YSI 400 vs 700 Temperature Probe: BMET Connector & Specification Guide

This guide covers the technical comparison and selection between YSI 400 and YSI 700 temperature probe families. If you are troubleshooting a temperature probe that is already giving errors or inaccurate readings, see our temperature probe error troubleshooting guide — that page will refer you back here if the root cause turns out to be a YSI series mismatch. For multi-brand accessory compatibility checking, see our patient monitor accessory compatibility matrix.

1. What Are YSI 400 and YSI 700 Temperature Probes?

YSI (Yellow Springs Instrument) is the dominant thermistor standard used in clinical patient temperature monitoring. Nearly all patient monitors that measure temperature — whether bedside, transport, or anesthesia — use a thermistor element based on one of two YSI families: the 400 series or the 700 series.

The "400" and "700" designations refer to the thermistor's resistance-temperature curve — essentially, how the electrical resistance of the thermistor changes as temperature changes. The monitor's firmware contains a lookup table for one specific curve. When a probe with the correct thermistor is connected, the monitor reads the resistance, maps it against the lookup table, and displays an accurate temperature. When a probe with the wrong thermistor type is connected, the monitor still reads the resistance, still maps it against its lookup table — but produces an incorrect temperature because the resistance-to-temperature relationship is wrong.

2. Why YSI 400 and 700 Are Electrically Incompatible

The incompatibility is in the thermistor element itself — the physical component inside the probe tip whose resistance changes with temperature. YSI 400 series uses a thermistor with a nominal resistance of 2,252 Ω at 25 °C. YSI 700 series uses a thermistor with a nominal resistance of approximately 1,000 Ω at 25 °C.

Because the resistance values at every temperature point are different between the two series, a monitor calibrated for YSI 400 will misinterpret YSI 700 resistance values (and vice versa). The error is not linear — it varies across the temperature range, making simple offset correction impossible.

The key point: this is not a connector compatibility issue — it is an electrical characteristics issue. Even if you physically connect a YSI 400 probe to a YSI 700 input using an adapter cable, the readings will still be wrong. The adapter cable changes the plug shape, not the thermistor physics. For more on cable and connector electrical standards, see our medical cable specifications guide.

3. Resistance-Temperature Curves Compared

The following table shows the resistance values at clinically relevant temperature points for both YSI 400 and YSI 700 series. The magnitude of the difference at each point illustrates why cross-series substitution produces incorrect readings.

4. Key Specifications: Side-by-Side Comparison

5. Monitor Compatibility by Brand and Model

The following table maps major patient monitor brands and model families to their YSI series requirement. Use this as a quick reference when ordering probes. For brand-specific accessory details beyond temperature, see our Philips & GE monitor service guide and Mindray monitor technical resources.

6. How to Identify Which YSI Series Your Monitor Requires

There are three reliable methods to determine which YSI series a monitor uses, listed from most definitive to most practical:

Method 1: Service Manual Lookup

The monitor's service manual specifies the thermistor type in the temperature measurement section. Look for terms like "YSI 400 compatible," "2.252k NTC at 25 °C," or "YSI 700 compatible," "1k NTC at 25 °C." This is the authoritative source.

Method 2: Resistance Measurement

If you have a known-good probe that currently works correctly with the monitor, measure its resistance with a multimeter at room temperature (25 °C):

• ~2,252 Ω at 25 °C → YSI 400 series
• ~1,000 Ω at 25 °C → YSI 700 series

The measurement does not need to be exact — the two values are far enough apart that even an approximate reading clearly distinguishes the series. Use a digital multimeter on the 20kΩ range for best resolution.

Method 3: Adapter Cable Identification

The adapter cable between the monitor and the probe often indicates the YSI series in its part number or specification sheet. For example, Philips adapter cable 21082A is specified for YSI 400 probes. If the adapter cable documentation specifies a YSI series, use that as your guide.

7. Connector Types and Adapter Cable Considerations

Temperature probes connect to patient monitors through brand-specific adapter cables. The adapter cable has a monitor-end connector (proprietary to each brand) and a probe-end connector (typically a standard 2-pin or 1/4" phone-style jack). The YSI series is determined by the probe's thermistor, not by the adapter cable connector.

Standard Probe Connector Types

• 6.35mm (1/4") phone plug — Used by many YSI-standard reusable probes. Two-conductor connection.
• 2-pin mini connector — Used by Mindray, Comen, Edan, and some Dräger models.
• Brand-specific round connector — Nihon Kohden, Biolight, and some Philips direct-connect probes.

Adapter Cable Part Numbers by Brand

An adapter cable cannot convert between YSI 400 and YSI 700 compatibility. Adapter cables change the physical connector form factor only. If your monitor requires YSI 700 probes, you must use probes with YSI 700 thermistors — no cable will make a YSI 400 probe work correctly.

8. Reusable Probe Selection Guide by Brand

The following table lists compatible reusable temperature probes organized by monitor brand and probe type. All listed probes use YSI 400 series thermistors (the standard for current monitors).

9. Disposable Probe Selection Guide

Disposable temperature probes use the same YSI thermistor elements as reusable probes. They are used in operating rooms, ICUs, and situations where infection control requires single-patient use. The YSI series must match the monitor just as with reusable probes.

For infant incubator and warmer-specific temperature probes, see our infant incubator temperature probe collection.

10. Common Mistakes and How to Avoid Them

Mistake 1: Assuming All Temperature Probes Are Interchangeable

This is the most common and most dangerous mistake. The probes look identical. They connect with the same cable. But if the thermistor series is wrong, the reading is wrong — silently. Always verify the YSI series before installing any temperature probe on a patient.

Mistake 2: Thinking an Adapter Cable Converts YSI Series

Adapter cables change the connector shape. They do not change the thermistor resistance curve inside the probe. A YSI 400 probe connected through any adapter cable to a YSI 700 monitor will still produce incorrect readings.

Mistake 3: Not Labeling Probes by YSI Series

In mixed-fleet hospitals (e.g., bedside monitors using YSI 400 and legacy anesthesia equipment using YSI 700), probes can migrate between departments. Label all probes with their YSI series using color-coded tags or permanent markers. This simple step prevents the most common temperature probe failure mode.

Mistake 4: Replacing Probes Without Checking the Monitor's Series

When a temperature probe fails, the technician replaces it with the nearest available probe from stock. If the stock includes both YSI 400 and YSI 700 probes (which can look identical), a mismatch can occur. Organize your probe inventory by YSI series and clearly separate the two types.

For a complete walkthrough of temperature probe troubleshooting — including wire damage, connector contact failures, and out-of-range readings — see our temperature probe error troubleshooting guide. For overall preventive maintenance procedures including temperature probe inspection, refer to our patient monitor PM checklist.

Frequently Asked Questions