yes, but only if you use a true RMS AC/DC clamp meter with millivolt resolution. The coil current is a pulsed DC square wave at 6.25 Hz typically. Clamp one of the coil wires and look for a stable reading between 125 mA and 500 mA depending on the model. If the current jumps or drops to zero, the fault is active.
Does F70 damage the flow sensor or the transmitter?
The error itself does not cause damage. It is a protective response. However, if the root cause is a shorted coil wire that keeps drawing excessive current, the transmitter coil driver can overheat and fail. So investigate the error within a few days.
We need a replacement sensor quickly. What details should we send?
Send a photo of the nameplate, the pipe size (DN), liner material, electrode material, and process temperature range. Also tell us the cable length you need. We keep common sizes in stock for fast dispatch. Send your enquiry to [email protected] or contact us via WeChat: +86 13512131526. Silver Automation Instruments ships to over 40 countries including Australia, Mexico, Nigeria, and India.